Ecology

Laboratory Manual
Ecology
Laboratory Manual

Darrell S. Vodopich
Baylor University
ECOLOGY LABORATORY MANUAL

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ISBN 978–0–07–338318–7
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Unless otherwise credited all photos © Darrell Vodopich.

Some of the laboratory experiments included in this text may be hazardous if materials are handled improperly or if proce-
dures are conducted incorrectly. Safety precautions are necessary when you are working with chemicals, glass test tubes, hot
water baths, sharp instruments, and the like, or for any procedures that generally require caution. Your school may have set
regulations regarding safety procedures that your instructor will explain to you. Should you have any problems with materials
or procedures, please ask your instructor for help.

www.mhhe.com
Contents

Preface vii Exercise 9

Welcome to Field Work and the Ecology Microcommunity Assessment 81
Laboratory ix
Exercise 10
Sampling a Plant Community 91
Exercise 1
The Nature of Data 1 Exercise 11
Sampling Animal Communities 105
Exercise 2
The Process of Science 11 Exercise 12
Species Diversity 115
Exercise 3
Soil Analysis 21 Exercise 13
Primary Productivity in an Aquatic Community 127
Exercise 4
Oxygen and Carbon Dioxide Cycling 33 Exercise 14
Competition 137
Exercise 5
Population Growth 45 Exercise 15
Natural Selection 149
Exercise 6
Age Distribution and Survivorship 51 Exercise 16
Adaptations of Vertebrates to Their
Exercise 7 Environment 159
Terrestrial Plant Community Assessment 63
Exercise 17
Exercise 8 Adaptations of Plants to Their Environment 165
Stream Ecosystem Assessment 69

TOC–1

v
Preface
I designed this manual to survey basic field and laboratory
techniques for an introductory ecology course. The exper-
iments and procedures are safe, easy to perform, and target
• First and foremost, instructors at any school with access
to a stream, a pond, some woodlands, and some grass-
lands can do these exercises. Specialized habitats are not
the needs of undergraduate classes. The manual includes needed and equipment is minimal.
photographs, traditional topics, and a few broad-based • This is an introductory manual. It assumes no previous
exercises targeted for the wide context of life science. Each knowledge of ecology, but it works best if the students
exercise has multiple, discrete procedures that help instruc- have completed introductory biology.
tors tailor the exercise to students’ needs, the style of the • The procedures work. The techniques and instructions
instructor, and the time and facilities available. in the manual and the accompanying resource guide
are detailed and straightforward, enough so that a
knowledgeable teaching assistant can set up and super-
TO THE STUDENT vise the techniques. However, this is not an autotuto-
rial manual. Students are often directed to speak with
This manual introduces you to a part of life science that you
their instructor. No manual can replace a real, live
probably don’t know much about. As you learn about ecol-
instructor.
ogy, you’ll spend equal time observing plants and animals
• A few of the exercises are purposely designed for ses-
around you and figuring out how to quantify their distribu-
sions in the lab without field work. Real-world teaching
tion, abundance, and interactions. Don’t hesitate to exceed
situations occasionally call for indoor sessions during
the observations outlined in the procedures—your future as
the semester.
a scientist depends on noticing things that others may over-
• Most exercises include enough variety of procedures to
look. In other words, don’t underestimate the role of simple
allow the instructor to pick and choose. The procedures
observation to support well-designed data collection and
are stand-alone, and customizing within or between
analysis. Now is the time to sharpen your skills in science
exercises is rather easy.
with a mix of work and relaxed observation. Have fun, and
• Each exercise involves a clear and rather singular con-
learning will come easily. Also, remember that this manual
cept. The introduction highlights that concept, and
is designed with both instructors and students in mind. Go
does it concisely. One exercise cannot completely sur-
to your instructors often with questions—their experience is
vey a broad topic such as gas exchange, competition, or
a valuable tool that you should use as you work.
population growth, but I hope they clearly compliment
the students’ accompanying lecture course.
TO THE INSTRUCTOR • The math required of the student does not overwhelm
observations of ecology and adaptations of real organ-
It has always bothered me that available ecology lab manu- isms. Ecology is certainly a quantitative science, and
als are overwhelming in presentation. They are either too tables with calculations abound throughout the manual.
customized to use in different environments, too mathemat- But a purposeful balance is struck between the need for
ical to let the biology of organisms and their environment quantification and trying to avoid massive calculations
shine through, or too vague to clearly lay out the steps of and statistics that mask learning of fundamental data-
fundamental field and lab procedures we often use to answer collection techniques.
ecological questions. So, as many of us have done, I designed • Questions are included with each procedure to bring
my own. I hope you find it useful. students into the interpretive stage of ecological studies.
This manual is all about observing the natural world, Be sure to examine the “Questions for Further Thought
asking questions, learning quantitative field techniques, and and Study” in each exercise. They can expand students’
melding these activities into good science. The following perceptions that each exercise has broad applications to
design features of each exercise support this goal: their world.

P–1

vii
 In summary, this manual’s straightforward approach
emphasizes experiments and activities that optimize stu-
dents’ investment of time and your investment of supplies,
equipment, and preparation. Remind your teaching assis-
tants that discussions and interactions between student and
instructor are major components of a successful laboratory
experience.

REVIEWERS

We thank the following reviewers for their helpful com-

ments and suggestions during the preparation of this labora-
tory manual.
David L. Boose, Gonzaga University
Scott Burt, Truman State University
James R. Curry, Franklin College
Elizabeth L. Rich, Drexel University

Darrell S. Vodopich

P–2

viii Preface
Welcome to Field Work
and the Ecology Laboratory

W elcome to ecology. Reading your textbook and

attending lectures are certainly important ways to
learn about ecology, but nothing can replace the impor-
these exercises most useful and enjoyable, follow these
guidelines:

tance of the laboratory. In lab you will get hands-on experi- THE IMPORTANCE OF COMING TO CLASS
ence with what you’ve heard and read about ecology—for
example, you’ll observe organisms, learn techniques, test The procedures in this manual are designed to help you
ideas, collect data, and draw conclusions about what you’ve experience ecology firsthand. To do that, you must attend
learned. You’ll do ecology. class. If you want to do well in your ecology course, you’ll
You will enjoy the exercises in this manual—they’re need to attend class and pay attention. To appreciate the
interesting, informative, and best of all they will intro- importance of class attendance for making a good grade in
duce you to field work. I’ve provided questions to test your ecology course, examine figure 1, a graph showing how
your understanding of what you’ve done. In some of the students’ grades in an introductory biology course relate to
exercises, I have also asked you to devise your own experi- their rates of class attendance. Data are from a general biol-
ments to answer questions that you have posed. To make ogy class, University of Minnesota, 2003.
100

B
80

D
60
Grade (%)

40

20

0
0 20 40 60 80 100
Attendance (% of classes attended)

W–1

ix
BEFORE COMING TO LAB SAFETY IN THE LABORATORY

Read the exercise before coming to lab. This will give you a The laboratory safety rules listed in table 1 will help make
general idea about what you are going to do, and why you’re lab a safe place for everyone to learn ecology. Remember, it
going to do it. Knowing this will not only save time, it will is much easier to prevent an accident than to deal with its
also help you finish your work. consequences.
Read the laboratory safety rules listed in table 1. If you
do not understand them, or if you have questions, ask your
WHEN IN LAB instructor for an explanation. Then complete table 1 and
sign the statement that is at the bottom of page xiii.
1. Don’t start the exercise until you’ve discussed the
exercise with your laboratory instructor. She/he will
give you specific instructions about the work ahead AFTER EACH LABORATORY
and tell you if the exercise will be modified.
2. Work carefully and thoughtfully, and stay focused Soon after each lab, review what you did. What questions
as you work. You can get your work done within the did you answer? What data did you gather? What conclu-
allotted time if you are well prepared and stay busy. sions did you make?
Also note any questions that remain. Try to answer
3. Discuss your observations, results, and conclusions
these questions by using your textbook or by visiting the
with your instructor and lab partners. Perhaps their
library. If you can’t answer the questions, discuss them with
comments and ideas will help you better understand
your instructor.
what you’ve observed.
Welcome to ecology!
4. Always follow instructions and safety guidelines pre-
sented by your instructor.
5. If you have questions, ask your instructor.

W–2

x Welcome
Table 1
Laboratory Safety Rules
Why is this rule important?
Rule What could happen if this rule is not followed?

Behave responsibly. No horseplay or fooling around while in lab.

Do not bring any food or beverages into lab, and do not eat, drink,
smoke, chew gum, chew tobacco, or apply cosmetics when in lab. Never
taste anything in lab. Do not put anything in lab into your mouth. Avoid
touching your face, chewing on pens, and other similar behaviors while
in lab.

Unless you are told otherwise by your instructor, assume that all chemicals
and solutions in lab are poisonous and act accordingly. Never pipette by
mouth. always use a mechanical pipetting device (e.g., a suction bulb) to
pipette solutions. Clean up all spills immediately, and report all spills to
your instructor.

Read the labels on bottles and know the chemical you are dealing with.
Do not use chemicals from an unlabeled container, and do not return
excess chemicals back to their container.

Unless your instructor tells you to do otherwise, do not pour any solutions
down the drain. Dispose of all materials as per instructions from your
instructor.

If you have long hair, tie it back. If you are using open flames, roll up loose
sleeves. Wear contact lenses at your own risk; contacts hold substances
against the eye and make it difficult to wash your eyes thoroughly.

Treat living organisms with care and respect.

Your instructor will tell you the locations of lab safety equipment, including
fire extinguishers, fire blanket, eyewash stations, and emergency showers.
Familiarize yourself with the location and operation of this equipment.

Notify your instructor of any allergies to latex, chemicals, stings, or other

substances.

If you break any glassware, do not pick up the pieces of broken glass with
your hands. Instead, use a broom and dustpan to gather the broken glass.
Ask your instructor how to dispose of the glass.

Unless told by your instructor to do otherwise, work only during regular,

assigned hours when the instructor is present. Do not conduct any
unauthorized experiments; for example, do not mix any chemicals without
your instructor’s approval.

Do not leave any experiments unattended unless you are authorized to

do so. If you leave your work area, slide your chair under the lab table.
Keep walkways and desktops clean and clear by putting books, backpacks,
and so on along the edge of the room, in the hall, in a locker, or in an
adjacent room.

Know how to use the equipment in lab. Most of the equipment is

expensive; you may be required to pay all or part of its replacement cost.
Keep water and solutions away from equipment and electrical outlets.
Report malfunctioning equipment to your instructor. Leave equipment in
the same place and condition that you found it. If you have any questions
about or problems with equipment, contact your instructor.

Know what to do and whom to contact if there is an emergency. Know

the fastest way to get out of the lab. Immediately report all injuries—no
matter how minor—to your instructor. Seek medical attention
immediately if needed. If any injury appears to be life-threatening, call
911 immediately.

At the end of each lab, clean your work area, wash your hands thoroughly
with soap, slide your chair under the lab table, and return all equipment
and supplies to their orignal locations. Do not remove any chemicals or
equipment from the lab.

W–3

Welcome xi
 Name _________________________________________

Lab Section _________________________________________

Your lab instructor may require that you submit this page at the end of today’s lab.

1. In the space below, write an analysis of the data shown in figure 1 (page ix).

After completing table 1, sign this statement:

2. I have read, understood, and agree to abide by the laboratory safety rules described in this exercise and discussed by my
instructor. I know the locations of the safety equipment and materials. If I violate any of the laboratory safety rules, my
instructor may remove me from the lab.

____________________________________________
Signature

____________________________________________
Name (printed)

____________________________________________
Date

W–5

Welcome xiii
exercise one

The Nature of Data 1

Objectives Statistics, such as a mean derived from samples, esti-
mate variables of the entire population. For example, we
As you complete this lab exercise you will: measure the lengths of 20 sampled oak leaves and calculate
1. Calculate measures of central tendency for a data a sample mean. This mean estimates the mean length of all
set and the different perspectives they provide on the leaves in the population of oaks. We will never know
the same data set. the exact mean for the entire population so we use the sam-
2. Understand the relationship between the mean ple mean (x) to estimate the population mean (m). Sample
and the variation for replicate values of a data set. statistics are estimates of population statistics.
3. Examine the frequency distribution for three data The major terms to quantitatively describe a set of data
sets. are:
4. Calculate the confidence intervals surrounding population—the entire collection of possible measure-
the mean of replicate values. ments about which we wish to draw conclusions. All
frogs in a pond, or all possible measurements of a for-
est’s soil nitrate content, are examples of populations.
sample—a measurement(s) representing all possible
E cologists collect data . . . and a lot of it. The data are
extensive because (1) natural processes are com-
plex; (2) ecological processes involve many variables; and
measurements of a parameter of a population. It is a
subset of all possible measurements in a population.
variable—a measurable characteristic of a biological
(3) each variable can vary greatly. For example, plant entity. It may vary from one organism to the next, one
growth is a complex process, the number of factors influ- environment to the next, or one moment to the next.
encing growth is immense, and factors such as rainfall vary statistic—an estimate of a parameter based on a repre-
from day to day. To understand this kind of complexity and sentative sample of a population. The mean of a set of
variation, ecologists analyze data to search for patterns and values is a statistic.
relationships among variables. In other words, they look at
the nature of their data.
DATA SETS
POPULATIONS AND SAMPLES Data are usually organized into a data set, defined as a series
OF POPULATIONS of repeated measures of one or more variables. A variable
might be the number of eggs in a robin’s clutch, the con-
Researchers gather data to describe and learn about large centration of nitrate in the soil, or the monthly rainfall on
populations. Unfortunately, most populations are too big a prairie. You will quickly learn that the most noticeable
for us to measure a variable for every member of the popu- attribute of a data set is its variation.
lation. For example, oak trees are too numerous for us to
measure the length of all of their leaves. Instead, we sample Procedure 1.1
the population of oak leaves. Sampling means that we take Examine variation within a data set.
a relatively small number of measurements that represent
the entire population. The characteristics we measure, such 1. Examine the data set in table 1.1.
as leaf length, are variables, and the values for a variable 2. These data are measurements of the length of
are our data. Ecologists use these data to calculate statistics 20 leaves randomly selected from an oak tree. Notice
such as means, variances, etc. that describe our sample. that all leaves are not the same length.

1–1

1
3. Use the blanks provided in table 1.1 to rearrange and Procedure 1.2
record the data from lowest to highest value. Examine measures of central tendency of an example
data set.
Questions 1
Do the leaf lengths shown in table 1.1 appear to be simply 1. Calculate and record the mean of the data set in
random numbers? If not, what pattern or tendency do you table 1.1.
detect? _________________________________________
Questions 2
________________________________________________ Are any of the leaf measurements the same as the mean?
What factors might cause variation in leaf length for an oak ________________________________________________
tree? ___________________________________________
How many leaves were longer than the mean? __________
________________________________________________
________________________________________________
How would you sample leaves to test one of those influences
How many leaves were shorter than the mean? _________
on leaf length? ___________________________________
________________________________________________
________________________________________________
Does the mean always describe the “typical” measurement?
Why or why not? _________________________________
TABLE 1.1
________________________________________________
A SAMPLE DATA SET OF 20 REPLICATE
MEASUREMENTS OF OAK-LEAF LENGTHS ________________________________________________
Oak Leaf Lengths:
2. Determine and record the median and mode of the
78 69 62 74 69 51 45 40 9 64 data set in table 1.1.
____ ____ ____ ____ ____ ____ ____ ____ ____ ____
65 64 61 69 52 60 66 71 72 27 Questions 3
____ ____ ____ ____ ____ ____ ____ ____ ____ ____ Notice that in the sample, the mean differs from the
median. What is responsible for this difference between the
Measures of Central Tendency: mean and the median? _____________________________
Mean __________ Median __________ Mode __________ ________________________________________________
How would the mean change without the 9-mm leaf?
MEASURES OF CENTRAL TENDENCY ________________________________________________
________________________________________________
The most likely “pattern” revealed by examining the data set
in table 1.1 is the central tendency of the values. They are How would the median change without the 9-mm leaf?
not spread out randomly. They tend to be clustered around a
________________________________________________
central value somewhere in the 50s, 60s, or 70s rather than
randomly scattered from 1 to 100. That shouldn’t surprise ________________________________________________
you—oak leaves don’t grow randomly. Their development
How would the mode change without the 9-mm leaf?
has a pattern.
The three most common measures of central tendency ________________________________________________
are mean, median, and mode. The mean (x) is the arithme-
________________________________________________
tic average of a group of measurements. It is the sum of all
the values ( a xi) divided by the number of values (N).
mean x a xi@N VARIATION WITHIN A DATA SET
The median is the middle value of a group of measure-
ments that have been ranked from lowest to highest or Measures of central tendency don’t fully describe varia-
highest to lowest. The mode is the value that appears most tion within a data set. Examine the two small sets listed in
often in the data set. table 1.2.
The mean is the most common measure of central Notice that the mean and the median are informative, but
tendency, but the median and mode are sometimes useful they do not describe variation in the data. The stream fish data
because they are less sensitive to extreme values. To appre- set has considerably more variation even though the mean is
ciate the differences among the measures of central ten- the same as for the pond fish data set. Variation is best quanti-
dency, complete Procedure 1.2. fied by range, variance, standard deviation, and standard error.

1–2

2 Exercise 1
 TABLE 1.2 Number of Pond Deviation
Fish Collected Mean from the mean (Deviation)2
TWO SAMPLE DATA SETS WITH DIFFERENT xi x 1 xi 2 x 2 1 xi 2 x 2 2
LEVELS OF VARIATION
25 30 5 25
Number of Fish Collected in Five Replicate Seine-Net Samples:
28 30 2 4
Pond Fish Data Set: 25 28 30 32 35 30 30 0 0
mean (x) 30; median 30 32 30 2 4
range ______________ variance ______________ 35 30 5 25

standard deviation ______________ standard error ______________ Sum of squared deviations 58

Variance 14.5
Stream Fish Data Set: 10 20 20 25 75
mean (x) 30; median 20
range ______________ variance ______________
The formulae for the sum of squared deviations and the
standard deviation ______________ standard error ______________ variance are:
N

a 1 xi 2 x 2
2
The range is the difference between the smallest and sum of squared deviations 58.0
i51
the largest values of the data set—the wider the range, the
sum of squared deviations
greater the variation. The range of the pond fish data set variance 58/4 14.5
N21
is 25–35 10; the range of the stream fish data set is
10–75 65. The mean number of fish per sample is the where
same for both data sets, but the ranges indicate much more N total number of samples
variation among the stream samples. Notice that the range x the sample mean
can be artificially inflated by one or two extreme values, xi measurement of an individual sample
especially if only a few samples were taken. This formula for sum of squared deviations is really
Questions 4 quite simple. The formula 1 xi 2 x 2 2 is the squared deviation
N
Examine the values for the stream and pond fish data sets. from the mean for each value. The summation sign ( a )
In which data set is the variation most influenced by a sin- i51
gle value? _______________________________________ means to sum all the squared deviations from the first one
(i 1) to the last one (i N). The sum of squared devi-
________________________________________________ ations (58) divided by the number of samples minus one
What is the best way to collect data and prevent a single (5 1 4) equals the variance. The variance for these
sample from skewing the measures of central tendency and data is 58/4 14.5.
variation? _______________________________________ Variance is a good measure of the dispersion of values
about the mean. A second, and more commonly cited, mea-
________________________________________________ sure of variation is the standard deviation. The standard
Could two samples have the same mean but different ranges? deviation (S) equals the square root of the variance. For our
Explain. ________________________________________ example data set:

________________________________________________ standard deviation (S) "variance "14.5 3.8

Could two samples have the same range but different means? Standard deviation is usually reported with the mean
Explain. ________________________________________ in statements such as, “The mean number of fish per sample
was 30 3.8.” The standard deviation helps us understand
________________________________________________
the spread of values in a data set. For normal distributions
of measurements, the mean 1 SD includes 68% of the
Variance measures how data values vary about the mean. measurements. The mean 2 SD includes 95% of the mea-
Variance is much more informative than the range, and is surements (fig. 1.1).
easy to calculate (see the following example). First, calculate Another useful measure of the spread of data about a
the mean. Second, calculate the deviation of each sample mean (i.e., variation) is the standard error (Sx). This value
from the mean. Third, square each deviation. Then sum the measures the error from having a limited sample size (N).
deviations. The summation is called the sum of squared devia- Clearly, a small sample (N) has more sampling error than
tions (or sum of squares). Finally, divide the sum of squared does a large sample. The term “sampling error” doesn’t mean
deviations by the number of data points minus one to calcu- that we have done something wrong. But we must document
late the variance (S2). The example uses the pond fish data sampling error to use later in our calculations of confidence
set (table 1.2). Record the calculated values in table 1.2. in the sample mean.

1–3

The Nature of Data 3

 High Percent of area distribution. A normal distribution is common for ecologi-
cal data—most values are near the mean and fewer values
are at the extremes. Many variables are normally distrib-
uted, and many statistical tests used by ecologists assume
that the variable is normally distributed.
Proportion

Procedure 1.4
68.3% Examine a frequency distribution of heights.
95.5% 1. Examine the data presented in figure 1.2 for the
± 1 S.D. height of 119 female college students.
99.7%
± 2 S.D.
± 3 S.D. Questions 6
Low Are the heights distributed as you expected? How so?
−3 −2 −1 0 1 2 3

Mean ________________________________________________
Standard deviations (S.D.) from mean
________________________________________________
Figure 1.1
Normal distribution graph. Do you see evidence of central tendency in this data set?
________________________________________________
Do the data appear normally distributed? ______________
Standard error is calculated as:
________________________________________________
Sx 5 S^ 5 1.70
"N
In what way do the data deviate from normality? ________
where
S standard deviation ________________________________________________
N total number of samples ________________________________________________

Procedure 1.3 2. Examine the mean, median, and mode provided for
Calculate four measures of variation. the data set in figure 1.2.

1. Complete table 1.2 by calculating the range, Question 7

variance, standard deviation, and standard error of One criterion for a normal distribution is that the mean,
the stream fish data set. median, and mode are equal. Are they equal for the data in
figure 1.2? _______________________________________
Question 5
The range indicates greater variation in the stream fish data ________________________________________________
set. Do the other three measures of variation also indicate 3. Calculate and record the variance, standard
greater variation? _________________________________ deviation, and standard error for the data. The mean
and sum of squared deviations are provided to speed
your calculations.
FREQUENCY DISTRIBUTION
Procedure 1.5
Some data sets are better understood if they are displayed in Calculate and graph the frequency distribution for a data
a graph called a frequency distribution (fig. 1.2). Frequency set for mosquito larvae occurrence in tree-hole cavities.
distributions summarize data at a glance and reveal subtleties
that might not be apparent in calculations of central tendency. 1. Some mosquito species lay their eggs in tree-hole
The abscissa (x axis) is plotted as Data Class and the ordinate cavities that hold small volumes of water (< 1 L).
(y axis) as Frequency of Occurrence in each Data Class. The Examine the data in figure 1.3 showing the densities
raw data for the frequency distribution in figure 1.2 are below of mosquito larvae (number per liter) found in
the graph. The shape of the curve reveals the nature of varia- 108 tree-hole cavities.
tion in the data set. A broad and flat curve reveals high varia- 2. Plot a frequency distribution in the graph provided in
tion. A narrow and high-peak curve reveals less variation. figure 1.3.
Frequency distributions often have gradually tapering 3. Calculate and record in figure 1.3 the mean, median,
“tails” of frequencies toward each end of the curve (fig. 1.2). mode, variance, standard deviation, and standard
These tails produce a “bell-shaped” curve called a normal error for this data set.

1–4

4 Exercise 1
 25

20
Frequency of Occurrence

15

10

0
55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74
Data Class (in.)

Frequency of
Raw Data (height in inches) Data Class Occurrence
Height (in.) Number of Students
56 62 64 65 66 68 55 0
57 62 64 65 66 68 56 1
57 62 64 65 66 68 57 2
58 62 64 65 66 68 58 3
58 62 64 65 66 68 59 2
58 63 64 65 66 68 60 3
59 63 64 65 66 68 61 6
59 63 64 65 66 68 62 8
60 63 64 65 66 68 63 11
60 63 64 65 67 69 64 17
60 63 64 65 67 69 65 20
61 63 64 65 67 69 66 16
61 63 64 65 67 69 67 11
61 63 65 66 67 70 68 9
61 63 65 66 67 70 69 4
61 63 65 66 67 70 70 5
61 64 65 66 67 70 71 1
62 64 65 66 67 70 72 0
62 64 65 66 67 71 73 0
62 64 65 66 67 74 0
Sum of Squared
N Mean Median Mode Deviations
119 64.6 65 65 1078.6 Variance Std. Dev. Std. Error

Figure 1.2
A frequency distribution of 119 height measurements of college-aged women.

1–5

The Nature of Data 5

 1.0

0.9

0.8

0.7
Frequency of Occurrence

0.6

0.5

0.4

0.3

0.2

0.1

0.0
0−10 11−20 21−30 31−40 41−50 51−60 61−70 71−80 81−90 91−100

Data Class (mosquitoes L–1)

Frequency of
Raw Data (mosquitoes L 1) Data Class Occurrence
Number of
1
Mosquitoes L Cavities
68 0 50 12 42 24 22 11 0–10 36
24 15 8 29 5 2 99 38 11–20 25
58 56 21 11 32 18 7 23 21–30 17
54 4 26 93 13 29 10 30 31–40 9
3 15 10 1 14 26 0 19 41–50 7
10 5 51 3 2 9 5 17 51–60 6
13 19 71 60 20 20 1 0 61–70 2
7 49 73 17 63 48 14 1 71–80 4
11 44 28 75 30 2 8 12 81–90 0
36 13 17 9 27 6 46 6 91–100 2
8 4 16 53 16 15 41
5 22 35 9 7 34 37
19 21 18 74 33 3 8
39 4 25 5 2 28 31 Mean Median Mode

Figure 1.3
Survey of the density (mosquito larvae L 1) of the mosquito, Aedes triseriatius, occurring in tree-hole cavities (N 108).

1–6

6 Exercise 1
Questions 8 The 95% confidence interval surrounding a population
Is this variable normally distributed? __________________ mean is calculated as:
________________________________________________ m x t0.05(Sx)

How many values were greater than the mean? How many where
were less? _______________________________________ m population mean
x sample mean
________________________________________________
t0.05 value from student’s t table at the 95% confidence
What value best describes the central tendency of this data level
set? ____________________________________________ Sx standard error

________________________________________________ The value of t0.05 is selected from a student’s t table

available in most statistics textbooks. The appropriate
student’s t value is determined by the degrees of freedom
(N 1) and the confidence level, which in this case is 95%
Procedure 1.6
( 0.05 probability that the population mean occurs out-
Collect an original data set and calculate its measures of side the range). For example, consult a student t table and
central tendency and variation. you will find that the appropriate t0.05 value for a sample of
1. Follow your instructor’s directions to gather an 30 (N 30) and for a 95% confidence interval is 2.045.
original data set. To calculate the 95% confidence interval using the oak
2. Record the raw data in figure 1.4. leaf measurements in table 1.1, first calculate the mean and
standard error (see table 1.1).
3. Calculate and record in figure 1.4 the mean, median,
mode, variance, standard deviation, and standard x mean _____
error for these data. Sx standard error _____
N 20
Questions 9
DF degrees of freedom (N 1) 19
Is the variable in your original data set normally distributed?
A student’s t table shows that the critical value
________________________________________________
of t0.05 is 2.09.
________________________________________________
Therefore, the 95% confidence intervals surrounding the
How many values were greater than the mean? _________ mean of 20 oak leaf samples is:
How many were less? ______________________________ m x 2.09(Sx) _______

What value best describes the central tendency of this data With this confidence interval, we can state that there
set? ____________________________________________ is a 95% probability that the population mean of the oak
leaves is somewhere between _____ and _____.
________________________________________________
Procedure 1.7
POPULATION MEANS AND Calculate the confidence intervals for example and
CONFIDENCE INTERVALS original data sets.

Population statistics can never be known exactly because we 1. Calculate the 95% confidence interval for the
only measure samples of the population, not every member. population mean of stream fishes per seine haul from
Therefore, values such as the population mean (m) must be table 1.2.
estimated by the sample mean (x). If variation is low, then x t0.05(Sx) ____ ____
we have high confidence in the sample mean as an estima-
tor of the population mean. 2. Calculate the 95% confidence interval for the
A confidence interval is a range of values within which population mean of pond fishes per seine haul from
the true population mean occurs with a particular probabil- table 1.2.
ity. Ecologists usually express their sample means with 95% x t0.05(Sx) ____ ____
level of confidence, also called a 95% confidence interval.
3. Calculate the 95% confidence interval for the
For example, a sample mean (x) may be 64.6 cm (see fig-
population mean height of female students whose
ure 1.2 for a sample of height measurements for college-aged
sample is presented in figure 1.2.
women). After calculations, we are 95% confident that the
population mean lies between 64.0 and 65.1. x t0.05(Sx) ____ ____

1–7

The Nature of Data 7

 25

20
Frequency of Occurrence

15

10

Data Class

Frequency of
Raw Data Data Class Occurrence

Mean Median Mode

Variance Std. Dev. Std. Error

Figure 1.4
Original data set for analysis.

1–8

8 Exercise 1
Questions for Further Thought and Study

1. Replicate samples are central to good experimental design. How would the frequency distribution of heights in
figure 1.2 differ if only five or six measurements were made?

2. Do you suspect that any biological variables have a perfectly normal distribution? Why or why not?

3. What is the relationship between variation in a data set and the width of the confidence intervals surrounding the
estimate of the population mean?

1–9

The Nature of Data 9

exercise two

The Process of Science 2

Objectives
As you complete this lab exercise you will:
1. Define science and understand the logic and
sequence of the scientific method.
2. Develop productive observations, questions, and
hypotheses about the natural world.
3. Calculate the range, mean, and standard devia-
tion for a set of replicate measurements.
4. Design and conduct a controlled experiment to
test a null hypothesis.

T he term science brings to mind different things to differ-

ent students. To some students, science is a book. To
others, it’s a microscope, a dissected frog, or a course that
you take. In fact, it’s none of these things. A good defini- Figure 2.1
Science is the process of learning about the natural world. Gathering
tion of science for biological research is the orderly process of
repeated and unbiased measurements (data) is the engine for testing
posing and answering questions about the natural world through hypotheses and answering questions. This student is recording the
repeated and unbiased experiments and observations. This defi- presence or absence of pollinators to answer questions about the daily
nition emphasizes that science is a process rather than a timing of pollination.
book, or a course, or a list of facts. Science is not a “thing.”
It’s a way of doing things—a way of learning and knowing Question 1
about the natural world (fig. 2.1). What practices besides science are used among world cul-
Our definition also emphasizes that people do science tures to address questions of the natural world? __________
by asking questions and then doing experiments to answer
________________________________________________
those questions. Asking questions and being curious are part
of human nature, and science is a human activity. Like any Questioning and testing are a part of science that enable
human task, it takes practice to do science effectively. us to systematically sift through natural variation to find
Finally, our definition emphasizes that science is a tool underlying patterns. The natural world includes much varia-
for learning about the natural world. It is ineffective for tion, and learning biology would be relatively easy if simple
moral choices, ethical dilemmas, and untestable ideas. For observations accurately revealed patterns of the natural world.
example, the scientific method cannot tell us if pollution is But they usually don’t—nature is too complicated to rely solely
good or bad. It can tell us the environmental consequences on simple observation. We certainly can learn much about our
of pollution, but whether these consequences are “good” or environment by looking around us. But, casual observations
“bad” is a judgment that we make based on our values, not are often biased and misleading because nature varies from
on science. Although this is an important limitation of the time to time and from organism to organism. Biologists need
scientific method, science remains one of the most powerful a structured and repeatable process for testing ideas about the
ways of learning about our world. variation in nature. Science is that process.

2–1

11
Question 2
What factors might cause variation in measurements such
as the heights of 10-year-old pine trees, or the kidney filtra-
tion rates of 10 lab mice? ___________________________
________________________________________________
The process of science deals with variation through an
organized sequence of steps that maintain as much objectivity
and repeatability as possible. Although these loosely organized
steps, sometimes called the scientific method, vary from situa-
tion to situation, they are remarkably effective for research and
problem solving. Typical steps in the process of science are:
• Make observations
• Pose and clarify testable questions
• Formulate hypotheses
• Conduct experiments to gather data
• Quantify and summarize the data Figure 2.2
Pill bugs are excellent experimental organisms for testing hypotheses
• Test the hypotheses about microenvironments under logs and within leaf litter. They are
• Answer the questions and make conclusions crustaceans, not insects. Unlike most crustaceans, they are terrestrial
rather than aquatic.

DEVELOPMENT OF OBSERVATIONS,
QUESTIONS, AND HYPOTHESES Observation 2: Fungi grow on leftover bread more
Make Insightful Observations than on leftover meat.
2. Which of these observations is most useful for
Good scientists make insightful observations. But that’s not research? Why? ______________________________
as easy as it seems. Consider these two observations:
___________________________________________
Observation 1: Fewer elk live in Yellowstone National
3. Insert the more insightful observation in Work-
Park than in the past.
sheet 1.
Observation 2: The density of elk in Yellowstone 4. Pill bugs (sometimes called roly-poly bugs) are good
National Park has declined during the consecutive model organisms for research. They often find food
dry years since the reintroduction of the native wolf and shelter where fungi are decomposing leaf litter
population. (fig. 2.2). We may be interested in whether pill bugs
are attracted to leaves or to fungi growing on the
Which of these two observations is the strongest and most leaves’ surface. Consider this observation.
useful? Both of them may be true, but the second one is
much more insightful because it provides a context to the Observation 1: Pill bugs often hide under things.
observation that elk populations are declining. It suggests 5. Propose a more productive observation for a study of
a relevant factor—the introduction of the wolf popula- pill bug feeding.
tion—as a productive topic for investigation. It also suggests Observation 2: ______________________________
a relationship between the density of elk and variation in
the local environment. ___________________________________________
For the remainder of this exercise you will simultane- 6. Record Observation 2 in Worksheet 2 on page 19.
ously develop questions, hypotheses, and designs for two You may revise this later.
experiments—an experiment involving yeast nutrition (see
Worksheet 1 on page 18) and an experiment investigating
food preferences for pill bugs (see Worksheet 2, page 19). Pose and Clarify Testable Questions
Procedure 2.1 Productive observations inspire questions. Humans think in
Make productive observations. terms of questions rather than abstract hypotheses or num-
bers. Phrasing a good question takes practice and experience,
1. Consider the following two observations.
and the first questions that capture our attention are usually
Observation 1: Fungi often grow on leftover food. general. For example, “Which nutrients can yeast most

2–2

12 Exercise 2
readily metabolize?” is a general question that expands the treatment variables, and implies how they will be compared.
observation posed in Procedure 2.1. This question is broad A hypothesis is a statement rather than a question, and your
and the type of question that we ultimately want to under- data analysis ultimately determines whether you reject your
stand. Record this as General Question in Worksheet 1. hypothesis or accept it (or more formally stated, fail to reject
Broad questions are important, but they are often your hypothesis). Accepting a hypothesis does not necessar-
vague. Therefore, scientists usually refine and subdivide ily mean that it is true. More specifically, it means that you
broad questions into more specific ones. For example, a do not have enough evidence to reject it.
more specific question is “What classes of biological mol- An example hypothesis is:
ecules are most readily absorbed and metabolized by
yeast?” Record this as Specific Question 1 in Worksheet 1. The mean number of eggs produced per clutch by
A further clarification might be “Does yeast absorb eagles nesting within 10 km of the coast of Alaska
and metabolize carbohydrates better than it absorbs and is not significantly different from the mean number
metabolizes proteins?” This is a good, specific question produced by eagles nesting more than 10 km from
because it clearly refers to organisms, processes, and likely the coast.
variables. It also suggests a path for investigation—that is, it Hypotheses are either accepted or rejected. There are no
suggests an experiment. Record this as Specific Question 2 “partial truths” or “middle ground.” This may seem like a
in Worksheet 1. rather coarse way to deal with questions about subtle natural
Procedure 2.2 processes, but using controlled experiments to either accept
or reject a hypothesis is proven and effective. The heart of
Pose and refine questions.
science is gathering and analyzing experimental data that
1. Examine these two questions. lead to rejecting or accepting hypotheses relevant to the
Question 1: Do songbird populations respond to the questions we want to answer.
weather? In this exercise you are going to do science as you inves-
tigate yeast nutrition and then experiment with food choice
Question 2: Do songbirds sing more often in warm
by pill bugs. As yeasts ferment their food, CO2 is produced
weather?
as a byproduct. Therefore, you can measure the growth of
2. Which of these questions is the most useful for yeast by the production of CO2 (fig. 2.3).
further investigation? Why? ____________________ A hypothesis related to our question about yeast growth
___________________________________________ might be:
3. Examine the following general question, and record H0: CO2 production by yeasts that were fed sugar is
it in Worksheet 2. not significantly different from the CO2 production
General Question: What influences the distribution by yeasts that were fed protein.
of pill bugs?
A related alternative hypothesis can be similarly stated:
4. Propose a specific question that refers to the food
of pill bugs as a variable, and record it here and in Ha: Yeasts produce more CO2 when fed sugar than
Worksheet 2. You may revise this later. when fed protein.
Specific Question 1 ___________________________ The first hypothesis (H0) is a null hypothesis because it
___________________________________________ states that there is no difference. This is the most common
way to state a clear and testable hypothesis. Researchers
5. Propose a more specific question about pill bugs eating find it more useful to associate statistical probabilities with
leaves, as opposed to pill bugs eating fungi growing null hypotheses rather than with alternative hypotheses. It
on leaves. Record it here and in Worksheet 2. You is usually more appropriate to accept or reject a hypothesis
may revise this later. with statistics when the hypothesis proposes no effect rather
Specific Question 2 ___________________________ than when there is an effect. Your instructor may elaborate
on why researchers test null hypotheses more effectively
___________________________________________ than alternative hypotheses. Record the null hypothesis in
Worksheet 1.
A null hypothesis should be testable and well written.
Formulate Hypotheses
In our experiment, the null hypothesis (1) specifies yeast as
Well-organized experiments require that questions be the organism, population, or group that we want to learn
restated as testable hypotheses. A hypothesis is a statement about; (2) identifies CO2 production as the variable being
that clearly states the relationship between biological vari- measured; and (3) leads directly to an experiment to evalu-
ables. A good hypothesis identifies the organism or process ate treatment variables and compare means of replicated
being investigated, identifies the response variables and measurements.

2–3

The Process of Science 13

 5. Propose a more effective null hypothesis. Be sure that
it is null, that it is testable, and that it includes the
variable you will control in an experiment.
Hypothesis 2 (H0): ___________________________
___________________________________________
6. Record your null hypothesis in Worksheet 2.

EXPERIMENTATION AND DATA

ANALYSIS: YEAST NUTRITION
Gather Experimental Data
To test our hypothesis about yeast growth we must design
a controlled and repeatable experiment. The experiment
suggested by our specific question and hypothesis involves
offering a sugar such as glucose to one population of yeast,
offering protein to another population of yeast, and then
measuring their respective growth rates. Fortunately, yeast
grows easily in test tubes. As yeast grows in a closed, anaero-
bic container it produces CO2 in proportion to how readily
Figure 2.3 it uses the available food. You can easily measure CO2 pro-
These tubes of yeast are fermenting nutrients in solution. The CO2 duction by determining the volume of CO2 that accumu-
produced by the yeasts accumulates in the inverted test tube and lates at the top of an inverted test tube.
indicates the yeasts’ rate of metabolism. From right to left, the tubes Experiments provide data that determine if a null
have abundant nutrients, low nutrients, and a control with no added
nutrients, respectively. hypothesis should be accepted or rejected. A well-designed
experiment links a biological response to different levels of
the variable being investigated. In this case, the biological
response is CO2 production indicating growth. The levels of
the variable are sugar and protein. These levels are called
treatments, and in our experiment they include glucose,
Procedure 2.3 protein, and a control. The treatment variable being tested
Formulate hypotheses. is the type of food molecule (i.e., protein, sugar), and the
response variable is the CO2 production that indicates yeast
1. Examine the following two hypotheses. growth.
Hypothesis 1: Songbirds sing more when the weather A good experimental design compensates for natural
is warm. variation. It should (1) include replications; (2) test only
one treatment variable; and (3) include controls. Replica-
Hypothesis 2: The number of bird songs heard per tions are repeated measures of each treatment under the
hour during daylight temperatures above 80°F is not same conditions. Replications effectively deal with naturally
significantly different from the number heard per hour occurring variation. Usually, the more replications, the bet-
at temperatures below 80°F. ter. Your first experiment today includes replicate test tubes
2. Which of these hypotheses is the most useful for of yeast, each treated the same. Good design also tests only
further investigation? Why? ____________________ one treatment variable at a time.
Good experimental design also requires controls. Con-
___________________________________________
trols are standards for comparison, and they verify that the
3. Which of these hypotheses is a null hypothesis? biological response we measure is a function of the variable
Why? ______________________________________ being investigated and nothing else. They are replicates
___________________________________________ with all of the conditions of an experimental treatment
4. Examine the following hypothesis. except the treatment variable. For example, if the treatment
is glucose dissolved in water, then a control has only water
Hypothesis 1: Pill bugs prefer leaves coated with a (i.e., it lacks glucose, the treatment variable). This verifies
thin layer of yeast. that the response is to glucose and not to the solvent.

2–4

14 Exercise 2
Procedure 2.4 four control replicates. To calculate the means for
Conduct an experiment to determine the effects of food the four replicates, sum the four values and divide
type on yeast growth. by 4.
3. The CO2 production for each glucose and protein
1. Label 12 test tubes as C1–C4, G1–G4, and P1–P4.
replicate must be adjusted with the control
See Worksheet 1, page 18.
mean. This ensures that the final data reflect the
2. Add 5 mL of water to test tubes C1–C4. These are effects of only the treatment variable and not
control replicates. the solvent. Subtract the control mean from the
3. Add 5 mL of 5% glucose solution to test tubes G1–G4. CO2 production of each glucose replicate and
These are replicates of the glucose treatment. each protein replicate, and record the results in
4. Add 5 mL of 5% protein solution to test tubes P1–P4. Worksheet 1.
These are replicates of the protein treatment. 4. Record in Worksheet 1 the range of adjusted CO2
5. Completely fill the remaining volume in each tube production for the four replicates of the glucose
with the yeast suspension provided. treatment and of the protein treatment.
6. For each tube, slide an inverted, flat-bottomed test 5. Calculate the mean CO2 production for the four
tube down over the yeast-filled tube. Hold the yeast- adjusted glucose treatment replicates. Record the
filled tube firmly against the inside bottom of the mean in Worksheet 1.
cover tube and invert the assembly. Your instructor 6. Calculate the mean CO2 production for the four
will demonstrate how to slip this slightly larger empty adjusted protein treatment replicates. Record the
tube over the top of each yeast tube and invert the mean in Worksheet 1.
assembly. If done properly, no bubble of air will be 7. Refer to the description of standard deviation in
trapped at the top of the tube of yeast after inversion. Exercise 1, and calculate the standard deviation for
7. Place the tubes in a rack and incubate them at 50°C the four adjusted glucose treatment values, and for
for 40 minutes. the four adjusted protein treatment values. Record
8. After 40 minutes, measure the height (mm) of the the two standard deviations in Worksheet 1.
bubble of accumulated CO2. Record your results in
Worksheet 1.
Test the Hypotheses
Our hypothesis about yeast growth is tested by comparing
Analyze the Experimental Data the mean CO2 production by yeast that was fed glucose to
Analysis begins with summarizing the raw data for biologi- the mean CO2 production by yeast that was fed protein.
cal responses to each treatment. The first calculation is the However, simply determining if one mean is higher than
mean (x) of the response variable (mm CO2) for replicates the other is not an adequate test because natural varia-
of each treatment and controls. The mean represents the tion always makes the two means at least slightly different
central tendency of all measurements (replicates) of the even if the two treatments have the same effect on yeast
response variable. Later, the mean of each treatment will growth. Therefore, the means must be compared to deter-
be compared to determine if the treatments had different mine if the means are not just different, but are signifi-
effects. cantly different. To be significantly different means that
The second step in data analysis is to calculate varia- the differences between means are due to the treatment,
tion within each set of replicates. The simplest measure and not just due to natural variation. If the difference is
of variation is the range, which is the highest and lowest significant, then the null hypothesis is rejected. If the
values in a set of replicates. A wide range indicates much difference is not significant, then the null hypothesis is
variation. The standard deviation (SD), another informa- accepted. Testing for significant differences is usually done
tive measure of variation, summarizes the variation just as with statistical methods.
the range does, and the standard deviation is less affected Statistical methods calculate the probability that the
by extreme values. Refer to the description in Exercise 1 for means are significantly different. We will use a simple
calculating the standard deviation. method to test for a significant difference between the means
of our two treatments. We will declare that two means are
Procedure 2.5 significantly different if the 95% confidence intervals sur-
Quantify and summarize the data. rounding the two means do not overlap. Review Exercise 1 for
the calculations of a 95% confidence interval. This simple
1. Examine your raw data in Worksheet 1. criterion will suffice as a test for significant differences. Your
2. Calculate and record in Worksheet 1 the mean of instructor may choose to present a more rigorous statistical
the response variable (CO2 production) for the test for significance.

2–5

The Process of Science 15

 For example, consider these two means and their 95% 6. The General Question is “Which nutrients can
confidence intervals: yeast most readily metabolize?” After testing the
hypotheses, are you now prepared to answer this
meana xa 10 m0.05 10 2.5
General Question? Why or why not? ______________
meanb xb 20 m0.05 20 5
___________________________________________
Are meana and meanb significantly different according to our 7. Record your best response to the General Question in
test for significance? Yes, they are because the confidence Worksheet 1.
interval 7.5 4 12.5 does not overlap 15 4 25.

Procedure 2.6 EXPERIMENTATION AND DATA

Testing the hypothesis. ANALYSIS: PILL BUG FOOD
PREFERENCE
1. Consider your null hypothesis and the data presented
in Worksheet 1.
In the previous procedures, you developed and recorded
2. Calculate and record in Worksheet 1 the 95% observations, questions, and hypotheses related to pill bug
confidence interval for the glucose mean. food preference. Pill bugs may be attracted to dead leaves
3. Calculate and record in Worksheet 1 the 95% as food, or they may be attracted to fungi growing on the
confidence interval for the protein mean. leaves as food. Leaves dipped in a yeast suspension can sim-
4. Do the confidence intervals surrounding the two ulate fungi growing on leaves. Use the following procedures
treatment means overlap? Record your answer in as a guide to the science of experimentation and data analy-
Worksheet 1. sis to test your hypothesis recorded in Worksheet 2.
5. Are the means for the two treatments significantly
different? Record your answer in Worksheet 1. Procedure 2.8
6. Is your null hypothesis accepted or rejected? Record Design an experiment to test pill bug food preference.
your answer in Worksheet 1. 1. Design an experiment to test your hypothesis in
Answer the Questions Worksheet 2 about pill bug food preference. To do
this, specify:
The results of testing the hypotheses are informative, but it
still takes a biologist with good logic to translate these results Experimental setup ___________________________
into answers for our specific and general questions. If your ___________________________________________
specific questions were well-stated, then answering them
based on your experiment and hypothesis testing should be ___________________________________________
straightforward. Treatment 1 to be tested _______________________

Procedure 2.7 Treatment 2 to be tested ______________________

Answer the questions: yeast nutrition. Control treatment ___________________________
1. Examine the results of hypothesis testing presented Response variable ____________________________
in Worksheet 1.
Treatment variable _ __________________________
2. Specific Question 2 is “Does yeast absorb and
metabolize carbohydrates better than it absorbs Number of replicates _________________________
and metabolizes proteins?” Record your answer in
Means to be compared ________________________
Worksheet 1.
2. Conduct your experiment and record the data in
3. Does your experiment adequately answer this
Worksheet 2.
question? Why or why not? ____________________
3. Analyze your data. Record the control means and the
___________________________________________ adjusted treatment means in Worksheet 2.
4. Specific Question 1 is “What classes of biological 4. Calculate the ranges and standard deviations for your
molecules are most readily absorbed and metabolized treatments. Record them in Worksheet 2.
by yeast?” Record your best response in Worksheet 1. 5. Test your hypothesis. Determine if the null
5. Does your experiment adequately answer Specific hypothesis should be accepted or rejected. Record
Question 1? Why or why not? ___________________ the results in Worksheet 2.
___________________________________________
___________________________________________

2–6

16 Exercise 2
Procedure 2.9 4. After testing the hypotheses, are you now prepared
Answer the questions: pill bug food preference. to answer your General Question “What influences
the distribution of pill bugs?” Why or why not?
1. Examine the results of your hypothesis testing in
___________________________________________
Worksheet 2.
2. Record your answer to Specific Question 2 in ___________________________________________
Worksheet 2. Does your experiment adequately ___________________________________________
answer this question? Why or why not? ___________
5. Record your response to the General Question in
___________________________________________ Worksheet 2.
3. Record your best response to Specific Question 1 in
Worksheet 2. Does your experiment adequately
answer this question? Why or why not? ___________
___________________________________________

2–7

The Process of Science 17

 Worksheet 1: Process of Science: Nutrient Use in Yeast

OBSERVATION

QUESTIONS
General Question:

Specific Question 1:

Specific Question 2:

HYPOTHESIS H0:

EXPERIMENTAL DATA: Nutrient Use in Yeast

TREATMENTS TREATMENTS MINUS CONTROL x–

Control Glucose Protein Glucose CO2 Protein CO2

CO2 CO2 CO2 Production Production
Production Production Production Adjusted for Adjusted for
Rep. (mm) Rep. (mm) Rep. (mm) the Control –x the Control –x

C1 ______ G1 ______ P1 ______ ______ ______

C2 ______ G2 ______ P2 ______ ______ ______
C3 ______ G3 ______ P3 ______ ______ ______
C4 ______ G4 ______ P4 ______ ______ ______

Control x– ______ Glucose Protein

x– ______ x– ______
Glucose range Protein range
______ ______ ______ ______
Glucose Protein
SD ______ SD ______

TEST HYPOTHESIS
Glucose treatment: mglucose 95% confidence interval mglucose _______

Protein treatment: mprotein 95% confidence interval mprotein _______

Do the 95% confidence intervals surrounding the means of the two treatments overlap? Yes ______ No ______

Are the means for the two treatments significantly different? Yes ______ No ______

Is the null hypothesis accepted? ______ or rejected? ______

ANSWER QUESTIONS
Answer to Specific Question 2:

Answer to Specific Question 1:

Answer to General Question:

2–8

18 Exercise 2
 Worksheet 2: Process of Science: Food Preference by Pill Bugs

OBSERVATION

QUESTIONS
General Question:

Specific Question 1:

Specific Question 2:

HYPOTHESIS H0:

EXPERIMENTAL DATA: Food Preference by Pill Bugs

TREATMENTS TREATMENTS MINUS CONTROL x–

Treatment 1 Treatment 2
Adjusted for Adjusted for
Rep. Control Rep. Treatment 1 Rep. Treatment 2 the Control –x the Control –x

1 ______ 1 ______ 1 ______ ______ ______

2 ______ 2 ______ 2 ______ ______ ______
3 ______ 3 ______ 3 ______ ______ ______
4 ______ 4 ______ 4 ______ ______ ______

Control x– ______ Treat 1 Treat 2

x– ______ x– ______
Treat 1 Treat 2
Range ___ ___ Range ___ ___
Treat 1 Treat 2
SD ______ SD ______

TEST HYPOTHESIS
For Treatment 1: mtreat 1 95% confidence interval mtreat 1 _______

For Treatment 2: mtreat 2 95% confidence interval mtreat 2 _______

Do the 95% confidence intervals surrounding the means of the two treatments overlap? Yes ______ No ______

Are the means for the two treatments significantly different? Yes ______ No ______

Is the null hypothesis accepted? ______ or rejected? ______

ANSWER QUESTIONS
Answer to Specific Question 2:

Answer to Specific Question 1:

Answer to General Question:

2–9

The Process of Science 19

Questions for Further Thought and Study

1. Newspaper articles often refer to a discovery as “scientific,” and claim it was proven “scientifically.” What is meant by
this description?

2. Experiments and publications in science are usually reviewed by other scientists. Why is this done?

3. Have all of our discoveries and understandings about the natural world been the result of applying the scientific
method? How so?

4. Suppose that you hear that two means are significantly different. What does this mean?

5. Can means be different but not significantly different? Explain your answer.

2–10

20 Exercise 2
exercise three

Soil Analysis 3
Objectives
As you complete this lab exercise you will:
1. Examine and compare soil horizons.
2. Measure the vertical temperature gradient, soil
pH, density, dry weight, organic content, and
moisture content for samples along a soil profile.
3. Measure the content of major nutrients in
selected soil horizons.
4. Determine the particle size distribution of a soil
sample.

T errestrial plants and animals depend on soil as more

than a convenient place to live. It’s a storehouse of
valuable resources. It’s a place to find water, to find valuable
nitrogen and phosphorus, and to find food. It’s a place to
hide, a place to decompose, and a place that changes faster
than most people realize. And, like every other aspect of
the environment, it varies from place to place. In terrestrial
habitats, soils, along with climate, determine the variety
and success of all of the residents (fig. 3.1).
Soil is old. It began forming soon after the Earth was
formed more than 4.5 billion years ago. Early on, the Earth was
Figure 3.1
a harsh place—“land” was little more than a mixture of igne-
Soil’s origin from weathered rocks and subsequent mixing with
ous, sedimentary, and metamorphic rocks. Slowly these rocks organic material typically produces a diverse environment of
were transformed into soil by glaciers, wind, and rain, and later particles, clumps, detritus, minerals, and microorganisms.
by the activities of organisms. This conversion of rock into
soil is called weathering, and it is constant, but slow—so slow
that in eastern North America, abundant, weathering rainfall
forms only 2 cm of topsoil every 200 years! Soil is a fragile and ture, but soil is more dynamic than it first appears. Climate
valuable resource that needs our protection. affects the rate of weathering of parent materials, the rate
Question 1 of leaching of organic and inorganic substances, the rate of
Can water actually break rocks? How so? _______________ erosion and transport of mineral particles, and the rate of
decomposition of organic matter. These processes produce a
________________________________________________ soil profile’s distinctive horizons.
The O horizon is the surface litter covering the soil. It
SOIL PROFILES AND HORIZONS is mainly composed of fallen leaves and is only a few centi-
meters thick. The deeper parts of the O horizon have highly
A vertical soil profile typically has several layers, called fragmented and partially decomposed organic matter. Frag-
horizons (fig. 3.2). A profile is a snapshot of soil struc- mentation and decomposition of the organic matter result

3–1

21
 O horizon—Surface litter
A horizon—Topsoil

B horizon—Subsoil

C horizon—Fragmenting
parent rock

Figure 3.2
Horizons of a verticle soil profile.

from activities of soil organisms, including bacteria, fungi, impenetrable bedrock of igneous, sedimentary, or metamor-
and animals ranging from nematodes and mites to burrow- phic rock.
ing mammals. The O horizon merges gradually with the A Regardless of the different properties of their horizons,
horizon. all soils contain the same five components: mineral parti-
The A horizon is topsoil and usually extends 10–30 cm cles, decaying organic matter (humus), air, water, and living
below the surface. In most fertile soils, the A horizon has a organisms. Differing amounts of these materials define the
pH near 7 and contains 10%–15% organic matter, which soil’s properties and the plants it can support.
makes the horizon a dark color. The A horizon contains a For this lab exercise, you will measure the primary char-
mixture of mineral materials such as clay, silt, and sand, and acteristics of a variety of soil samples. Your instructor will
organic material derived from the O horizon. design your class’s survey of various sites, horizons, depths,
The B horizon has larger soil particles than those in and replications. Each of the following procedures applies to
the A horizon and extends 30–60 cm below the soil surface. a single sample, but includes tables to record as many as four
This horizon contains progressively less organic matter and sets of values for each soil sample. Check with your instruc-
is therefore lighter in color than the overlying A horizon. tor about how many and which soil samples your group will
In many regions, the B horizon contains large amounts of process. Your instructor has selected two or more communi-
minerals and clay particles washed by rainfall from the A ties with contrasting soil types and soil profiles. Notice the
horizon. Mature roots commonly extend into the B horizon, differences in the plant communities of each site.
where minerals accumulate. The B horizon is often called
subsoil. Procedure 3.1
The C horizon occurs 60–120 cm below the surface
Examine and compare soil horizons.
and consists of weathering rock subject to the action of
frost, water, and the deeper penetrating roots of plants. This 1. Identify two (or more) local communities with
horizon usually lacks organic matter and is often called par- contrasting soil types. Consult with your instructor
ent material, because it is the raw material from which soil and with geological survey maps of your area to
forms. The C horizon extends to an underlying and often locate potential sampling sites.

3–2

22 Exercise 3
2. Expose a soil profile at each site by taking soil cores, Procedure 3.2
or by digging a hole to reveal a vertical profile about Measure the vertical temperature gradient in a soil
1 m deep. profile.
3. Measure and record in step 4 the depth (thickness)
1. Obtain a suitable thermometer—either a dial
of each horizon for each site.
thermometer with a metal “probe,” or a linear
4. Site________________________________________ thermometer with a metal jacket that can be pushed
Horizon ____________________________________ into the ground. Sophisticated thermistors on long
metal probes are sometimes used.
Depth _______________ cm
2. The probe of a dial thermometer will provide
Thickness ____________ cm temperature readings for air, soil surface, and shallow
depths. For deeper depths, use a flat, sharp-edged
shovel to dig a hole that exposes a vertical profile
Site _______________________________________ as deep as possible. Then quickly push the probe
horizontally 5–10 cm into the horizon at various
Horizon ____________________________________
depths (preferably every 15 cm).
Depth _______________ cm 3. Record in step 4 the temperature profile for a
Thickness ____________ cm sampling site.
4. Site_______________________________________

Site _______________________________________ Horizon ___________________________________

Horizon ____________________________________ Depth _______________ cm

Depth _______________ cm Temperature __________ ˚C

Thickness ____________ cm
Site_______________________________________
Horizon ___________________________________
Site _______________________________________
Depth _______________ cm
Horizon ____________________________________
Temperature __________ ˚C
Depth _______________ cm
Thickness ____________ cm Site_______________________________________

Questions 2 Horizon ___________________________________

The boundaries of horizons are not always distinct. What Depth _______________ cm
physical processes blend the boundaries? ______________
Temperature __________ ˚C
________________________________________________
What kinds of organisms and what biotic processes blend
Site_______________________________________
soil boundaries? ___________________________________
Horizon ___________________________________
________________________________________________
Depth _______________ cm
Temperature __________ ˚C
SOIL TEMPERATURE Questions 3
How would you design an experiment to determine the
Soil temperature is more important to the ecology of plants
depths of soil that are subject to daily temperature fluctua-
and animals than you might expect. Topsoil is a reasonably
tions? __________________________________________
good insulator and offers burrowing animals a cool refuge
from daytime heat, and a stable, insulated environment dur- ________________________________________________
ing harsh winter freezes. The lower horizons typically have
How might organisms take advantage of the insulating
a cooler and narrower temperature range. Insulation from
properties of soil? _________________________________
freezing air also keeps deeper soil moisture from freezing.
Plants cannot absorb frozen water. ________________________________________________

3–3

Soil Analysis 23
SOIL pH

Soil fertility is strongly affected by pH. The subtleties of soil

chemistry are beyond the scope of this exercise, but acidity
can strongly influence nutrient availability. Nutrients must
be in solution before plant roots can absorb them easily, and
nutrient solubility depends much on pH. Most minerals and
nutrients dissolve better in acidic soils than in neutral or
basic (alkaline) soils. The pH of soils ranges from roughly
3 (acidic peat bogs) to 10 (dry desert soils). Most farm soils
have a pH range of 4.5–9. A pH of 5–7 is optimum for most
plants.

Procedure 3.3
Measure soil pH
1. If a soil test kit is available (fig. 3.3), use it to
measure pH of a soil sample taken 5–10 cm deep, and
another taken 30–40 cm deep. This method relies on Figure 3.3
the color change of indicator chemicals. Record the This test kit provides the chemicals, mixing tubes, and solutions to
values in step 3. detect physical and chemical properties of soil.
2. If a pH meter is used rather than chemicals in a soil
test kit, then collect the two soil samples mentioned
in step 1. Mix equal volumes of soil and water in a
beaker and measure the pH of the suspension with
the pH meter. Record the values in step 3. Questions 4
3. Site _______________________________________ Examine some potting soil. What pH would you expect it to
have? Why? _____________________________________
Horizon ____________________________________
________________________________________________
Depth _______________ cm
What characteristics of potting soil make it a good medium
pH __________________ for cultured plants? ________________________________
________________________________________________
Site _______________________________________
Horizon ____________________________________ SOIL DENSITY
Depth ________________cm
Soil densities can vary considerably. The moisture content,
pH __________________ ratio of particle sizes, and organic content all affect density.

Procedure 3.4
Site _______________________________________ Measure soil density.
Horizon ____________________________________ 1. Obtain and weigh a 1-L beaker.
Depth ________________ cm 2. Fill the beaker with 1 L of water, and accurately mark
the water level on the side of the beaker. Empty and
pH __________________ dry the beaker.
3. Fill a 1-L graduated cylinder with 1 L of coarse sand.
4. Use a sharp-edged trowel to dig a soil sample of
Site _______________________________________
400–500 mL and put it in the beaker. Be careful not
Horizon ____________________________________ to compact the soil.
Depth _______________ cm 5. Weigh the beaker and soil sample and subtract the
weight of the beaker (step 1) to obtain the weight of
pH __________________ the soil sample. Record the weight in step 9.

3–4

24 Exercise 3
6. Slowly pour sand from the graduated cylinder over Depth ________________ cm
and around the soil sample in the beaker until the
Soil density _________ g L⫺1
total volume in the beaker reaches 1 L.
7. Read the remaining volume of sand in the cylinder. Beaker ________________ g
The volume of remaining sand equals the volume of Soil and beaker _________ g
the soil sample. Record soil volume in step 9.
8. Calculate and record in step 9 the density of the soil Soil __________________ g
sample as: Soil volume _________ mL
soil density (g L⫺1) ⫽ (weight of soil sample) /
(volume of soil sample) Questions 5
Do you notice any places near your study sites where soil has
9. Site _______________________________________
been significantly compacted? _______________________
Horizon ____________________________________
________________________________________________
Depth ______________ cm
How might compaction of soil affect its oxygen content?
Soil density _________ g L⫺1 Moisture content? Overall plant success? ______________
Beaker ________________ g ________________________________________________
Soil and beaker _________ g
Soil __________________ g SOIL MOISTURE CONTENT
Soil volume _________ mL Soil retains water. The amount of retained water is propor-
tional to the surface area of the soil’s particles—the larger
the total surface area, the greater the retention of water. Clay
Site _______________________________________ particles are smaller than sand and therefore have a much
Horizon ____________________________________ larger surface area per unit of soil volume than does sand.
Indeed, the surface of clay particles in the upper few centi-
Depth _______________ cm meters of soil in a 2-hectare cornfield equals the surface area
Soil density _________ g L⫺1 of North America. Because clay soils retain much more water
than do sandy soils, clays would seem ideal for plant growth.
Beaker ________________ g
But the small size of clay particles also results in being densely
Soil and beaker _________ g packed—so densely that the clay has low amounts of oxygen,
due to small air spaces. This density also retards the penetra-
Soil __________________ g
tion of water into the soil (e.g., water penetrates clay about
Soil volume __________mL 20 times slower than it penetrates sand). As a result, much of
the water that falls on clay soil runs off and is unavailable for
plant growth. Tightly packed clay can impede plant growth.
Site _______________________________________
Procedure 3.5
Horizon ____________________________________
Measure the fresh weight, dry weight, and moisture
Depth ________________ cm content of a soil sample.
Soil density _________ g L⫺1 1. Collect a soil sample ( 50 g) and seal it in a pre-
Beaker ________________ g weighed bag.
2. Weigh the bag with soil sample and subtract the
Soil and beaker _________ g weight of the bag to calculate the sample’s fresh
Soil __________________ g weight. Record this fresh weight in step 8.
3. Transfer the soil to an open, pre-weighted container
Soil volume _________ mL such as a small aluminum pan or glass dish. Break up
any chunks so that the soil dries evenly.
Site _______________________________________ 4. Dry the soil for 24 h at 110°C.
5. After drying, use tongs to place the pan and soil in
Horizon ____________________________________ a desiccator until it cools. Do not seal the desiccator
jar completely.

3–5

Soil Analysis 25
6. Weigh the cooled pan with soil. Subtract the weight Site _______________________________________
of the pre-weighed pan and record in step 8 the
Horizon ____________________________________
remaining dry weight of the soil.
7. Calculate and record in step 8 the percent moisture Depth _______________ cm
content as:
% moisture content = 100 ⭈ (fresh weight ⫺ dry Moisture content ________%
weight) / fresh weight
Collection bag __________ g
8. Site _______________________________________
Bag with soil sample _____ g
Horizon ____________________________________
Fresh soil ______________ g
Depth ______________ cm
Moisture content ______ % Drying pan _____________ g

Collection bag __________ g Pan with dried soil_______ g

Bag with soil sample _____ g Soil dry weight _________ g
Fresh soil ______________ g Question 6
Some desert sands receive significant rainfall but only sup-
Drying pan _____________ g
port desert plants. Why? ____________________________
Pan with dried soil_______ g
________________________________________________
Soil dry weight _________ g

Site _______________________________________ ORGANIC CONTENT AS ASH-FREE

Horizon ____________________________________ DRY WEIGHT
Depth _______________ cm Humus is the decomposing organic matter in soil. The
Moisture content ________% amount of humus in soil varies; heavily mineralized soils
contain 1%–10% humus, whereas organic soils typically
Collection bag __________ g contain about 30% humus. Most plants grow best in soil
containing 10%–20% humus. The most organic soils are
Bag with soil sample _____ g those of swamps and bogs, which may contain more than
Fresh soil ______________ g 90% humus. These soils are usually so acidic that decom-
posers grow poorly in them. As a result, swamp humus accu-
Drying pan _____________ g mulates faster than it is broken down.
The amount of humus in soil affects the soil and plants
Pan with dried soil_______ g in several ways:
Soil dry weight _________ g • Its lightweight and spongy texture increases the water-
retention capacity of the soil. Water absorption by
Site _______________________________________ humus decreases runoff, thereby slowing erosion.
• Most humus is rich in organic acids and tends to
Horizon ____________________________________ increase nutrient availability.
Depth _______________ cm • Humus swells and shrinks as it absorbs water and later
dries. This periodic swelling and shrinking aerates the
Moisture content ________% soil.
• Humus is a reservoir of nutrients for plants. Like time-
Collection bag __________ g
release vitamins, humus gradually releases nutrients as it
Bag with soil sample _____ g is degraded by decomposers.

Fresh soil ______________ g Procedure 3.6

Drying pan _____________ g Measure organic content.
1. Collect a soil sample (L 30 g) from a selected
Pan with dried soil_______ g
horizon and seal it in a plastic bag.
Soil dry weight _________ g

3–6

26 Exercise 3
2. Transfer the soil to an open container such as a small 10. Site _______________________________________
aluminum pan or glass dish. Break up any chunks so
that the soil dries evenly. Horizon ____________________________________
3. Dry the soil for 24 h at 110°C, and homogenize the Depth ______________________ cm
sample after drying.
4. Weigh and record in step 10 a ceramic crucible to Organic matter _______________ %
the nearest 0.1 mg. Then add 1–5 g of the oven-dried Collection bag _________________ g
soil sample.
5. Weigh the filled crucible and subtract the original Crucible _______________________g
crucible weight to obtain the oven-dry weight of the
Crucible with soil _______________g
sample. Record this weight in step 10.
6. Place the crucible in a muffle furnace and heat Oven dry soil ___________________g
to 500°C for 6 h. Do not exceed 500°C. If black,
unoxidized material still remains, then heat for two Crucible with ashed soil __________g
more hours (fig. 3.4). Ash free dry weight ______________g
7. Allow the furnace to cool for 3 h before removing
the crucible. Use tongs to remove the crucible and
Site _______________________________________
place it in a desiccator to attain room temperature.
Do not seal the desiccator completely. Horizon ____________________________________
8. Weigh the crucible with the ashed soil to the nearest
0.1 mg and subtract the original weight of the crucible Depth _______________________ cm
to determine the ash-free dry weight. Record these Organic matter ________________ %
weights in step 10.
9. Calculate and record in step 10 the percent organic Collection bag ________________ g
matter as:
Crucible _______________________g
percent organic matter ⫽ (oven dry weight ⫺ ash
free dry weight) / oven dry weight Crucible with soil ______________ g
Oven dry soil ___________________g

Crucible with ashed soil __________g

Ash free dry weight ______________g

Site _______________________________________

Horizon ____________________________________
Depth _______________________ cm

Organic matter ________________ %

Collection bag ________________ g

Crucible _______________________g

Crucible with soil ______________ g

Oven dry soil ___________________g

Crucible with ashed soil __________g

Ash free dry weight ______________g

Figure 3.4
Muffle furnaces commonly reach temperatures above 700˚F. Be
careful. High temperatures volatilize organic molecules and leave Site _______________________________________
behind inorganic ash. Samples are typically held in porcelain
crucibles that do not melt at high temperatures. Horizon ____________________________________

3–7

Soil Analysis 27
 Depth _______________________ cm Phosphate _________mg L⫺1
Organic matter ________________ % Nitrate ___________mg L⫺1

Collection bag ________________ g

Site _______________________________________
Crucible _______________________g
Horizon ____________________________________
Crucible with soil ______________ g
Depth _______________ cm
Oven dry soil ___________________g
Phosphate _________mg L⫺1
Crucible with ashed soil __________g
Nitrate ___________mg L⫺1
Ash free dry weight ______________g
Questions 7 Site _______________________________________
Did organic content relate to soil color? How so? ________
Horizon ____________________________________
________________________________________________
Depth _______________ cm
Consider the characteristics of soil discussed so far. How
would organic content rank among them as a predictor of Phosphate _________mg L⫺1
plant community success? __________________________ Nitrate ___________mg L⫺1
________________________________________________ Questions 8
Why does nutrient content only potentially determine soil
fertility? ________________________________________
NUTRIENT CONTENT
________________________________________________
Nutrient content obviously determines much about a soil’s How do your measurements of nutrient content relate to
fertility. Nitrates and phosphates often limit growth, and organic content of the soils you tested? ________________
their concentrations significantly influence a plant commu-
nity’s growth rate as well as which species are persistent and ________________________________________________
competitive. Nutrient measurements are best done with a
soil analysis kit (see fig. 3.3).
SOIL TEXTURE
Procedure 3.7
Weathering breaks rocks into progressively smaller par-
Measure the content of major nutrients in selected soil ticles. These particles consist of minerals, which are natu-
horizons. rally occurring inorganic compounds usually made of two
1. Obtain a LaMotte soil analysis kit. or more elements. All soils contain three kinds of soil par-
2. Select sites to obtain soil samples for comparison, ticles: sand, silt, and clay (fig. 3.5). Gravel (> 2.0 mm) may
and sample the horizons selected by your instructor. be mixed with the soil. Clays are the final product of weath-
ering and the smallest soil particles:
3. Follow the directions provided in the analysis kit to
measure phosphate and nitrate for each soil sample.
Type of Soil Particle Diameter of Soil Particle (mm)
Record your results in step 4.
4. Site _______________________________________ Sand 2–0.05
Silt 0.05–0.002
Horizon ____________________________________
Clay < 0.002
Depth _______________ cm
Phosphate _________mg L⫺1 Different kinds of soils contain different proportions of
sand, silt, and clay. Particle size is particularly important
Nitrate ___________mg L⫺1 because it affects moisture content. Coarse-grained soils
drain quickly—a good thing for some plant species and
not for others. Fine-grained soils retain moisture far longer,
Site _______________________________________ but their large surface-to-volume ratio causes nutrients to
Horizon ____________________________________ adsorb to the particles’ surfaces and thus lowers overall fer-
tility. Clay soils contain more than 30% clay particles, and
Depth _______________ cm sandy soils contain less than 20% silt and clay. Soils con-

3–8

28 Exercise 3
taining approximately equal mixtures of sand, silt, and clay 12. Record in step 20 the hydrometer value at the top
are called loams. Plants grow best in loams. of the meniscus 40 sec after placing it in sample.
Measure and record the water’s temperature.
Procedure 3.8 13. Remove and clean the hydrometer. Leave the
Determine the particle size distribution of a soil sample. cylinder and suspension undisturbed for 6 h. After
6 h gently add the hydrometer. Record in step 20
1. Collect a soil sample ( 75 g) from the horizons and
the 6-h hydrometer reading. Measure and record the
sites selected by your instructor.
water’s temperature.
2. Air dry the sample in an open container for 24 h.
14. Pass the entire suspension through a 0.053-mm sieve
3. Gently use a mortar and pestle or rolling pin to break with a gentle rinse.
any clods.
15. Transfer the retained sand to a pre-weighed, open
4. Pass the soil through a 2-mm sieve to remove gravel container suitable for oven drying. Oven dry 24 h at
and larger components. Weigh and record in step 20 110°C. Weigh the oven-dried sand and record the
the gravel retained by the sieve. value in step 20 as dry weight of sand.
5. Oven dry (110°C, 24 h) at least 50 g of the soil that 16. Calculate and record in step 20 the % sand as:
passed through the sieve.
% sand ⫽ (dry weight of sand ⫻ 100) / (dry weight
6. Add 50 g of the sample to a 1-L beaker. Record 50 g
of soil)
in step 10 as the oven-dry weight of soil. Add 2 g of
the detergent sodium hexametaphosphate and mix 17. Correct the 6-h hydrometer reading for temperature
with the soil. as:
7. Add 500 mL distilled water to the beaker. corrected 6-h hydr. reading = 6-h reading ⫹ 0.36 g
8. Use an electric mixer for 10 min to mix the L⫺1 for every 1° above 20°C
suspension and disperse the soil particles. Mix for 5
(If temperature is below 20°C, subtract 0.36 g L⫺1
more min if the soil is high in silt and clay.
for every 1° below 20°C).
9. Rinse ALL of the sample from the beaker, including
any settled sand, into a 1000-mL graduated cylinder. 18. Calculate and record in step 20 the % clay as:
Bring the total volume to 1000 mL by adding % clay ⫽ (corrected 6-h hydr. reading ⫻ 100) / (dry
distilled water. weight of soil)
10. Cap the cylinder and invert the suspension several 19. Calculate and record in step 20 the % silt as:
times. Avoid making suds at the surface.
% silt ⫽ 100 ⫺ (% sand ⫹ % clay)
11. Remove the cap and immediately add a soil hydro-
meter graduated in grams of soil per liter (g soil L⫺1).

inches 1/16 2/16 3/16

invisible at
this scale

Gravel Sand Silt Clay

mm 1 2 3 4 5

Figure 3.5
A comparison of gravel and soil particle sizes.

3–9

Soil Analysis 29
20. Site _______________________________________ Sand _________________%

Horizon ____________________________________ Clay _________________%

Depth _______________ cm Silt _________________ %

Sand _________________% Gravel ________________ g

Clay _________________% Oven dry wt of soil ______ g

Silt _________________ % 40-sec hydr. reading ______

Gravel ________________ g 40-sec temp ___________ ⬚C

Oven dry wt of soil ______ g 6-h hydr. reading _________

40-sec hydr. reading ______ 6-h temp _____________ ⬚C

40-sec temp ___________ ⬚C Oven drying container ___ g

6-h hydr. reading _________ Container with oven dried sand _____ g

6-h temp _____________ ⬚C Oven dried sand ________ g

Oven drying container ___ g

Site _______________________________________
Container with oven dried sand _____ g
Horizon ____________________________________
Oven dried sand ________ g
Depth _______________ cm

Site _______________________________________ Sand _________________%

Horizon ____________________________________ Clay _________________%

Depth _______________ cm Silt _________________ %

Sand _________________% Gravel ________________ g

Clay _________________% Oven dry wt of soil ______ g

Silt _________________ % 40-sec hydr. reading ______

Gravel ________________ g 40-sec temp ___________ ⬚C

Oven dry wt of soil ______ g 6-h hydr. reading _________

40-sec hydr. reading ______ 6-h temp _____________ ⬚C

40-sec temp ___________ ⬚C Oven drying container ___ g

6-h hydr. reading _________ Container with oven dried sand _____ g

6-h temp _____________ ⬚C Oven dried sand ________ g

Questions 9
Oven drying container ___ g
Does your temperature profile provide evidence for differ-
Container with oven dried sand _____ g ent particle size compositions having different temperature
insulating properties? ______________________________
Oven dried sand ________ g
________________________________________________
Considering soil particle composition and moisture reten-
Site _______________________________________
tion, why would it be somewhat misleading to state “John-
Horizon ____________________________________ son grass grows best with rainfall of 30 in. per year”? _____

Depth _______________ cm ________________________________________________

3–10

30 Exercise 3
Questions for Further Thought and Study

1. Northern soils may have a permafrost zone. Research this term. What factors would affect the depth of this zone?

2. Particle size distribution affects nutrient availability. How would you design an experiment to determine the optimum
particle distribution for fertile soil?

3. What are the most common soil amendments used by farmers? In other words, how do they manipulate their soil?

4. Speak to a local farmer. What are the best qualities and worst qualities of his soil?

3–11

Soil Analysis 31
exercise four

Oxygen and Carbon Dioxide Cycling 4

Objectives following procedure you will use the pH indicator phenol
red to detect uptake of CO2 by a photosynthesizing aquatic
As you complete this lab exercise you will: plant, Elodea. Phenol red (phenol-sulfonphthalein) is a pH
1. Detect the uptake of carbon dioxide during indicator that turns yellow in an acidic solution (pH < 7)
photosynthesis. and red in a neutral to basic solution (pH > 7).
2. Measure the production of carbon dioxide by a To detect CO2 uptake, you will put a plant into a solu-
variety of organisms during respiration. tion that you have made slightly acidic with your breath.
3. Measure dissolved oxygen in a water sample. Carbon dioxide in your breath dissolves in water to form
4. Measure simultaneous changes in carbon dioxide carbonic acid, which lowers the pH of the solution.
and oxygen during community respiration and
H2O CO2 H2CO3 H HCO3
photosynthesis. water carbon carbonic hydrogen bicarbonate
5. Measure the total oxygen demand to decompose a dioxide acid ion ion

sewage effluent sample. As a photosynthesizing plant fixes CO2 and removes it from
the solution, the pH rises. When the pH rises above 7, the
dissolved indicator turns red.

O rganic molecules of the biosphere are rich with carbon-

carbon bonds. These bonds hold the energy captured
by photosynthesis using sunlight as energy and carbon diox-
Procedure 4.1
Detect the uptake of CO2 during photosynthesis.
ide as raw material. Each year the world’s ecosystems use
1. Fill two test tubes half full with a dilute solution of
about 10% of the 700 billion metric tons of carbon dioxide
phenol red provided by your laboratory instructor.
to synthesize organic compounds. This carbon is constantly
cycled among living organisms and between organisms and 2. Blow slowly into each solution with a straw. Stop
their environment. Respiration and photosynthesis drive blowing bubbles immediately when the color
that cycling (fig. 4.1). changes to yellow. Excess carbonic acid unnecessarily
Carbon and oxygen cycling are directly linked lengthens the procedure.
(fig. 4.2). Photosynthesis uses CO2 to make organic mol- 3. Add pieces of healthy Elodea totaling about 10 cm to
ecules and liberates O2. Respiration uses O2 to accept elec- a tube. Pour off excess solution above the Elodea.
trons and oxidize carbon as it releases the energy of organic 4. Place both tubes in a bright, sunlit window or
molecules and stores it in ATP. Respiration releases CO2. L 0.5 m in front of a 100-watt bulb for 30–60 min.
The revolving uptake and production of oxygen and carbon 5. Observe the tubes every 10 min and note any color
dioxide are critical to the operation of ecosystems. This lab change in the solution.
exercise presents techniques commonly used to detect and Questions 1
measure O2 and CO2 uptake and release. What happens to the color of the indicator? ____________
________________________________________________
Why did the color change? _________________________
UPTAKE OF CARBON DIOXIDE DURING
PHOTOSYNTHESIS ________________________________________________
How does this change in color relate to the summary equa-
pH is a measure of the acidic or basic properties of a solu-
tion for photosynthesis? ____________________________
tion; pH 7 is neutral. Solutions having a pH < 7 are acidic,
and solutions having a pH > 7 are basic (fig. 4.3). In the ________________________________________________
4–1

33
 CO2 in atmosphere

Photosynthesis

Combustion of fuels in
industry, homes and cars
Animal, plant and
microbial respiration
Carbon in plants

Carbon in dead
organic matter
Exchange
between
water and
atmosphere
Food Chains
Carbon in animals

Dissolved CO2 and

HCO3 Oxidation of
methane

Carbon in fossil fuels

(coal, petroleum)

Photosynthesis
Food Chains Release of
methane
Conversion
by geological Carbon in algae and plants
processes

Figure 4.1
Photosynthesis by plants and algae captures carbon in the form of organic chemical compounds. Aerobic respiration by organisms and fuel
combustion by humans return carbon to the form of carbon dioxide or bicarbonate. Microbial methanogens living in oxygen-free microhabitats,
such as the mud at the bottom of the pond, might produce methane, a gas that would enter the atmosphere and then gradually be oxidized
abiotically to carbon dioxide (shown in inset).

Carbohydrates 2O2 2H2O Carbon dioxide

H C OH Respiration (oxidation) O C O

H C OH Photosynthesis (reduction) O C O

2O2 2H2O

Figure 4.2
Carbon dioxide and oxygen cycling.

PRODUCTION OF CO2 DURING AEROBIC their assimilated energy by maintaining internal homeostasis
RESPIRATION and highly responsive nervous systems. More sedentary, tol-
erant, less-active organisms with a simple or absent nervous
Cellular respiration rates are important to ecologists because system require far less energy, and expend a smaller percent in
they represent the expenditure of energy by an organism. respiration. Understanding respiration (and its release of CO2)
Active organisms with keen senses expend the majority of is part of understanding organism and community energetics.

4–2

34 Exercise 4
Stomach acid Carbonated beverages Acid rain Human blood Oceans Alkaline lake

1 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14 10-15 H+ ion concentration
(moles per liter)

Neutral

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 pH value
Acidic Basic
Most streams and rivers

Figure 4.3
The pH scale of hydrogen ion concentration.

Control
Procedure 4.2 uses the indicator phenolphthalein to CO2 H2CO3 CO2 H2CO3
detect changes in pH resulting from CO2 and carbonic acid
produced during cellular respiration (fig. 4.4). Phenolphtha- (a)
lein is red in basic solutions and colorless in acidic solutions.
pH indicator pH indicator pH indicator
Thus, you can monitor cellular respiration as a change in
pH due to carbonic acid production. In Procedure 4.2, you
will directly measure the volume and milligrams of CO2
produced by a respiring organism.
Questions 2
The organisms studied in Procedure 4.2 include a plant
(Elodea) and an animal (snail). Which do you think will
respire more? ____________________________________ (b)

Write your hypothesis here: _________________________

________________________________________________
NaOH

Procedure 4.2
Measure relative CO2 production by aerobic organisms.
Experimental Setup
1. Obtain 400 mL of culture solution provided by your pH 7
instructor. This solution has been dechlorinated and
adjusted to be slightly acidic.
(c) Control
2. Place 100 mL of this solution in one labeled beaker
for each organism tested (treatment beakers), plus
one control beaker (table 4.1).
3. Obtain the organisms listed in table 4.1 from
your instructor and determine the volume of each
NaOH NaOH
organism by following steps 4–6. Your instructor may
include additional organisms and may ask you to
work with mass rather than volume.
Determine Organism Volume by Water Displacement
4. Pour exactly 25 mL of water in a 50-mL graduated
cylinder.
5. Measure the volume of each organism being tested.
Place the organism in the cylinder and note the (d) Snail treatment Elodea treatment
increase in volume above the original 25 mL. This
increase equals the volume of the organism. Figure 4.4
Illustration of the steps of Procedure 4.2.

4–3

Oxygen and Carbon Dioxide Cycling 35

6. Record the volumes in table 4.1. Gently return each by 10 (to adjust for 100 mL volume) and then by 4
organism to the appropriate beaker. (to adjust for 15-min. incubation).
Incubate Experimental Treatments 17. Calculate the milligrams CO2 produced per mL
7. Cover each beaker with a plastic film or petri dish organism per hour by dividing mg CO2 produced by
top and set them aside on your lab bench. Place the the volume of the organisms.
beaker containing the Elodea in the dark by covering 18. Record in table 4.2 the data from all student groups
it with a coffee can or aluminum foil. in your class. Calculate the mean, standard deviation,
8. Allow the organisms to respire for 15 min. and 95% confidence interval (see Exercises 1 and 2)
9. Gently remove the organisms from the treatment for each of the two tested organisms.
beakers and return them to their original culture bowls.
Disturb the water in the beakers as little as possible. Questions 3
Titrate to Gather Your Raw Data Do the 95% confidence intervals surrounding the means of
10. Add four drops of phenolphthalein to the contents CO2 production by the two tested organisms overlap?
of each beaker. The solutions should remain clear ________________________________________________
because the solutions are acidic.
11. Obtain a burette or dropper bottle to dispense NaOH Before you gathered your data, you formulated a hypothesis
(0.0227 N). Add NaOH drop-wise to the contents of about the expected results. Do you reject or do you fail to
the control beaker. Swirl the contents of the beaker reject your hypothesis? _____________________________
after each drop. An accurate endpoint remains pink The last two columns in table 4.1 express results differently.
for only a moment. Do not over-titrate. Which is the most appropriate for assessing the CO2 pro-
12. Record in table 4.1 the volume (mL) of NaOH duction of an organism? ____________________________
required to reach the control beaker endpoint of
________________________________________________
phenolphthalein (i.e., make the solution pink).
Either read the volume directly from the burette, or What is your major conclusion from this procedure?
calculate volume as 20 drops per milliliter.
________________________________________________
13. Repeat steps 11 and 12 for the treatment beakers.
Record in table 4.1 the titration volume of NaOH to ________________________________________________
reach endpoint for each treatment beaker. What characteristics of the organisms most likely contrib-
Calculate Your Results uted to their differences in CO2 production rate? ________
14. For each treatment subtract the volume of NaOH
to reach endpoint for the control beaker from the ________________________________________________
volume of NaOH added to each treatment beaker ________________________________________________
and record these values in table 4.1.
Are your conclusions applicable to all plants and animals,
15. If 100 mL was the original beaker volume, then
or only to the organisms you tested? __________________
the milliliters of titrant (mL 0.0227 N NaOH) to
reach endpoint (calculated in step 14) equal the ________________________________________________
milligrams CO2 per liter in the solution. Record these
equivalent values in table 4.1 as concentration of What other organisms would you include in an expanded
CO2 produced by respiration. experiment? Why did you choose these organisms? ______
16. Calculate the milligrams of CO2 produced per hour ________________________________________________
by respiration by dividing the concentration per liter ________________________________________________

Table 4.1
Measurement of CO2 production during respiration

Volume Volume NaOH to

Total Volume NaOH Reach Endpoint Concentration of CO2 Produced by
1
of Organisms to Reach Minus Control CO2 Produced by Respiration per CO2 Produced mL
Organisms (mL) Endpoint Volume Respiration Hour Organism h 1
1 1 1 1
Beaker 1: Four snails _____ mL _____ mL _____ mL ___ mg CO2 L ___mg CO2 h ___ mg CO2 mL organism h
1 1 1 1
Beaker 2: Elodea _____ mL _____ mL _____ mL ___ mg CO2 L ___mg CO2 h ___ mg CO2 mL organism h
1 1 1 1
Beaker 3: _____ _____ mL _____ mL _____ mL ___ mg CO2 L ___mg CO2 h ___ mg CO2 mL organism h
Control beaker _____ mL

4–4

36 Exercise 4
 Table 4.2
1 1
CO2 produced mL ___ mg CO2 mL
Elodea h 1 snail h 1
Student group 1
Student group 2
Student group 3
Student group 4
Student group 5
Mean
Std. dev.
Confidence
interval

OXYGEN AND CARBON DIOXIDE

CYCLING VIA PHOTOSYNTHESIS AND
RESPIRATION

Aquatic samples are ideal for measuring gas exchange

because dissolved gases are handled more easily in the lab
than are atmospheric gases. Furthermore, a plankton com- Figure 4.5
munity and its microorganisms in lake water is a convenient Dissolved oxygen bottles are designed to hold a precise amount of
microcosm of both autotrophs and heterotrophs. water, minimize contact with air, and be chemically inert. Each
The ecological significance of dissolved gases in aquatic bottle has an identification number and a ground glass stopper that
seals the water from air when properly inserted.
systems is further magnified by the fact that gasses, espe-
cially oxygen, do not dissolve in high concentrations in
water, and warmer water holds less oxygen than does cold
water. Only 8 mg oxygen L 1 will dissolve in water at 25°C. 3. Fill a clean DO bottle from the flask and stopper
This translates into 8 parts per million and is far less than the bottle so no bubbles are trapped. Record the
the 200,000 ppm of oxygen in the atmosphere. Cold water ID number of this DO bottle for aerated water in
(4 C) can hold (i.e., is saturated by) 12.5 mg oxygen L 1. In table 4.3.
the next three procedures you will (1) refine your technique 4. Boil the remaining water in the flask for 5 min. This
of measuring dissolved oxygen; (2) learn to use a water sam- reduces the dissolved gases.
pler to gather lake water; and (3) measure the cycling of O2 5. Let the boiled water cool for 10–15 min and then
and CO2 within a plankton community. fill a second DO bottle. Fill the bottle gently to
A common method used to measure dissolved oxygen minimize mixing with air. Record the ID number of
(DO) in a water sample is the Winkler titration method. the DO bottle in table 4.3.
Although specialized electronic meters can measure DO, 6. Assemble the materials needed for a Winkler
the Winkler titration is simple and accurate. Measuring DO titration.
is an important part of procedures to detect oxygen produc-
7. Follow the steps listed in the boxed insert “Winkler
tion and uptake.
Titration: Chemistry and Procedure” to determine
the DO concentrations in the two water samples.
Procedure 4.3
The volume of each DO bottle allows three replicate
Practice the Winkler titration method for measuring 100-mL titrations.
dissolved oxygen.
8. Record your results in table 4.3.
1. Examine a 300-mL DO bottle (fig. 4.5). Its narrow Question 4
neck minimizes the water’s contact with air. Its Examine the data in table 4.3. How did boiling affect the
ground-glass stopper is precisely shaped to enclose dissolved oxygen concentration of the water sample?
exactly 300 mL.
________________________________________________
2. Fill a 1-L flask with tap water, and swirl it vigorously
to aerate the water. ________________________________________________

4–5

Oxygen and Carbon Dioxide Cycling 37

 Winkler Titration: Chemistry and Procedure
Materials: 2. Remove the DO bottle’s stopper and insert the tip of a pipet
Manganese sulfate solution Sodium thiosulfate, or PAO just below the water level, and add 2 mL MnSO4 solution.
Alkali-iodine-azide solution titrant 3. Insert the tip of a pipet just below the water level, and add
Sulfuric acid Burette, 20-mL 2 mL alkaline-iodine-azide solution.
Starch indicator solution Ring stand 4. Replace the stopper and invert the bottle 10 times to mix the
Volumetric pipets and bulbs 250-mL flask solution.
5. Allow the resulting precipitate to settle until the top third of
the sample is clear. Then invert the bottle 10 times again to
The Winkler titration procedure for measuring dissolved oxy-
remix the solution.
gen begins with collecting a water sample and adding a series
6. Allow the resulting precipitate to settle until the top third of
of chemicals (manganese sulfate, alkali-iodine-azide, and sul-
the sample is clear.
furic acid). The first chemical reaction forms a precipitate of
manganous hydroxide. 7. CAUTION Remove the stopper and add 2 mL concentrated
H2SO4 by releasing the acid from a pipet tip held against the
MnSO4 2 KOH S Mn(OH)2T K2SO4 inside of the upper lip of the sample bottle. Allow the acid
As this precipitate settles through the water sample, it quickly to flow down the neck of the bottle into the water sample.
absorbs dissolved oxygen molecules present according to the 8. Replace the stopper and invert the bottle 10 times to mix the
following equation: solution. The sample is now “fixed” and can be stored for
3 days with refrigeration before titrating.
2 Mn(OH)2 O2 S 2 MnO(OH)2
STEPS FOR SAMPLE TITRATION
Adding iodide and then acid releases iodine (I2) in amounts
equivalent to the O2 that was originally present. 1. Obtain the fixed sample to be titrated.

MnO(OH)2 2 KI H2O S Mn(OH)2 I2 2 KOH 2. Measure 100 mL of solution with a graduated cylinder and
pour it into a 250-mL flask.
A starch indicator is added. The iodine (I2) reversibly binds
with the starch and makes the solution dark blue-black. The 3. Add 1 mL of starch indicator solution.
iodine is quantitatively removed by slowly titrating with (add- 4. Slowly titrate drop-wise with 0.0125 N thiosulfate solution
ing) a standardized solution of sodium thiosulfate or PAO (or PAO) until the blue color first disappears. Disregard any
solution until the dark color of the solution disappears (titra- subsequent reappearance of the blue color.
tion endpoint). The amount of titrant used to remove the blue-
black color is directly proportional to the amount of I2, which is 5. Record the milliliters of titrant used.
directly proportional to the amount of O2 originally dissolved in 6. One milliliter of 0.0125 N titrant equates to a 1 mg of DO
the water sample. in a liter of sample. Convert the milliliters of titrant to mg
STEPS FOR SAMPLE FIXATION L 1 of DO. For example, 7.5 mL of titrant equates to 7.5 mg
1
DO L in lake water.
1. Obtain a water sample in a 300-mL, glass-stoppered DO
bottle.

Table 4.3
Titration volumes and dissolved oxygen concentrations for aerated and boiled
water samples.

DO Bottle ID Number ____ DO Bottle ID Number ____

Aerated Water Sample Boiled Water Sample
mL of DO (mg L 1) mL of DO (mg L 1)
titrant titrant
Replicate 1 ________ ________ ________ ________
Replicate 2 ________ ________ ________ ________
Replicate 3 ________ ________ ________ ________
x ________ x ________

4–6

38 Exercise 4
Procedure 4.4 12. Insert the ground-glass stopper into the bottle to seal
Collect an undisturbed lake-water sample for dissolved the 300-mL volume with no bubbles. The sample is
oxygen analysis. now ready for Winkler titration.
13. Empty the van Dorn sampler back into the lake.
1. Obtain a 300-mL DO bottle and ground-glass
stopper. Rinse the inside of the bottle with some of Question 5
the lake water being sampled. Why should the volume be overflowed three times while
2. Examine a van Dorn water sampler. Close the drain filling a DO bottle? _______________________________
valve at the base of the flexible drain tube (fig. 4.6). ________________________________________________
3. Your instructor will show you how to CAREFULLY
set the spring-loaded suction cups, and how to trigger
the release mechanism with a weighted messenger.
4. Cock the van Dorn sampler. Slowly submerge the Procedure 4.5
cocked van Dorn completely in lake water (or a sink Use light and dark DO bottles to measure simultaneous
full of water if you are practicing). Lower the van photosynthesis and respiration rates of a plankton
Dorn to 1-m depth as marked on the line. community.
5. Move the van Dorn sampler about 1 m horizontally 1. Discuss with your instructor where to sample a
to displace any previously disturbed water in the nearby lake with a rich plankton community. Your
cylinder. instructor may assign measurements for a different
6. Drop the messenger to close the cylinder and enclose depth to each group in the class.
the water sample. 2. Assemble six 300-mL DO bottles. Cover two of the
7. Raise the cylinder out of the water and rest it bottles ( DOdark bottles) completely (light-tight)
vertically on the edge of a solid surface so the drain with tinfoil (fig. 4.7).
valve is at the lower end. 3. Use a van Dorn sampler to take water samples
8. Insert the drain tube into the DO bottle so the end 0.25-m deep according to Procedure 4.4, and fill
of the tube touches the bottom of the bottle. the four light bottles and the two dark bottles with
9. Open the drain valve. If water doesn’t flow freely lake water. Use a small square of tinfoil to cover the
into the bottle, lift the edge of the upper suction cup stopper and neck of each dark bottle.
to break the seal and allow air flow.
10. Allow the water to overflow the DO bottle until the
volume of the bottle has been displaced three times.
11. As the water continues to flow, slowly pull the tube
out of the bottle.

Figure 4.7
These light and dark bottles contain lake water with plankton.
Figure 4.6 During incubation, photosynthesis and respiration change
A van Dorn water sampler effectively captures a water sample from the dissolved oxygen in the light bottle. Respiration decreases
a known depth when a heavy “messenger” travels down the rope to the dissolved oxygen change in the dark bottle. The rate of
trigger rubber cups and seal water inside a large cylinder. The water is photosynthesis is calculated by comparing changes in dissolved
brought to the surface and drained into a sample bottle by opening a oxygen in each bottle. A bottle is made “dark” by tinfoil or a
valve and hose on the side of the sampler. black plastic coating.

4–7

Oxygen and Carbon Dioxide Cycling 39

4. Designate two of the light bottles as DOinit bottles 13. Record the titration results for the three 100-mL
for initial dissolved oxygen measurement, and record aliquots from each bottle in table 4.4 as mL of
their ID numbers in table 4.4. titrant.
5. Designate two of the light bottles as DOlight bottles. 14. Calculate and record in table 4.4 the mean milliliters
They will allow light and photosynthesis during of titrant for each set of six values.
incubation. Record their ID numbers in table 4.4. 15. The mean milliliters of titrant equals the mg L 1 DO
6. Designate two of the dark bottles as DOdark for each treatment. Record these values in table 4.4.
bottles. They will not allow photosynthesis during 16. Calculate and record community respiration in
incubation. Record their ID numbers in table 4.4. table 4.4.
7. Suspend the four bottles (two light, two dark) community respiration DOinit DOdark
0.25 m beneath the surface from a flotation device 17. Calculate and record net and gross photosynthesis in
to incubate for 24 h. Your instructor may modify table 4.4.
the incubation time. net photosynthesis DOlight DOinit
8. Immediately after suspending the light and dark gross photosynthesis community respiration
bottles, fix the contents of the DOinit bottles net photosynthesis
according to the STEPS FOR SAMPLE FIXATION 18. Be prepared to combine the data for your assigned
in the boxed insert: Winkler Titration Chemistry depth with data of the other groups for an analysis of
and Procedure. the production-depth profile.
9. Return to the lab and titrate the two DOinit Questions 6
bottles according to the STEPS FOR SAMPLE If your class measured photosynthesis and respiration at dif-
TITRATION in the boxed insert: Winkler ferent depths, does the productivity rate reflect the depth of
Titration: Chemistry and Procedure. the water sampled? ________________________________
10. Record the titration results for the three 100-mL
________________________________________________
aliquots from each bottle in table 4.4 as mL of
titrant. If the biomass and number of planktonic animals far exceed
11. After 24 h incubation, retrieve the DOlight and the amount of algae, how would that affect each of the three
DOdark bottles. While in the field, fix their contents community variables calculated in steps 16 and 17?
according to the STEPS FOR SAMPLE FIXATION. ________________________________________________
12. Return to the lab and titrate the samples according
to the STEPS FOR SAMPLE TITRATION. ________________________________________________

Table 4.4
Data and calculations for measurement of photosynthesis and respiration in lake water

Bottle ID
Number mL Titrant for 100-mL Aliquot Dissolved Oxygen

DOinit bottle ______ ____ mL ____ mL ____ mL

DOinit bottle ______ ____ mL ____ mL ____ mL
1
x ____ ___ mg L DO DOinit
DOdark bottle ______ ____ mL ____ mL ____ mL
DOdark bottle ______ ____ mL ____ mL ____ mL
1
x ____ ___ mg L DO DOdark
DOlight bottle ______ ____ mL ____ mL ____ mL
DOlight bottle ______ ____ mL ____ mL ____ mL
1
x ____ ___ mg L DO DOlight

Community respiration Net photosynthesis Gross photosynthesis

______ mg O2 L 1 d 1
______ mg O2 L 1 d 1
______ mg O2 L 1 d 1

4–8

40 Exercise 4
BIOCHEMICAL OXYGEN DEMAND 5. Add 30 mL of sample to two DO bottles (1.0 dilution
factor). Label the sample bottles DILUTE0.10.
Not all communities are based on autotrophic plants. Some 6. Add 15 mL to two DO bottles (0.05 dilution factor).
communities with high inputs of organic matter and little Label the sample bottles DILUTE0.05. Record the
light for photosynthesis are heterotrophic and not driven bottle ID numbers in table 4.5. Your instructor may
by photosynthesis. They often experience critical oxygen recommend a different dilution scheme.
deficits from decomposition of their high organic content. 7. Fill the remaining volume of all DO bottles
Microbial communities such as lake sediments, leaf litter, (including the two control bottles) with aerated,
sewage effluent, and polluted lakes and rivers are typically demineralized water from the flask to slightly above
heterotrophic, and their O2 deficits and high organic con- the neck of each DO bottle and stopper them to
tents dictate community structure. Their organic load may capture 300 mL with no air bubbles.
be so significant that ecologists sometimes characterize it in 8. Use Winkler titration to determine the initial
terms of the amount of O2 required for its decomposition— dissolved oxygen concentration for one bottle of
sometimes called biochemical oxygen demand (BOD). each of the four pairs of samples including the
The organic content of heterotrophic communities controls. Record these initial values in table 4.5.
and its impact on CO2 and O2 cycling can be bioassayed 9. Incubate the remaining four samples at 20 C for
by measuring BOD. BOD is the amount of oxygen required 5 days.
by aerobic microorganisms to decompose the organic matter
10. After incubation, use Winkler titration to determine
in a sample of water. BOD is a common measure of organic
the dissolved oxygen concentration for each of the
pollution in a water sample or in a diluted sample of organic
four samples including the control. Record these
soil or sediment. A BOD assay measures the dissolved oxy-
incubation values in table 4.5.
gen consumed as microbes respire and break down organic
matter in the sample. 11. For each sample bottle, average the three 100-mL
A BOD procedure includes collecting water samples of replicate titrations, and record the mean in table 4.5.
a heterotrophic community, measuring the initial oxygen 12. Remember that the milliliters of titrant equals the
content, incubating the samples for 5 days, and measuring milligrams O2 per liter (mg O2 L 1) of the sample.
the final DO concentration. The difference in the initial Calculate and record the control adjustment as:
DO and the DO after incubation is the biochemical oxygen control adjustment x initial x incubated
demand. Most pristine rivers have a 5-day incubation BOD ________ mg L 1

of less than 1 mg O2 L 1. Moderately polluted rivers have 13. Calculate and record the changes ( ) in DO after
BODs from 2–8 mg O2 L 1. Wastewater coming into most incubation for the three dilutions as:
sewage treatment plants is about 200 mg L 1. Efficiently
For dilution 0.50,
treated sewage has a BOD value of about 20 mg L 1.
DO DILUTE0.50initial DILUTE0.50incubated
control adjustment
Procedure 4.6
For dilution 0.10,
Measure the biochemical oxygen demand in a highly
DO DILUTE0.10initial DILUTE0.10incubated
organic water sample.
control adjustment
1. Assemble eight numbered, 300-mL DO bottles with For dilution 0.05,
stoppers, and either graduated or volumetric pipets to DO DILUTE0.05initial DILUTE0.05incubated
measure 20–50 mL volumes, and materials needed for control adjustment
the Winkler oxygen method. 14. Discard data for any samples in which the
2. Fill a large (> 2 L) flask with demineralized water, and DILUTEincubated value was 0.0. These treatments
shake it vigorously to saturate it with dissolved oxygen. became anoxic (DO 0.0 mg L 1) during
3. Collect a suitable sample of highly organic sewage incubation and are inaccurate.
effluent. Your instructor will describe the risks and 15. Select the dilution that produced a DO drop ( DO)
proper procedures for handing samples of effluent. of 2–4 mg L 1 from its initial value. For this dilution,
4. To avoid the dissolved oxygen being completely calculate and record the final BOD value in table 4.5 as:
depleted during incubation, the sample should be BOD (DOinit DOincubated) / dilution factor
diluted. To do this, add 150 mL of sample to two DO
bottles (0.5 dilution factor). Label the sample bottles
DILUTE0.50.

4–9

Oxygen and Carbon Dioxide Cycling 41

Questions 7 Are heterotrophic communities naturally self-sustaining?
Does your effluent sample have a BOD similar to the 20 mg How so? ________________________________________
L 1 typical for other treated sewage samples? ____________
________________________________________________
________________________________________________
Oxygen and carbon dioxide cycling is critical to healthy
communities. Which of the two gases is likely the limiting
factor for organically polluted systems? ________________
________________________________________________

TABLE 4.5
DATA AND CALCULATIONS FOR MEASUREMENT OF BIOCHEMICAL OXYGEN DEMAND

Bottle
ID
Number mL titrant for 100-mL aliquot DO
CONTROLinit ______ ____ mL ____ mL ____ mL x initial ____ mL
CONTROLincub ______ ____ mL ____ mL ____ mL x incubated ____ mL
Control adjustment
control adjustment x initial mL x incubated mL ________ mg L 1

DILUTE0.50init ______ ____ mL ____ mL ____ mL x initial ____ mL

DILUTE0.50incub ______ ____ mL ____ mL ____ mL x incubated ____ mL
Dilution 0.50
1
DO DILUTE0.50initial DILUTE0.50incubated control adjustment DO ___ mg L

DILUTE0.10init ______ ____ mL ____ mL ____ mL x initial ____ mL

DILUTE0.10incub ______ ____ mL ____ mL ____ mL x incubated ____ mL
Dilution 0.10
1
DO DILUTE0.10initial DILUTE0.10incubated control adjustment DO ___ mg L

DILUTE0.05init ______ ____ mL ____ mL ____ mL x initial ____ mL

DILUTE0.05incub ______ ____ mL ____ mL ____ mL x incubated ____ mL
Dilution 0.05
1
DO DILUTE0.05initial DILUTE0.05incubated control adjustment DO ___ mg L

1
BOD ( DO)/dilution factor BOD ____ mg L

4–10

42 Exercise 4
Questions for Further Thought and Study

1. In what environments would oxygen cycling be unimportant?

2. How would you adapt the BOD procedure to determine the oxygen demand of solid sludge?

3. A surprising number of coral species and other reef invertebrates have algae growing symbiotically in their tissues.
What are some adaptive advantages to that relationship?

4. Oxygen dissolves more readily in cold water than in warm water. Yet, deep lake water is often oxygen poor. Why is
this the case?

4–11

Oxygen and Carbon Dioxide Cycling 43

exercise five

Population Growth 5
Objectives
As you complete this lab exercise you will:
1. Describe how populations grow.
2. Show the effects of resources and environmental

Number of Individuals (N)

conditions on population growth.
3. Graphically analyze how the human population is
increasing.

A population can grow incredibly fast. Abundant

resources of food, space, and nutrients usually produce
unlimited growth and a population growth curve with a geo-
metric or “J” shape (fig. 5.1). Life history traits that influ-
ence the speed of population growth include (1) survival
rates through reproductive age; (2) the age of first reproduc-
tion; (3) the number of offspring per generation; and (4) the
number of times the species reproduces in a lifetime. Time
The simplest population model is one based on unlim-
ited growth occurring in discrete pulses. An annual plant Figure 5.1
that reproduces once per year, or a protozoan that repro- A J-shaped, geometric curve.
duces asexually once every hour by dividing to make two, is
growing in discrete pulses according to the geometric popu-
lation growth model. We compute this growth as:
Nt ⫽ N0␭t ⫽ (1)(272) bacteria
⫽ 40,000,000,000,000,000,000,000 bacteria
where
⫽ 4.7 ⫻ 1021 bacteria
Nt ⫽ the number at time t
N0 ⫽ the number of individuals present initially
This many bacteria would weigh an incredible 2.3 mil-
␭⫽ average number of offspring left by an individual
lion kg (520 tons). But bacteria aren’t the only organisms
during one time interval
with high biotic potential. For example,
t ⫽ number of time intervals (generations)
• Oysters each produce about 50 million eggs per year.
This calculation shows how quickly populations can
• A single pair of Atlantic cod and their descendants
grow, and the result is shown in figure 5.1. Consider Esche-
reproducing without hindrance would completely fill
richia coli, a common bacterium that divides every 20 min in
the Atlantic Ocean in 6 years.
ideal conditions. In one day (1440 min), these bacteria can
go through 72 (1440/20) generations. Therefore, if we start • The 80 offspring produced every 6 months by a pair
our experiment with one bacterium, the number of bacteria of cockroaches produce 130,000 roaches in only 18
present after one day would be: months—enough to overrun any apartment!

5–1

45
Natural populations can grow at extraordinary rates dic- Y
tated by high ␭ values and short generation times, but only
for short periods and with unlimited resources.
Question 1

Population Size
In simple terms, why isn’t our world overrun with roaches
and Atlantic cod if they can reproduce so dramatically?
________________________________________________
________________________________________________

ENVIRONMENTAL RESISTANCE 0 1 2
X
AND CARRYING CAPACITY Time (hours)

Organisms in the “real world” do not usually reproduce at Figure 5.2

their maximum rates. Maximum rates are not sustainable Theoretical and actual population growth of E. coli.
because environmental resistance increases due to disease,
accumulation of waste products, lack of food, and other fac-
tors. Ultimately, the size and growth of a population is a
function of the environment as well as reproductive traits.
Question 2 How long did it take the actual population to double during
Environmental resistance slows growth. Which of the four early stages of the experiment? Middle stages? Later stages?
life-history traits listed at the beginning of this exercise
________________________________________________
would be influenced the most by environmental resistance?
________________________________________________
________________________________________________
When was growth of the actual population most rapid?
________________________________________________
________________________________________________
To understand the effect of environmental resistance
on growth rates, complete table 5.1. This table provides ________________________________________________
real data for a growing but limited population of bacteria.
At what stage was growth slowest? What factors likely lim-
For comparison, you must calculate the size of a theoretical
ited the growth? __________________________________
population of E. coli given unlimited resources. The popu-
lation potentially doubles every 20 min. After doing these ________________________________________________
calculations, plot the growth of the theoretical and actual
As population size goes up, the environmental resis-
populations on figure 5.2.
tance increasingly slows the growth rate until it reaches
Questions 3 zero and the population size remains constant. This is called
How did growth of the actual population compare with logistic population growth, and the model for this is a sig-
that of the theoretical population during early stages of the moid growth curve (fig. 5.3). Sustainable growth of a popu-
experiment? During later stages? _____________________ lation occurs when the birth rate equals the death rate. This
________________________________________________ population size is referred to as the carrying capacity of the

Table 5.1
Theoretical and actual growth of E. coli bacteria

Time Size of Population (103 bacteria per mL)

Generation Hours Minutes Theoretical Actual
1 0 0 8 8
2 0 20 16 15
3 0 40 32 28
4 1 0 _______ 48
5 1 20 _______ 120
6 1 40 _______ 220
7 2 0 _______ 221

5–2

46 Exercise 5
 Carrying Population Growth Growth stops; population Table 5.2
capacity: grows rapidly slows size stabilizes at carrying Growth of bacteria in a
theoretical capacity, K limited-nutrient medium
maximum
population Time (hours) Turbidity Intensity (0–10) Absorbance Value

0 _________ _________
4 _________ _________
8 _________ _________

K
12 _________ _________
24 _________ _________
Number of Individuals (N)

48 _________ _________

3. Visually quantify the relative turbidity of each

culture between 0 (clear) and 10 (most turbid).
4. Record your results in table 5.2.
5. If turbidometers are available, measure the
turbidity of the solutions according to procedures
demonstrated by your instructor. Spectrophotometers
may also be used at 600 nm. Record your results in
table 5.2.

Procedure 5.2
Time
Measure the effect of resources and environmental
Figure 5.3 conditions on the size of a bacterial population.
The theoretical sigmoid curve of population growth. The early 1. Examine cultures of E. coli grown for 10 days in the
lag and log phases closely represent geometric growth before
environmental resistance and limited resources become significant.
following environments:
Distilled water, pH 7
Nutrient broth, pH 3
Nutrient broth, pH 5
Nutrient broth, pH 7
environment. Population size remains near the carrying Nutrient broth, pH 9
capacity as long as limiting factors are constant. However, Nutrient broth, pH 11
that is rarely the case, and oscillations (occasional peaks and 2. Quantify the relative turbidity of each culture
crashes) typically occur, especially for populations regulated between 0 (clear) and 10 (most turbid). Record your
primarily by abiotic factors. results in table 5.3.
In the laboratory, you can measure the growth of real 3. If turbidometers are available, measure the
populations such as bacteria that reproduce quickly. As bac- turbidity of the solutions according to procedures
teria reproduce in a clear nutrient broth, the broth becomes demonstrated by your instructor. Record your results
turbid. You can’t accurately count individual bacteria in in table 5.3.
this broth, but you can measure the increase in turbidity of a
growing culture. More turbidity means more bacteria—tur-
bidity values roughly estimate population size.
Your instructor previously inoculated some test tubes of Table 5.3
culture media with E. coli, a common bacterium. At regular
Growth of bacteria in a
time intervals, some of the tubes were put into a refrigerator to limited-nutrient medium
stop growth. Examine the cultures according to Procedure 5.1.
Media Turbidity Intensity (0–10) Absorbance Value

Procedure 5.1 Distilled water, pH 7 _______ _______

Measure population growth of bacteria. Nutrient broth, pH 3 _______ _______
1. Discuss with your instructor proper technique for Nutrient broth, pH 5 _______ _______
safely handling bacteria. Nutrient broth, pH 7 _______ _______
2. Examine cultures of E. coli grown for 0, 4, 8, 12, 24, Nutrient broth, pH 9 _______ _______
and 48 h. Nutrient broth, pH 11 _______ _______

5–3

Population Growth 47
Question 4 Y
Compare your data for populations grown in nutrient broth
and in distilled water. Does the presence of nutrients ensure
rapid growth of bacteria? Why or why not? _____________

Number of Plants
________________________________________________

Procedure 5.3
Measure population growth of duckweed (Lemna).
1. During the first week of this term, your instructor
placed 10 duckweed (Lemna) plants in an illumi- X
nated aquarium. Each week since then, he or she Time (days)
counted the number of plants in the aquarium.
Figure 5.4
Those data are posted by the aquarium.
Population growth of duckweed (Lemna).
2. From now until the end of the term, count duckweed
plants in the aquarium each week. Plot your data in
figure 5.4.
Questions 5
Questions 6
What do you conclude about population growth of duck-
How does the shape of the graph in figure 5.5 compare with
weed? ___________________________________________
those you made for the bacteria? _____________________
________________________________________________
________________________________________________
What will eventually happen to the size of the population?
What do you conclude from your graph of human popula-
Why? ___________________________________________
tion growth? _____________________________________
________________________________________________
________________________________________________
The population data listed in Procedure 5.4 have tre-
GROWTH OF HUMAN POPULATIONS mendous implications. For example, if our population had
stabilized after World War II, today we could provide all
Our global population is growing extremely fast. of our energy needs (and have a higher standard of living)
without having to burn any coal or import any oil.
Procedure 5.4
Plot the historical growth of the human population.
1. Consider these data: 10,000

Year Human Population (millions)

8000 b.c. 5 8000
4000 b.c. 86
Population in Millions

a.d. 1 133
1650 545 6000
1750 728
1800 906
1850 1130 4000
1900 1610
1950 2400
1960 2998 2000
1970 3659
1980 4551
1990 5300 0
8000 6000 4000 2000 0 2000
2000 6200 B.C. B.C . B.C. B.C. A.D. A.D.

2040 13,000 (projected) Time

Figure 5.5
2. Plot these data in figure 5.5. Growth of the human population.

5–4

48 Exercise 5
 Another important feature of a population is its dou-
bling time. In 1850, the doubling time for the human pop-
6
ulation was 135 years. Today, the doubling time is about
40 years. Consequently, during that same 40 years we must
5
also double our resources if we want to maintain our cur-
Significant advances
rent standard of living. Improving our standard of living will in public health
require that we more than double our resources.

Billions of People
4
Industrial
Questions 7 Revolution
How does rapid population growth impact you? _________ 3
Bubonic plague
________________________________________________ “Black Death”
2
The doubling time for populations in developed countries
is about 120 years but in developing countries it is about
1
30 years. What is the significance of this? ______________
________________________________________________ 0
4000 B.C. 3000 B.C. 2000 B.C. 1000 B.C. 0 1000 2000
Interestingly, the birthrate among Americans has
Year
climbed to its highest level since 1971 according to recent
data (2006) from the National Center for Health Statistics. Figure 5.6
The birthrate hit 2.1 in 2006, which means that each female History of human population size. Temporary increases in death rate,
theoretically has 2.1 offspring. At this rate, each generation even severe ones like the Black Death of the 1400s, have little lasting
equally replaces itself. For industrialized countries, this is a impact. Explosive growth began with the industrial revolution in the
rather high birthrate (table 5.4). 1700s, which produced a significant long-term lowering of the death
rate. The current population exceeds 6 billion, and at the current rate
will double in 39 years.
Table 5.4
Average number of births for every
woman in selected developed countries
Country Birthrate

United States 2.1

France 2.0 • In the 6 seconds it takes you to read this sentence, 18
Australia 1.8 more people will be added to our population. Each of
United Kingdom 1.9 these people eats food, generates wastes, and, in his or
Germany 1.3
her own way, affects our Earth.
Russia 1.3 • At our present rate of growth, in 2000 years our
Japan 1.3 population will weigh as much as the entire earth. And
4000 years later, it would weigh as much as the visible
universe.
In contrast to growth in the United States, the global
Question 8
population has increased explosively during the past three
Can the current growth rate of humans continue? Why or
centuries (fig. 5.6). Although the birthrate has remained con-
why not? ________________________________________
stant (at about 30 per 1000), the death rate has fallen from
about 30 per 1000 per year to its current level of about 13 per ________________________________________________
1000 per year. The difference between birthrates and death
What will happen when our population exceeds the Earth’s
rates (17 per 1000) means that the human population is grow-
carrying capacity? _________________________________
ing at a rate of about 1.7% per year. Here’s what that means:
________________________________________________
• Each hour, the world’s population grows by 11,000.
Each year, the world’s population grows by about How might the growth of the human population affect the
90,000,000. growth of other populations? ________________________
• Each year, there are about 90 million more people on ________________________________________________
Earth. That annual increase in our population equals
How does the growth of the human population affect eco-
the combined population of Great Britain, Ireland,
systems? ________________________________________
Iceland, Belgium, The Netherlands, Sweden, Norway,
and Finland. ________________________________________________

5–5

Population Growth 49
Questions for Further Thought and Study

1. How can a population be slowed by its own numbers?

2. Some people are now realizing the significance of population growth. Although this exercise treated the problem only
in biological terms, the reality of population growth is much more complex because it involves political, social, and
economic problems. What are some of these problems? How do they affect you now? How will they affect you later in
life (e.g., when you want to retire)?

3. Should we do anything to slow the “population explosion”? If so, what? If not, why?

4. From a purely ecological standpoint, can the problem of world hunger ever be overcome by improved agriculture
alone? What other components must a hunger-control policy include?

5. How are problems such as deforestation, pollution, and world hunger linked to population growth?

6. The late Garrett Hardin, a famous biologist, wrote that “Freedom to breed will bring ruin to us all.” Do you agree with
him? Explain your answer.

5–6

50 Exercise 5
exercise six

Age Distribution and Survivorship 6

Objectives Plants offer no parental care beyond a seed’s food supply and
seed coat protection. What effect would a lack of extended
As you complete this lab exercise you will: parental care have on early survivorship? ______________
1. Construct and examine age pyramids for growing
and nongrowing populations. ________________________________________________
2. Construct survivorship curves for organisms with In this lab exercise you will examine and graph age distri-
contrasting life histories. butions. Then you’ll gather data sets of birth and death in-
3. Gather and use cemetery demography data to formation from a cemetery to construct survivorship curves
assess the survivorship, mortality, and age distri- of a past human population.
bution of a local human population.

M ost natural populations include members of differ-

ent ages. Maturing individuals encounter mortality
factors and survivorship rates that vary among young, old,
AGE DISTRIBUTION

The number of individuals in each age class quantifies a

and all ages in between. To examine survival from birth to population age distribution. Age distributions interest ecol-
death, ecologists assemble age distribution data in a fairly ogists because they reveal much about a population’s poten-
standardized format called a life table. Life tables are con- tial for change and which stages of life are most subject to
structed with rows of data for each age interval from birth to mortality factors.
death. The initial data typically include the percent of the A population’s age distribution can be graphed as the
population in each age class and allow us to calculate sur- percent of the population occupying each age class (fig-
vival rates (and reciprocal mortalities). The ultimate value ure 6.1). This graph is sometimes called an age pyramid
of life tables is to reveal which life stages are experiencing because it typically has a broad base of young individuals
mortality pressures and to assess the potential for population and a narrow apex of old individuals.
growth. Questions 2
Questions 1 For each of the age pyramids in figure 6.1, what age classes
Are populations that suffer early and high rates of mortality include the youngest 50% of the entire population? ______
necessarily limited in their growth? ___________________
________________________________________________
________________________________________________
________________________________________________
How might a population adapt or compensate for high mor-
tality of its young stages? ___________________________ The death rate of the population of Sweden equals the
birthrate, and growth is zero. What characteristics of its age
________________________________________________ pyramid reflect that fact? ___________________________
During which life stages of a large mammal such as a bear ________________________________________________
would you suspect that mortality factors such as disease and
predation are highest, and therefore survival is lowest?
________________________________________________
________________________________________________

6–1

51
Sweden Procedure 6.1
b = birthrate = 0.010 The age distribution and zero per Construct age pyramids for the age distribution data of
d = death rate = 0.010 capita rate of increase (r) indicate
b – d = r = 0.000 that Sweden’s population is stable. three known populations.
85+ Males Females 1. Examine the three data sets presented in table 6.1.
80+
75−80 Assume that the data provided is only for females,
70−74 and that males are equal in number.
65−69
60−64 2. For each data set calculate and record in table 6.1
55−59
Age (years)

50−54 the Percent in Each Age Class.

45−49
40−44
3. Construct in figure 6.2 an age pyramid for each of the
35−39 data sets in table 6.1. Plot Age Class vs Percent in Each
30−34
25−29 Age Class.
20−24
15−19
Questions 3
10−14 Which of the three pyramids shown in figure 6.2 has the
5−9
0−4 broadest base? ____________________________________
8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8
________________________________________________
Percent of Population
How do these pyramids compare to those for human popula-
Hungary tions shown in figure 6.1? ___________________________
b = birthrate = 0.010 Hungary’s age distribution and
d = death rate = 0.013 negative r indicate a declining ________________________________________________
b – d = r = −0.003 population.
85+
80+
Males Females AGE-SPECIFIC SURVIVORSHIP
75−80
70−74 With each passing age interval, individuals encounter mor-
65−69
60−64
tality factors associated with that age. Not all individuals
55−59 alive at the beginning of an age class will survive to the next
Age (years)

50−54
45−49 class. In other words, survival rate is age specific. Ecologists
40−44 portray age-specific survivorship with a survivorship curve
35−39
30−34 (figure 6.3). For this graph, the log10 of the number of survivors
25−29
20−24
is the dependent variable on the y axis. Age divided into age
15−19 classes is the independent variable on the x axis.
10−14
5−9
Survivorship curves provide an informative view of
0−4 a lifetime of varying survivorship rates. The slope of the
8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 curve at any age class reflects survivorship. For some spe-
Percent of Population cies, survival is high during the early and mid stages of life
Rwanda and low (high mortality) late in life. Other species experi-
b = birthrate = 0.040 Rwanda’s age distribution and
ence roughly the same mortality and survivorship through-
d = death rate = 0.016 high r indicate a rapidly growing out life. Still other populations experience high mortality
b – d = r = −0.024 population. during early age classes. These life-history strategies produce
85+ Males Females type I, II, and III curves, respectively (figure 6.4).
80+
75−80 Questions 4
70−74
65−69 Which type of survivorship curve is typical for a human
60−64 population? ______________________________________
55−59
Age (years)

50−54
45−49 ________________________________________________
40−44
35−39 Would you expect the survivorship curves to vary between
30−34
25−29
developed and undeveloped countries? How so? _________
20−24
15−19 ________________________________________________
10−14
5−9 Survivorship rates are calculated from age distribution
0−4
data gathered either by (1) following a group, or cohort, of
8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8
“new-born” individuals (age class 0) as they pass through
Percent of Population
successive age classes; or (2) counting all individuals in
Figure 6.1 each age class in a single static observation. To help com-
Age distributions for human populations in countries with stable, pare populations of different sizes, the raw counts for each
declining, and rapidly growing populations. Data from the U.S. age class are usually standardized as a proportion of 1000
Bureau of the Census, International Database 2006. individuals at the beginning of age class 0.
6–2

52 Exercise 6
 Table 6.1
Three data sets of age distributions including American robins (Farner, 1945), Dall mt.
sheep (Deevey, 1947), and simulated data for oak trees

Robins Dall Mt. Sheep Oak Simulated Data

Age Class Observed Percent in Age Class Observed Percent in Age Class Observed Percent in
(yrs) Individuals Each Age Class (yrs) Individuals Each Age Class (yrs) Individuals Each Age Class
x ax x ax x ax
0 303 0 1000 0 1246
1 150 1 801 1 67
2 69 2 789 2 12
3 30 3 776 3 3
4 11 4 764 4 3
5 3 5 734 5 2
6 2 6 688 6 1
7 0 7 640 7 0
8 571
9 439
10 252
11 96
12 6
13 3
14 0

Age Class

Males Females
8

30 20 10 0 10 20 30
Percent of Population

Figure 6.2a
Labeled axes for student-constructed age pyramid for robins (data from table 6.1).
6–3

Age Distribution and Survivorship 53

 Age Class

Males Females
14

13

12

11

10

7 6 5 4 3 2 1 0 1 2 3 4 5 6 7
Percent of Population
Figure 6.2b
Labeled axes for student-constructed age pyramid for mt. sheep (data from table 6.1).
Age Class

Males Females
8

50 40 30 20 10 0 10 20 30 40 50
Percent of Population
Figure 6.2c
Labeled axes for student-constructed age pyramid for oak seedlings (data from table 6.1).

6–4

54 Exercise 6
 3
In type I survivorship,
juvenile survival is high
and most mortality occurs
Log10 Number of Survivors among older individuals.

1,000

2 I In contrast,
individuals in a

Number of Survivors
population with
type II survivorship
100
die at equal rates,
regardless of age.

1
II

10 Individuals showing
type III survivorship
die at a high rate as
juveniles and then at
much lower rates
0
III later in life.
0 5 10 15 20 25 30 35 40 1

Age (years) Young Old

Age
Figure 6.3
Constant rate of survival for a common mud turtle population Figure 6.4
(data from Deevey 1947, Baker, Mewaldt, and Stewart 1981, Frazer, Theoretical types I, II, and III survivorship curves.
Gibbons, and Greene 1991).

Question 5 During which of the first 5 years is mortality the least?

Which of the two methods of gathering age and survivor-
________________________________________________
ship information assumes that the mortality rates remain
stable through time? _______________________________ ________________________________________________
________________________________________________ Which of the three species has a type I curve? Type II curve?
Type III curve? ___________________________________
Procedure 6.2 ________________________________________________
Plot survivorship curves for three populations.
1. Examine the age distribution data in table 6.2. The CEMETERY DEMOGRAPHY
ax column is the number of individuals alive at the
beginning of the age class. Survivorship curves reveal changing survival and mortality
2. The nx column is the number of individuals alive at rates during a lifetime. Survivorship data for human popula-
the beginning of each age class. These numbers are tions are easily gathered because we leave behind accurate
standardized to 1000 for the first age class. Calculate records of birth and death dates on gravestones in cemeteries.
and record each nx value as: Dates on these gravestones allow us to track survivorship of
cohorts of humans that passed through decades of mortality.
nx (ax /a0) 1000
To gather data and construct survivorship curves for a
For example, nx for age class 1 is (150/303) 1000 local human population, your instructor has selected one or
495. more cemeteries with gravestones and birth dates routinely
dating back 100 years. You will work in teams and gather
3. Calculate and record in table 6.2 the log10 nx for each data for two cohorts. A cohort is a group of humans born
age class. in the same decade. Your class will gather birth year, death
4. Construct in figure 6.5 a survivorship curve for each of year, and gender information from L 100 grave stones for
the three data sets by plotting log10 nx versus age class. the birth dates in the 1870s and 1890s.
Questions 6
Examine the slope of each segment of the three survivorship
curves. During which year of the first 5 years is mortality the
greatest for each species? ___________________________
________________________________________________

6–5

Age Distribution and Survivorship 55

 Table 6.2
Three age distributions including data for American robins (Farner, 1945), Dall mt.
sheep (Deevey, 1947), and a simulated population of oak trees

Robins Dall Mt. Sheep Oak Simulated Data

Individuals Alive Individuals Alive Individuals Alive
at Beginning at Beginning at Beginning
Age of Age Class Age of Age Class Age of Age Class
Class Observed (standardized to Class Observed (standardized to Class Observed (standardized to
(yrs) Individuals 1000) (yrs) Individuals 1000) (yrs) Individuals 1000)
x ax nx log10 nx x ax nx log10 nx x ax nx log10 nx
0 303 1000 3.00 0 1000 1000 0 1246 1000
1 150 1 801 1 67
2 69 2 789 2 12
3 30 3 776 3 3
4 11 4 764 4 3
5 3 5 734 5 2
6 2 6 688 6 1
7 0 7 640 7 0
8 571
9 439
10 252
11 96
12 6
13 3
14 0

3.0 3.0
Log10 Number of Survivors

Log10 Number of Survivors

2.0 2.0

1.0 1.0

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Age Interval Age Interval
(a) (b)

Figure 6.5
Student-constructed survivorship curves for (a) robins; (b) Dall mt. sheep; and (c) oak simulated data. Data are from table 6.2.

6–6

56 Exercise 6
 3.0 5. Examine data summary tables 6.4, 6.5, and 6.6.
Notice that each age class includes 10 years.
6. For table 6.4, calculate the number of deaths during
each age class (i.e., dx) and record them in the
column for your cohort and your team number. For
example, to determine dx of Age Class 0–9, count
Log10 Number of Survivors

2.0
the number of individuals who died between 0 and 9
years of age.
7. Repeat step 6, but record in table 6.5 only the data
for males.
8. Repeat step 6, but record in table 6.6 only the data
for females.
1.0
Summarize the data from class teams
9. For tables 6.4, 6.5, and 6.6, record values for the
remaining three dx columns in each of the tables from
the summary data sheets of the other three teams.
10. For each Age Class, sum across the four values from
the four teams and record the total for the Age Class
0 1 2 3 4 5 6 7 in column TOTAL Number of Deaths in Age Class dx.
Age Interval Repeat for tables 6.4, 6.5, and 6.6.
(c)
11. Sum down the column the values of the TOTAL
Number of Deaths in Age Class dx. For table 6.4,
it should equal 100 (i.e., the total number of
Procedure 6.3 gravestones recorded by all four teams). Record this
Gather cemetery demography data. value for ax of Age Class 0–9.
Survey gravestones 12. Repeat step 11 for tables 6.5 and 6.6. The sum,
1. Locate a large cemetery with gravestones showing however, will not equal 100 for either table.
birth dates in the 1870s and 1890s. These decades 13. For tables 6.4, 6.5, and 6.6, calculate the remaining
include the cohorts you will follow through time. values for Observed Number Alive at the Beginning of
Examine table 6.3 for recording your raw data. Make the Age Class ax values as:
multiple copies of table 6.3 for all members of your ax ax – dx
1 1
team.
2. Divide into eight teams. Your instructor will assign 14. For tables 6.4, 6.5, and 6.6, calculate values for the ax
four teams to each of the two cohorts (decades) Standardized to 1000 nx column as:
investigated. Each team will record data from 25 nx (ax / ax ) (nx )
1 1
gravestones.
3. Walk the cemetery and read gravestones to find 15. For tables 6.4, 6.5, and 6.6, calculate values for the
individuals born in the decade your team was SURVIVORSHIP CURVE log10 (nx) column.
assigned. Record in table 6.3 the birth year, death year, 16. Exchange data with the teams working on the other
and gender of 25 people born in that decade. If the cohort.
gender is not apparent by the first name, then skip Plot survivorship for each cohort and gender
that gravestone. Don’t count gravestones that other 17. Plot two survivorship curves (combined genders) in
teams have already recorded. figure 6.6. One curve is for the 1870 cohort, and one
4. On your raw data sheet (table 6.3), calculate and is for the 1890 cohort.
record for each individual his or her Age at Death by 18. Plot two survivorship curves in figure 6.7. One curve
subtracting Birth Year from Death Year. is for 1870 females and one curve is for 1870 males.
19. Plot two survivorship curves in figure 6.8. One curve
is for 1890 females, and one curve is for 1890 males.

6–7

Age Distribution and Survivorship 57

 Table 6.3
Raw data for cemetery demography

Team _______ Cohort ________

Record Number Birth Year Death Year Gender Age at
Death

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

6–8

58 Exercise 6
Table 6.4
Summary life table for gender-combined data from cemetery demography

______Cohort
TEAM 1 TEAM 2 TEAM 3 TEAM 4 TOTAL Observed Number
Number of Number of Number of Number of Number of Alive at the
Deaths in Deaths in Deaths in Deaths in Deaths in Beginning of the ax standardized SURVIVORSHIP
Age Class Age Class Age Class Age Class Age Class Age Class Age Class to 1000 CURVE
(yr) dx dx dx dx dx ax nx log10(nx)
0–9 1000 3.0
10–19
20–29
30–39
40–49
50–59
60–69
70–79
80–89
90–99
100–110

Table 6.5
Summary life table for the male survivorship data

______Cohort
TEAM 1 TEAM 2 TEAM 3 TEAM 4 TOTAL
Number Number Number Number Number Observed Number
of Male of Male of Male of Male of Male Males Alive at the
Deaths in Deaths in Deaths in Deaths in Deaths in Beginning of the ax standardized SURVIVORSHIP
Age Class Age Class Age Class Age Class Age Class Age Class Age Class to 1000 CURVE
(yr) dx dx dx dx dx ax nx log10(nx)
0–9 1000 3.0
10–19
20–29
30–39
40–49
50–59
60–69
70–79
80–89
90–99
100–110

6–9

Age Distribution and Survivorship 59

 Table 6.6
Summary life table for the female survivorship data

______Cohort
TEAM 1 TEAM 2 TEAM 3 TEAM 4 TOTAL
Number Number Number Number Number Observed Number
of Female of Female of Female of Female of Female Females Alive at the
Deaths in Deaths in Deaths in Deaths in Deaths in Beginning of the ax standardized SURVIVORSHIP
Age Class Age Class Age Class Age Class Age Class Age Class Age Class to 1000 CURVE
(yr) dx dx dx dx dx ax nx log10(nx)
0–9 1000
10–19
20–29
30–39
40–49
50–59
60–69
70–79
80–89
90–99
100–110

3.0 3.0
Log10 Number of Survivors

Log10 Number of Survivors

2.0 2.0

1.0 1.0

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100
Age Interval Age Interval

Figure 6.6 Figure 6.7

Survivorship curves for combined-gender. Data from table 6.4. Survivorship curves for data for males from table 6.5.

6–10

60 Exercise 6
 3.0 Questions 7
Could a survivorship curve ever go up from one age interval
to the next? Why? Or why not? ______________________
________________________________________________
For which cohort did 50% of the population live the
Log10 Number of Survivors

2.0
longest? (log10 500 2.7) ___________________________
________________________________________________
How does the survivorship of males compare with that of
females? _________________________________________

1.0
________________________________________________
Did the later cohort have longer survivorship? __________
________________________________________________
What are some possible factors responsible for the different
survivorship of the later cohort? _____________________

0 10 20 30 40 50 60 70 80 90 100
________________________________________________
Age Interval During which age intervals was survivorship the greatest?
Figure 6.8 ________________________________________________
Survivorship curves for data for females from table 6.6.
________________________________________________
What are the weaknesses inherent in using a single cem-
etery to characterize survivorship of a population? _______
________________________________________________

6–11

Age Distribution and Survivorship 61

Questions for Further Thought and Study

1. For what kinds of species would longevity correlate with population growth? For what kinds would it not correlate?

2. During which age interval would life-saving medical advances have the most impact on potential population growth?
Why?

6 –12

62 Exercise 6
exercise seven

Terrestrial Plant Community Assessment 7

Objectives
As you complete this lab exercise you will:
1. Characterize a terrestrial community as to observ-
able physical factors, plant dominance, and
interactions.
2. Quantify the distribution and abundance of plants
in a community.
3. Use transect data to quantify the importance of
different plant species in a plant community.

E cological communities are extraordinarily complex. The

plant community you observe at any given time is the
product of interactions among (1) plants and their physical
surroundings; (2) different species of plants; and (3) plants
and animals (fig. 7.1). The engine driving these interactions
Figure 7.1
is the flow of energy captured by green plants and passed to
This terrestrial community is a diverse and interacting mix of
consumers and decomposers. plants, animals, and microorganisms. They interact with the abiotic
This exercise cannot explain all of the processes occur- environment and compete for nutrients, light, moisture, and shelter.
ring in a plant community, but it can guide you through some
basic observations that characterize and distinguish com- (2) the diversity of the community; and (3) the
munities. Do not underestimate the importance of qualita- interactions among organisms.
tive observations in understanding communities followed by
designing quantitative studies to address specific hypotheses.
PHYSICAL FACTORS
QUALITATIVE COMMUNITY Observations
ASSESSMENT 1. What levels of light intensity occur throughout the
community? ________________________________
Procedure 7.1
Observe and assess the ecological characteristics of a ___________________________________________
terrestrial community. 2. Does the community include shade-tolerant as well
as shade-intolerant plants (i.e., are some plants doing
1. Locate and visit a terrestrial community designated well in the shade and some better in full sunlight)?
by your instructor.
2. Characterize the community according to the criteria ___________________________________________
and questions that follow. After you’ve answered ___________________________________________
the questions, discuss your observations with your 3. How does light differ among vertical levels of
instructor and other groups. Be prepared to use vegetation? _________________________________
your observations as a basis for describing (1) your
assessment of energy flow through the community; ___________________________________________

7–1

63
4. What is the temperature 2 m above ground? _______ 2. Which plant species are most abundant (numbers)?
___________________________________________ ___________________________________________
5. What is the temperature at the soil surface? _______ ___________________________________________
___________________________________________ 3. Which plant species are most abundant (biomass)?
6. How much and in what direction does the ground ___________________________________________
slope? _____________________________________
___________________________________________
___________________________________________
4. What general categories of plant types (shrubs, trees,
7. How would you characterize the soil? Loam? Clay? etc.) are apparent? ____________________________
Sand? _____________________________________
___________________________________________
___________________________________________
5. What is the vertical distribution of vegetation?
8. What is the nature of the groundcover? Grasses? Bare
soil? _______________________________________ ___________________________________________

___________________________________________ ___________________________________________
9. Is there a layer of leaf litter on the ground? ________
Interpretations
___________________________________________
10. Is the environment generally moist, moderate, or 1. Would you describe this community as diverse? Why
dry? _______________________________________ or why not? _________________________________

___________________________________________ ___________________________________________
2. What comparable community in your local area
Interpretations would you consider to be more diverse? Less diverse?
1. How might shade affect the temperature of the ___________________________________________
community? ________________________________
___________________________________________
___________________________________________ 3. What observations led to your conclusion for the
2. How might different amounts of light at different previous question? ___________________________
vertical levels within the community be important?
___________________________________________
___________________________________________ 4. Are there specific factors that make your comparison
___________________________________________ community more or less diverse? Human impact?
3. What parts of the community might be cooler than Stressful environmental factors? Geology? _________
others? Why could this be important? ____________ ___________________________________________
___________________________________________
4. Why would ground slope be important? __________ INTERACTIONS AMONG ORGANISMS
___________________________________________ Observations
5. Based on your observations of slope and soil type,
would you expect the soil to retain moisture? ______ 1. What evidence do you see of resident vertebrates?
___________________________________________ ___________________________________________
6. How long has this community been left to develop ___________________________________________
without disturbance? That is, what is the apparent 2. What evidence do you see of resident invertebrates?
age of the community? ________________________
___________________________________________
___________________________________________
___________________________________________
PLANT DOMINANCE 3. What evidence do you see of plant-animal
interactions? ________________________________
Observations
___________________________________________
1. Most plant communities are dominated by one, two,
4. What evidence do you see of plant-plant
or three species. Is the plant community dominated
interactions? ________________________________
by a single species? Two species? ________________
___________________________________________
___________________________________________
7–2

64 Exercise 7
5. What adaptations do the plants have to discourage side or within the community. Randomly choose
herbivores? _________________________________ points along this line as starting points to lay out
perpendicular transects.
___________________________________________
Questions 1
6. Do you see any obvious or subtle evidence of
What concepts or ideas should govern the placement of
competition by plants for available resources? ______
your transect to obtain a representative sample of the com-
___________________________________________ munity? _________________________________________
________________________________________________
Interpretations
Are there any “wrong” places to put a transect? Why or why
1. If you don’t see any vertebrates, does that mean they not? ____________________________________________
are not around? Explain your answer. ____________
________________________________________________
___________________________________________
2. Reexamine the observations that you just listed. How 4. You and your lab partners will work on a single
would each observation affect the type and growth of transect. Stretch the measuring tape on the ground
plants in the community that you studied? ________ to establish a transect.
5. Divide the transect into 1-m or 5-m intervals to
___________________________________________
facilitate frequency calculations.
3. What kinds of competitive interactions are apparent
6. At the top of table 7.1 record the total transect
in the community? ___________________________
length (Ltotal) and total number of intervals (Itotal).
___________________________________________ 7. For the first interval, identify plants that touch,
4. What kinds of mutually beneficial interactions are overlie, or underlie the transect line. Treat bare
apparent in the community? ___________________ ground as a “species.”
___________________________________________ 8. In your field notes record each type (species) of
plant. Also, for each species record the total length
of the line intercepted by all individuals of that
QUANTITATIVE COMMUNITY species. For plants that overhang the line, record the
ASSESSMENT length of the line’s imaginary vertical plane that the
plant intercepts.
Ecologists have developed a variety of techniques to mea- 9. Repeat steps 7 and 8 for each transect interval.
sure the numbers, densities, and distributions of organisms 10. When all plants from all intervals have been
in terrestrial plant communities. One common technique recorded in your field notes, summarize your data in
is to count organisms within randomly distributed quadrats table 7.1.
(sometimes called plots) of uniform size. See Exercise 10 for
11. Sum the values in each of the three data columns of
more information about using quadrats. Another common
table 7.1 to calculate Ntotal, Ftotal, and Ctotal. Record the
technique is the line-intercept method. In this method, a
calculations at the bottom of each column.
transect, or line, is laid out within the community. Organ-
12. Use the data in table 7.1 to calculate Density
isms in contact with this line are counted and measured. Cal-
culations based on measurements from these line transects or and Relative Density for each species within the
community. Record your results in table 7.2.
quadrats reveal the relative abundances, frequencies, and dis-
tributions of the plant species that compose the community. Densityi ni / Ltotal
Relative Densityi ni / Ntotal
Procedure 7.2
13. Use the data in table 7.1 to calculate the following
Assessing a community with the line-intercept method.
Frequency and Relative Frequency for each species
1. With the help of your instructor, locate a suitable within the community. Record your results in table 7.2.
field site with a plant community to be examined.
Frequencyi fi / Itotal
2. Obtain a measuring tape 10–15 m long, a meter stick, Relative Frequencyi fi / Ftotal
and a notepad. If a measuring tape is unavailable, use
a measured piece of string or rope. 14. Use the data in table 7.1 to calculate Coverage
3. Assess the general layout of the community to be and Relative Coverage for each species within the
sampled. With the aid of your instructor, decide on community. Record your results in table 7.2.
a reasonable set of criteria to govern the placement Coveragei ci / Ltotal
of a transect for each group of students. One Relative Coveragei ci / C
common method is to establish a baseline along one

7–3

Terrestrial Plant Community Assessment 65

15. Use the data in table 7.1 to calculate the Importance Questions 2
Value for each species within the community. Record What is the meaning of an importance value? __________
your results in table 7.2.
________________________________________________
importance value of speciesi relative densityi
Why would we calculate this in addition to density, cover-
relative coveragei relative frequencyi
age, and frequency? _______________________________
________________________________________________

Table 7.1
Summary of data for plant species occurring along a transect

Ltotal total length of transect __________ Itotal total number of intervals __________

ni Total Number of fi Number of Intervals ci Total Length of

Individuals Encountered in Which Species i Transect Intercepted for
Speciesi for Entire Transect Occurs All Intervals

Ntotal Total of all Ftotal Total of all Ctotal Total length of

individuals ____ frequencies ____ transect intersected ____

Table 7.2
Relative values of each species in a selected community using parameters
of the line-intercept method

Relative Relative Relative Importance

Speciesi Density Density Frequency Frequency Coverage Coverage Value

7–4

66 Exercise 7
Questions for Further Thought and Study

1. Diverse plant communities have species representing a variety of plant types such as grasses, shrubs, succulents,
hardwood trees, softwood trees, vines, ferns, and so on. What factors increase a community’s diversity? Age of the
community? Energy input? Moisture? Nutrients? Disturbance? Human activity? How do they do so?

2. What characteristics of a community might make it more resilient than other communities after disturbance?

3. What characteristics indicate that a community has not been disturbed for a few years?

7–5

Terrestrial Plant Community Assessment 67

exercise eight

Stream Ecosystem Assessment 8

Objectives
As you complete this lab exercise you will:
1. Define a stream’s drainage basin and explain how it
is influenced by surrounding land.
2. Prepare a linear morphometric map of the stream.
3. Measure and record water temperature, water
velocity, and stream discharge along the stream
channel and shoreline.
4. Assess sediment particle size, primary producers,
benthic invertebrates, and fish populations for
riffles versus pools.

R ivers and streams drain rain and melting snow from ter-
restrial ecosystems. This runoff water eventually collects
in small rivulets that join to form a network of channels
that drain the landscape. Quiet pools in the channels may
have a current velocity of only a few millimeters per sec-
ond, whereas water in the riffles may flow at 6 m per sec.
(fig. 8.1). This continuous movement of flowing water is
the most prominent characteristic of a stream. Its current
delivers food, removes wastes, renews oxygen, and strongly
affects the size, shape, and behavior of organisms. The vol-
ume of water per hour flowing past a point of a stream is its
Figure 8.1
discharge.
Pools and riffles of a stream are formed by variation in sediment
A stream basin is the area of land drained by a network erosion and deposition. Although they share the same flowing water,
of rivers and streams. Streams and rivers can be classified by these microenvironments vary in current speed, substrate type, and
stream order according to where they occur in the drain- resident species. This photo shows pools separated by rocky, shallow
age network. Headwater streams are first order. A stream riffles.
formed by the joining of two first-order streams is second
order. A third-order stream results from the joining of two
second-order streams and so on. A lower-order stream join- so thorough along some streams that little photosynthesis
ing a higher-order stream does not raise the order of the by aquatic primary producers occurs. Shading lessens down-
stream below the junction (fig. 8.2). stream as stream width increases.
Streams are more strongly impacted by their surround- In this lab exercise you will assess some major physical
ing environment than you think. First- and second-order and biotic characteristics of a local stream segment. You’ll
streams are generally shaded by riparian vegetation occur- characterize the pattern of surrounding land use, as well as
ring along the shorelines and forming a transition between flow dynamics, sediment variation, invertebrate communi-
the aquatic and terrestrial environments. Shading may be ties, and fish communities within riffles and pools.

8–1

69
 species occurring per linear 10 m of stream reach.
Count only trees with a trunk diameter of > 6 cm
1st order at breast height (DBH). DBH refers to the stem (or
trunk) diameter at 1.5 m above the ground. For this
exercise, the entire population of large trees along the
stream reach can be counted. In Exercise 10 you will
learn how to sample larger populations.
2nd order
species A, number per 10 m _______
rd
3 order species B, number per 10 m _______
species C, number per 10 m _______
8. Many streams are shaded by expansive and
overgrown riparian vegetation. Walk the length of
the sample area and estimate the areas of water’s
total surface receiving full sunlight, partial sunlight,
and no direct sunlight.
% area full sunlight _______
% area partial sunlight _______
% area no direct sunlight _______
9. If time permits, visit the stream 2 hours after sunrise,
th
4 order midday, and 2 hours before sunset to determine
variation in how much of the water’s surface receives
full sunlight.
Questions 1
What order stream includes your sampling site? _________
Figure 8.2
A drainage network illustrating stream-order classification for a ________________________________________________
fourth-order watershed.
What is the primary land use in the drainage basin? ______
PHYSICAL CHARACTERISTICS ________________________________________________
Procedure 8.1 Does the land use in the immediate area differ from that of
Define the drainage basin and the influence of the entire basin’s? How so? __________________________
surrounding land.
________________________________________________
1. Locate on a county map a stream with an accessible
Which would have the greater impact on the stream’s wa-
50-m sampling site, and identify the surrounding
ter quality: the drainage basin or the land immediately sur-
streams and rivers.
rounding the stream sampling area? How so? ____________
2. Follow on the map the upstream path of the stream
from your sampling site, and generally define the ________________________________________________
boundaries of the stream’s drainage basin. Riparian vegetation is a buffer zone between the stream and
3. Use the scale on your map and the locations of the surrounding land. How extensive is the riparian buffer
the surrounding rivers and streams to estimate the at your sampling site? ______________________________
drainage basin area in square kilometers (or miles).
________________________________________________
4. Determine the land use in the drainage basin either
by discussing the area with your instructor, locating a What signs of erosion are apparent along the riparian zone?
map that shows land-use patterns, or driving through
________________________________________________
the area.
5. Determine the land-use pattern within the 150-m ________________________________________________
wide border along each side of your sampling area. Are the riparian trees different species from the trees away
6. Survey the riparian vegetation. The riparian from the water’s edge? _____________________________
vegetation (vegetation directly influenced by the
stream) may extend as far as 20 m from the water’s ________________________________________________
edge. Do you detect differences in vegetation among areas with
7. Identify three, four, or five of the most common different amounts of direct sunlight? How so? ___________
riparian tree species, and count individuals of these
________________________________________________
8–2

70 Exercise 8
Rivers and streams often divide along their lengths into shoreline. Direct a team member where to put a
pools and riffles. Riffles have rapid flow and are shallow temporary flag marking the first of a series of 5-m
enough for the bottom sediment to cause noticeable turbu- intervals.
lence. Pools are deeper areas of slower flow. 4. Wade 5 m farther down the center of the stream.
Face the same shoreline and direct a team member
Procedure 8.2 where to put a temporary flag marking that 5-m
interval.
Prepare a linear morphometric map of the stream.
5. Continue to walk the center line of the entire stream
1. Obtain graph paper with uniform boxes. Each box and mark 5-m intervals along the same shoreline.
represents a scaled distance of 1 m or other value as Bends in the stream result in markers closer together
determined by the instructor (fig. 8.3). than 5 m on the inside of a curve, and more than 5 m
2. Draw a straight line spanning 50 units along the apart on the outside of the curve. That’s okay. They
middle of the long axis of the paper. This line indicate a 5-m interval along the center of the stream.
represents the middle of the stream at all points 6. Return to the upstream end of the stream and the
along the stream. first marker. Measure the width of the stream at
3. Begin at the upstream end of the 50-m stream that marker. Indicate on your map half that width
reach and wade down the middle of the stream (adjusted for scale) on either side of the center line.
channel (equidistant from each shoreline). Face For example, a 10-m width has two points five boxes
perpendicular to the channel and toward one on either side of the end of the center line.
7. At the next shoreline marker, measure the width of
Upstream the stream, and record that span across the center
line of your map.
17⬚ 17⬚
8. Repeat step 7 for all shoreline markers to the
downstream end of the stream.
9. On your map, connect the dots along each side of
the centerline to outline the two shorelines with a
smooth line.
14⬚ 17⬚ Temp. Profile
16⬚
10. With your instructor’s aid, identify riffles and pools.
Shade and label the areas of your map that are
15⬚ obvious riffles or obvious pools.
14⬚
Pool Transect Question 2
Do the riffles and pools locations relate to stream widths?
14⬚ 14⬚ Velocity = 6 m min–1
X.S. Area = 40.5 m2
How so? ________________________________________
Discharge = 194 m3 min–1
________________________________________________
Pool

Procedure 8.3
15⬚ 15⬚
Measure and record water temperature along the stream
Temp. Profile channel and shoreline.
17⬚
17⬚ 1. Obtain an electronic thermometer with a probe
Riffle Transect
(thermistor) that can be submerged (fig. 8.4).
Velocity = 70 m min–1 Measure and record on your map the air temperature.
19⬚ 17⬚ X.S. Area = 3.5 m2
Discharge = 196 m3 min–1 2. In the middle of the channel across from the initial
upstream marker, measure the temperature within
3–4 cm of the bottom. Record this value on your
map at the upstream end of the centerline.
3. At the initial upstream marker, measure the water
20⬚ 18⬚
temperature at each shoreline. Place the thermistor
within 0.3 m of the water’s edges. Record these two
Figure 8.3 values on your map.
Example map of a stream segment. Temperatures at shoreline and
midstream intervals are indicated, along with temperatures of two
4. Repeat steps 2 and 3 to take and record three
verticle profiles. The positions and characteristics of a pool transect temperature readings at each 5-m interval transect
and a riffle transect are indicated. Each block is 1 square meter. along the stream.

8–3

Stream Ecosystem Assessment 71

Figure 8.4
This digital thermometer has a thermistor probe at the end of a cable to lower to a known depth. This
student is recording the temperature at the surface of shallow water.

5. Construct a vertical temperature profile of a pool. To Examine the shoreline temperatures closely. Do you detect
do this, find the deepest part of the stream reach and any areas that the temperature is cooler than the midchan-
take temperature readings at four equally distributed nel water? What might account for this? _______________
depths from surface to bottom. Indicate on your map
________________________________________________
the location measured, and record the values.
6. Construct a vertical profile of temperature for a
riffle. To do this, record the temperature at the Procedure 8.4
water surface and at the sediment surface. If possible, Measure and graph a depth profile for pools and riffles.
push the thermistor probe below the surface of the 1. Obtain a meter stick, measuring tape, and graph
sediment and measure and record the temperature. paper divided into uniform blocks.
Record on your map the location and temperatures.
2. Review your stream map and locate the major riffles
Questions 3 and pools.
Is there a downstream temperature gradient? ___________
3. With your instructor’s guidance, choose a
________________________________________________ representative pool and a representative riffle
appropriate for constructing a depth profile.
Why is shoreline water typically warmer than midchannel
water? __________________________________________ 4. Measure the stream width from shoreline to
shoreline across the middle of the pool. Determine
________________________________________________ an appropriate scale value for one block on your

8–4

72 Exercise 8
 Question 4
If the water entering the stream segment equals that amount
of water leaving the segment, then would you expect the
cross-sectional area to be the same at all points (transects)
along the stream? Why or why not? __________________

Pool Transect ________________________________________________

Surface

5.3 m Procedure 8.5

Measure water velocity and stream discharge.
1. Locate the stream transects that were profiled in
1 block = 1 m2
Blocks = 29 Procedure 8.4.
Subblocks = 23
2. Obtain an orange (fruit) and a stopwatch. Verify that
Cross−sectional area = 40.5 m2 the orange floats with only a small portion above
Velocity = 6 m min−1
Discharge = 194 m3 min−1 water. If not, try another orange.
3. At the first stream transect, mark a 5–10-m length of
midchannel stream flow; the longer the segment, the
Figure 8.5 better, as long as the stream dynamics are the same
Example map of pool transect. Each block is 1 square meter. Depth throughout the length.
readings were taken at 2-m intervals.
4. Release the orange in the water a few meters
upstream from the beginning of the segment. Start
timing when it reaches the beginning of the segment.
Stop timing when the orange reaches the end of the
measured segment.
graph paper so the width of the pool spans the entire 5. Calculate the velocity of water flow in meters per
width of the graph paper (fig. 8.5). minute. Record this value on your linear stream map
5. Represent the stream’s surface width as a horizontal as well as on the depth profile for that transect.
line across the graph paper spanning the number of 6. Repeat steps 3–5, but measure velocity as close to the
boxes appropriate to the map’s scale. shorelines as possible rather than midchannel.
6. Divide the width of the stream into 10–15 uniform 7. Calculate discharge (m3 min⫺1) by multiplying the
intervals, and use the meter stick to measure the cross-sectional area (m2) of the stream transect
depth of water at each interval. Record each depth (Procedure 8.4, step 8) times the midchannel
as a point the appropriate distance below the surface velocity (m min⫺1) (Procedure 8.5, step 5). Multiply
line on your depth-profile graph. this value by 0.8 to adjust for using only the
7. Map the bottom of the stream profile by connecting midchannel velocity.
the points below the surface with a smoothly drawn 8. Record the discharge value alongside midchannel
line. velocity measurements on your maps and depth
8. Calculate a transect’s cross-sectional area of the profiles.
stream: 9. Repeat steps 3–8 for each transect profile that you
a. Determine the area represented by a single block. constructed in Procedure 8.4.
For example, if the side of a block represents
0.5 m, then one block is 0.5 ⫻ 0.5 ⫽ 0.25 m2. Questions 5
b. Count the number of blocks completely enclosed Are midchannel velocities greater than velocities near the
within the lines of the stream cross section. shoreline? Why or why not? ________________________
c. Count the number of blocks that the bottom line ________________________________________________
subdivides.
d. Sum the number of whole blocks (step b) and one A rain event during the rainy season results in greater ve-
half the number of intersected blocks (step c). locity and discharge than a rain event during the dry season.
e. Multiply the sum of blocks by the area of a single Why? __________________________________________
block (step a). Record this value on your depth- ________________________________________________
profile map as the cross-sectional area of the
stream transect being measured. Are the discharge volumes at each of the transect profiles
the same? Should they be? How so? ___________________
9. Repeat steps 4–8 for the pool and riffle designated by
your instructor. ________________________________________________

8–5

Stream Ecosystem Assessment 73

What might account for variation in discharge volumes Procedure 8.6
from one part of the stream to another? _______________ Assess sediment particle size for riffles versus pools.
________________________________________________ 1. Locate the stream transects with profiles and
Some streams are fed primarily by runoff, whereas others are calculated discharges. Select a riffle transect and a
fed by groundwater. How would this affect turbidity of the pool transect.
water? __________________________________________ 2. If the typical particle size at the center of the riffle
transect exceeds 1 cm, then randomly select 20–30
________________________________________________ particles from a 0.25-m2 plot (0.5 m ⫻ 0.5 m),
Do you detect any areas of the stream that have significantly measure their greatest dimension, and determine the
clearer water indicative of a groundwater seepage feeding median size.
the stream? ______________________________________ 3. If the typical particle size at the center of the riffle
transect is less than 1 cm, then use a trowel to
________________________________________________
remove the sediment to a depth of 6 cm from a
The sediments of rivers and streams are more dynamic 400-cm2 plot (20 cm ⫻ 20 cm) in the middle of the
than you might expect. Inorganic and organic materials stream channel. Place the sediment in a bucket.
from the surrounding landscape continuously wash, fall, 4. Rinse the collected sediment through a series of
or blow into rivers. At the same time, flow and turbulence stacked sediment sieves to separate it into size classes
erode bottom sediments and keep them in suspension, par- (fig. 8.6).
ticularly during floods. Areas of rapid flow, such as riffles, 5. Examine the sieves and record in table 8.1 the
transport small sediment particles downstream and leave sequence of mesh sizes from largest to smallest.
only the coarsest sediment. In pools, the slow current allows
small particles to settle out of the water column.

Figure 8.6
Washing a sediment sample through individual or stacked sieves separates soil particles
into known size classes. Soil water retention depends as much on diversity of particle sizes
as it does on the mean particle size.

8–6

74 Exercise 8
6. Rinse the sediment retained on each sieve into 2. Estimate the percent of sediment covered by
clean, pre-weighed, labeled jars. Dry at 105°C for periphyton in riffles ______ and in pools ______.
24 h and weigh the sediment representing each size 3. Estimate the percent of sediment covered by detritus
class. Record the weight of each sediment size class for riffles ______ and for pools ______.
in table 8.1. 4. Estimate the percent of sediment covered by vascular
7. Calculate for each sediment size class the percent of plants for riffles ______ and for pools ______.
the total sample weight and record in table 8.1. Questions 7
8. Repeat steps 1–7 for each transect. Which segments of the stream have the most extensive
Questions 6 periphyton community, pools, or riffles? _______________
What is the relationship between sediment particle size and
________________________________________________
stream velocity? __________________________________
What is the relationship between reception of direct sun-
________________________________________________
light and extent of plant and periphyton coverage?
Some large particles (rocks) are apparently too large for the
________________________________________________
stream to have moved them. How did they get there?
________________________________________________
________________________________________________
Does a vascular plant stand or area of periphyton require
________________________________________________
continuous, direct sunlight? _________________________
Some land uses promote erosion. Does the clarity of the wa-
________________________________________________
ter at your sampling site indicate upstream erosion?
Rivers and streams can be divided vertically into the water
________________________________________________
surface, the water column, and the bottom, or benthic, zone.
________________________________________________ The benthic zone includes the surface of the stream bottom
and the porous interior of the substrate through which sur-
face water routinely flows.
BIOTIC CHARACTERISTICS
Procedure 8.8
As with terrestrial ecosystems, an aquatic system is an amal-
gam of species constrained by physical factors, biotic fac- Assess the benthic invertebrate population.
tors, and balancing interactions. Assessing a stream’s biota 1. Examine the parts and dimensions of a Surber
typically requires sampling its vascular plants, invertebrates sampler (fig. 8.7). Select a representative pool and
(insects and crustaceans), fish, and periphyton (the micro- riffle to sample for invertebrates. The depth should
floral community growing on firm substrate). be less than the height of the sampler.
2. Randomly select five sites to take a 1-ft2 benthic
Procedure 8.7 sample within the pool or riffle.
Assess the primary producer population. 3. For the first site, face upstream and lower the Surber
1. Inspect periphyton, detritus, and vascular plant sampler to the stream bottom with the mouth facing
organic matter visible on the sediment or growing upstream. The flowing water should expand the
from the sediment. catch net.

Table 8.1
Sediment particle size distribution for a midchannel sample from a riffle and a pool

Pool Transect Riffle Transect

Sediment Particle Dry Weight (g) Percent of Total Sediment Particle Dry Weight (g) Percent of Total
Size Range (mm) Sample Dry Size Range (mm) Sample Dry
Weight Weight

8–7

Stream Ecosystem Assessment 75

Figure 8.7
A Surber sampler depends on a steady water flow to sweep dislodged invertebrates into a trailing net.
Be patient and thorough when digging, stirring, and scrubbing the gravel and rocks of a stream bottom.

4. Firmly hold the frame against the substrate. Pick 9. Examine figure 8.8 to determine the taxonomic
up and hold the large rocks in front of the net and order for each sorted group of insects or the major
brush their surface thoroughly to dislodge clinging invertebrate taxon for that group.
invertebrates. Be sure that the material dislodged 10. Use aluminum foil to form a small weighing pan for
from the rocks flows into the Surber net. Put the each taxon, weigh the pan, and add the organisms.
brushed rocks to the side. 11. Dry the organisms for 24 h at 105°C, weigh the pan
5. After all the large rocks have been brushed, use a trowel with organisms, and subtract the original weight of the
to stir the sediment within the square-foot frame. Stir pan to determine the dry weight of the organisms.
the sediment thoroughly to a depth of 5–10 cm. 12. Record in appropriate places on your map the dry
6. Lift the sampler while retaining the caught organisms weight (g m⫺2) of invertebrates for each of the riffles
in the net, and repeat steps 3–5 for all five replicate and pools sampled.
samples.
7. When all five replicates have been taken, empty Questions 8
What adaptations of the invertebrate bodies do you see that
the pooled samples from the net into a wide-mouth
jar. Return the live sample to the lab for sorting. If might help them live in a fast current? ________________
necessary, preserve the sample with 40% isopropyl ________________________________________________
alcohol.
Are riffle invertebrates more diverse than pool invertebrates?
8. At the lab, pour the contents of the sample jar from
each pool or riffle sampled into large, shallow, white ________________________________________________
trays. Use forceps to remove the macroinvertebrates ________________________________________________
(> 3 mm) and sort them in petri dishes by general
body shape.

8–8

76 Exercise 8
 Damselfly larva Mayfly larva Stonefly larva Dragonfly larva Water beetle adult

Water beetle larva Midge larva

Caddisfly larvae Fly larva Blackfly larva

Planaria flatworm Mosquito larva

Planorbid snail Annelid worm Water strider adult Predaceous water bug adults

Turbinate snail Crayfish Scud shrimp

Figure 8.8
Common invertebrates of stream sediments.

8–9

Stream Ecosystem Assessment 77

Procedure 8.9 f. To finish a seine haul, sweep the net toward and
Assess and compare the fish populations of riffles versus onto the shoreline rather than lifting the net out
those of pools. of the water while you stand in the stream.
g. Handle the fish as little as possible. All fish must
1. Obtain a 12-ft (or longer) fish seine net. Choose a be returned to the water alive.
representative pool and riffle to sample.
4. Have a team of students prepared to count the
2. Your instructor will demonstrate how to effectively fish immediately when the net is brought to the
use a fish seine (fig. 8.9). shoreline.
3. Remember these tips: 5. After each seine haul, count and record the number
a. Sweep the seine upstream rather than down- of each type (species) of fish.
stream.
6. For each species, measure the length of 10
b. Hold the seine poles so you push the bottom end
representative fish to calculate a mean length.
in front of you rather than walking backwards and
pulling the seine. 7. Sketch a lateral view of each species to assess the
c. Be safe. Move quickly but step carefully and don’t general shape with special attention to the ratio of
overwhelm your seining partner. length to height.
d. While seining with your partner, separate the two 8. Take two or three seine hauls from each pool and
poles only about one-half to two-thirds the total riffle. Combine the data for the pool seine hauls and
length of the seine. For example, the poles of a for the riffle hauls.
12-ft seine should be kept no more than 6–9 ft
apart.
Questions 9
e. Keep the lower, weighted edge of the seine against
Does fish shape correlate with current speed of the immedi-
the sediment. As you move, bump the ends of
ate environment? _________________________________
the poles along the bottom of the sediment. This
keeps the seine low in the water column. ________________________________________________

Figure 8.9
Effective hauling of a seine net depends on using the net as a sack that trails behind the poles rather than
a stretched, flat net. Always keep the net moving, and keep its bottom edge dragging on the stream or lake
bottom.

8–10

78 Exercise 8
What is the relationship between areas of high current ve- What are the likely features of that micro-environment
locity and fish diversity? ____________________________ (pool or riffle) that supports higher fish densities? _______
________________________________________________ ________________________________________________
Which environment—pool or riffle—supports the greatest If fish densities are greater in pools (or in riffles), is it a con-
apparent fish species diversity? ______________________ sequence of water velocity? _________________________
________________________________________________ ________________________________________________

8–11

Stream Ecosystem Assessment 79

Questions for Further Thought and Study

1. Would you expect a more rapidly flowing stream to be more diverse? Why?

2. Nutrient and carbon cycling readily occurs within a relatively closed ecosystem. Do streams fit this model of recycling
within a closed system? Can you argue that it fits as well as doesn’t fit?

3. Water flow can be considered the defining characteristic of a stream. Yet some ecologists say variation in water flow is
the defining feature. What might be their logic?

8–12

80 Exercise 8
exercise nine

Microcommunity Assessment 9
Objectives
As you complete this lab exercise you will:
1. Collect, examine, and count invertebrates in leaf
litter and lichen communities.
2. Pose a research question about an invertebrate
microcommunity.
3. State a hypothesis about the ecology of the
community.
4. Gather and analyze the data to test your
hypothesis.

S mall communities often go unnoticed. We usually think

of communities that cover areas large enough for us
to walk through and support organisms we can easily see.
But within these large communities are small, inconspicu-
ous, and thriving microcommunities that deserve a closer
look (fig. 9.1). A community is an association of species
that live together. Composition of the community is deter-
mined by its species interactions and by characteristics of
the environment.
In this lab exercise you will sample two microcommu-
nities: arthropods in woodland leaf litter and invertebrates
in lichens. Leaf litter in a woodland includes a variety of
Figure 9.1
small arthropods and members of minor phyla. Lichens
This woodland community is dense enough to produce much litter
might seem like a rather desolate place for a community of and open enough for lichens to prosper.
microorganisms, but rotifers, nematodes, tardigrades, and
protozoa come to life when a lichen is hydrated. After you
perfect techniques for collecting, counting, and assessing
resident invertebrates in lichen communities and then in
leaf litter, you will propose and test hypotheses about their
ecology. lichens include nematodes, rotifers, and tardigrades. A sin-
gle lichen commonly accommodates as many as 5–10 spe-
cies of tardigrades, rotifers, nematodes, mites, small insect
LICHEN COMMUNITY larvae, and various protozoa. Many of these well-adapted
animals survive periodic drying in lichens by entering a dor-
Lodged within lichens (fig. 9.2) is a surprising diversity mant, metabolic state called cryptobiosis. During crypto-
of microscopic (< 1 mm) invertebrates and protozoa that biosis, microinvertebrates survive by suspending all but the
await reviving. The most common invertebrate residents of most vital life functions.

9–1

81
Figure 9.2
This lichen initially appears dry and lifeless, but this combination of algae and fungi is extremely
tolerant of desiccation. Addition of water reanimates a variety of microorganisms living on the lichen.

(a)

(b)

Figure 9.3
Typical inhabitants of lichens include (a) nematodes;
(b) rotifers; and (c) tardigrades.
(a) Nancy Kokalis-Burelle, USDA-ARS; (b) Courtesy of David Mark
(c) Welch/MBL; (c) © CH Diagnostic Inc., Fisheries & Oceans, Canada

9–2

82 Exercise 9
Tardigrades Procedure 9.1
Sample a lichen microcommunity.
Tardigrades (phylum Tardigrada) are unusual and rarely seen
even when they are active in a moist environment. These 1. Scrape four replicate lichen samples from tree bark,
microscopic animals range from 0.1 to 1.0 mm long and live or cut the lichen and bark away from the branch if
in lichens, mosses, and wet leaf litter. Tardigrades, commonly the lichen does not release easily. Record where the
called “water bears,” become active only when surrounded samples were taken (table 9.1, Location).
and rehydrated by water. Reanimated tardigrades cling to 2. If the lichens are still attached to tree bark, trim the
substrate and search slowly for food. When their surround- bark so the remainder is covered 100% by lichens.
ing water evaporates, tardigrades eliminate as much as 90% Place the replicate samples in plastic bags labeled as
of their body water and assume a desiccated form called a Rep 1, 2, 3, and 4 and return them to the laboratory.
“tun.” This loss of body water, called anhydrobiosis, leads to 3. For each replicate lichen, trim the edges so it will fit
a cryptobiotic state in which these organisms can survive for into a petri dish.
months or even years until reanimated with water.
4. Determine the area of each lichen.
a. Outline each lichen on graph paper with lines
Nematodes
marking four squares per centimeter (10 squares
Nematodes (phylum Nematoda), commonly called round- per in.).
worms, include 12,000 recognized species. They are remark- b. For each replicate lichen, count all squares com-
ably abundant and diverse in marine, freshwater, terrestrial, pletely enclosed in its outline. Record the number
and parasitic habitats. Most nematodes are microscopic in table 9.1.
(< 1 mm) and live in soil and sediment. A scoop of fer- c. Count all squares the outline subdivides. Record
tile soil may contain more than a million nematodes. They the number in table 9.1.
are slender, long, and rather featureless worms that feed on d. For each outline, calculate and record in table
detritus and cellular fluids of plants and animals. 9.1 the Total Squares by adding the number of
complete squares to half the number of subdivided
Rotifers squares.
e. Divide the Total Squares by 16 to convert to Area
Rotifers (phylum Rotifera) are small (< 0.5 mm), bilater-
of Lichen (cm2). Record the areas of each lichen in
ally symmetrical, aquatic animals with a crown of cilia at
table 9.1 and the bottom of table 9.3.
their heads. Their active cilia filter organic particles from
the environment as food. Most of the 2000 species of this 5. Label a petri dish for each lichen replicate and
interesting phylum live in freshwater, soil, or damp crevices fill each dish half full with filtered pond water (or
of plants and lichens. Rotifers are a significant member of distilled water). Invert each lichen replicate (lichen
lake zooplankton communities. side down) in its petri dish. Make sure the entire
The following procedure is a protocol for sampling lichen surface is submerged.
lichens and assessing the resident community. This protocol 6. After 24 h remove the lichen from each dish and
will be part of your experimental design to test a hypothesis use a dissecting microscope to scan the petri dish
that you and your team propose in Procedure 9.2. contents at 50⫻. Search for moving invertebrates
and record the number and species in table 9.2.

Table 9.1
Calculation of the area of replicate lichen samples
Location ___________________

Number of Completely Number of Divided Lichen Area

Enclosed Squares Squares Total Squares (total squares / 16)
Replicate 1 cm2
Replicate 2 cm2
Replicate 3 cm2
Replicate 4 cm2

9–3

Microcommunity Assessment 83
7. Use a Pasteur pipet to remove each invertebrate and 14. Calculate and record in table 9.3 the Density of Total
place it in a labeled vial of 50% ethanol for later Invertebrates as the sum of the mean densities of all
study or mounting. invertebrate species.
8. Return the lichen to the water. 15. Calculate and record in table 9.3 Shannon’s Diversity
9. After 24 h more, repeat steps 6–8 and record in of all invertebrates. See Exercise 12 for steps to
table 9.2 the invertebrates found during second 24h. calculate diversity.
10. After 24 h more, repeat steps 6–8 and record in Questions 1
table 9.2 the invertebrates found during third 24h. What group of invertebrates was the most abundant in your
11. Examine your raw data set in table 9.2. For each lichen samples? Least abundant? ____________________
replicate and each species, record in table 9.3 the ________________________________________________
sum of individuals (from table 9.2) collected at all
three time intervals (24 h, 48 h, and 72 h). Was your measure of lichen surface area accurate? Is there a
better way? ______________________________________
12. Calculate and record in table 9.3 the density (number
cm⫺2) of each species of invertebrate in each replicate ________________________________________________
by dividing the number of each invertebrate species
Lichen communities, like all organisms, encounter an
by the area of the lichen replicate.
array of physical, chemical, and biological conditions that
13. Calculate and record in table 9.3 the Mean Densities vary from one environment to another. Good research
of invertebrates by summing the four replicate involves identifying ecological conditions (variables) likely
densities of each invertebrate species and dividing to impact the community and then testing the effects of
by 4. those variables.

Table 9.2
Data for microinvertebrates extracted from four replicate lichen samples

Invertebrates during 1st 24h Invertebrates during 2nd 24h Invertebrates during 3rd 24h

Rep 1 Rep 2 Rep 3 Rep 4 Rep 1 Rep 2 Rep 3 Rep 4 Rep 1 Rep 2 Rep 3 Rep 4
Species A
Species B
Species C
Species D
Species E

Table 9.3
Data sheet for calculation and analysis of density and diversity of microinvertebrates
extracted from a lichen sample

Mean Density Shannon

Total Invertebrates Recovered after 72 h Density of Invertebrates (sum of Wiener
(summed from table 9.2) (number cm⫺2 lichen) reps 1–4 / 4) Diversity (H⬘)
Rep 1 Rep 2 Rep 3 Rep 4 Rep 1 Rep 2 Rep 3 Rep 4
Species A (pA)(loge pA)
Species B (pB)(loge pB)
Species C (pC)(loge pC)
Species D (pD)(loge pD)
Species E (pE)(loge pE)
Lichen area
(cm2) Density of Total Invertebrates = _____ H⬘ _____

9–4

84 Exercise 9
Procedure 9.2
Design and test a hypothesis involving factors that affect
the density and diversity of microinvertebrates in a
lichen community.
1. Review Exercises 1 and 2.
2. After discussion with your lab group, list 5–10
ecological factors likely to influence the density
and diversity of invertebrates in a lichen
microcommunity.
1. _________________________________________
2. _________________________________________
3. _________________________________________
4. _________________________________________
5. _________________________________________
3. Identify the ecological variable you want to
investigate. _________________________________
___________________________________________
4. Pose a general research question that relates this variable
to the density and/or diversity within contrasting lichen
communities. State your question here: _____________ Figure 9.4
Leaf litter is a remarkably hospitable habitat for large and small
___________________________________________
invertebrates. Leaf litter is moist, insulating from the heat of direct
___________________________________________ sunlight, and rich in organic detritus as food for decomposers.
5. Pose a testable hypothesis that will provide at least a
partial answer to your question. State that hypothesis
here: _______________________________________ LEAF LITTER ARTHROPOD
___________________________________________ COMMUNITIES
___________________________________________ Decaying leaf litter with abundant organic material sup-
6. Discuss with your lab group and instructor an ports a microcommunity of invertebrates (fig. 9.4). This
experimental design to test your hypothesis. web of invertebrates is supported by detritus (decompos-
7. Design your raw data sheet and analysis sheet (similar ing organic matter) and by bacteria and fungi feeding on
to tables 9.2 and 9.3), and conduct your experiment. the detritus. Invertebrates, especially small arthropods, are
Questions 2 abundant grazers on the detritus and microbial decomposers
Did you accept or reject your hypothesis? ______________ and may number into thousands per square meter. As you
might guess, the density and diversity of arthropods in leaf
What was the answer to your question? ________________ litter and the upper surface of the soil vary significantly with
________________________________________________ temperature, moisture, and organic input.
Ecologists extract small arthropods from a sample of leaf
Was your experimental design adequate to answer your litter with a Berlese funnel (fig. 9.5). A sample of leaf litter
question fully? ____________________________________ is placed on a screen below a light bulb. The bulb’s light and
________________________________________________ warmth causes the arthropods to move down through the
sample, fall through the screen, and slide down the funnel
How would you improve your experimental design? ______ into a collecting vial with preservative.
________________________________________________
Procedure 9.3
________________________________________________ Collect leaf litter arthropods from a woodland forest
Good research usually leads to further questions. How would floor.
you expand your research to better answer your question? 1. Assemble four Berlese funnels, and locate a wood-
________________________________________________ land sampling site with moist and abundant leaf litter
(> 2.5 cm thick).
________________________________________________
9–5

Microcommunity Assessment 85
 3. Return to the lab and put the samples on the screens
in the funnels. Some material may fall through
immediately. Gather the fallen material and put it
on top of the leaf litter so it won’t fall through again.
4. Place a collecting vial with 50% ethanol under each
funnel spout, and turn on the light bulb. Do not
overheat the samples.
5. Allow the invertebrates to collect in the vials for
12 h.
6. Replace the sample vial for each funnel every 12 h
for 36 total h. If the sample is particularly wet, you
may need to flip the sample on the screen rather
than let the upper surface burn before the sample
dries to the bottom.
7. Sort the collected arthropods into major groups
(fig. 9.6). Record in table 9.4 the number of each
species collected.
8. Examine your raw data set in table 9.4. For each
replicate and each invertebrate species, record in
table 9.5 the Total Arthropods Recovered at 12 h,
24 h, and 36 h.
9. Calculate and record in table 9.5 the Density of
Arthropods for each species in each replicate by
dividing the number collected of that species by
the sample area of the replicate.
10. Calculate the mean densities of arthropods by
Figure 9.5 summing the four replicate densities of each
A Berlese funnel uses heat from a light bulb to drive invertebrates arthropod species and dividing by four.
toward the bottom of a leaf litter sample where they fall through a
screen into a preserving solution.
11. Calculate and record in table 9.5 the Density of Total
Arthropods as the sum of the mean densities of all
invertebrate species.
2. Collect in plastic bags four replicate samples of leaf 12. Calculate and record in table 9.5 Shannon’s Diversity
litter from patches (about 200 cm2) of woodland of all invertebrates. See Exercise 12 for steps to
forest floor. Collect deep enough to include any loose calculate diversity.
soil to a depth of about 1 cm. A reasonable sample
size (area) depends on the amount your Berlese
funnel can accommodate.

Table 9.4
Number and species of arthropods collected from four replicate samples of leaf litter

Arthropods during 1st 12h Arthropods during 2nd 12h Arthropods during 3rd 12h
Rep 1 Rep 2 Rep 3 Rep 4 Rep 1 Rep 2 Rep 3 Rep 4 Rep 1 Rep 2 Rep 3 Rep 4
Species A
Species B
Species C
Species D
Species E
Species F
Species G

9–6

86 Exercise 9
 Silverfish Pill bug Leaf bug Beetle Roach

Thrip Termite Spider Ant Snail

Daddy long legs Tick Mite Pseudoscorpion Scorpion

Cricket Grasshopper Leafhopper

Centipede Millipede Springtail

Figure 9.6
Representative soil invertebrates.

9–7

Microcommunity Assessment 87
 Table 9.5
Data sheet for calculation and analysis of density and diversity of arthropods
extracted from a leaf litter sample

Mean Density Shannon

Total Arthropods Recovered Density of Arthropods (sum of reps Wiener
(summed from table 9.1) (number cm⫺1 sample) 1–4 / 4) Diversity (H⬘)
Rep 1 Rep 2 Rep 3 Rep 4 Rep 1 Rep 2 Rep 3 Rep 4
Species A (pA)(loge pA)
Species B (pB)(loge pB)
Species C (pC)(loge pC)
Species D (pD)(loge pD)
Species E (pE)(loge pE)
Species F (pF)(loge pF)
Species G (pG)(loge pG)
Sample area
(cm2) Density of Total Arthropods = _____ H⬘ _____

Questions 3 4. Pose a general research question that relates this

What arthropods were most common in leaf litter? ______ variable to the density and/or diversity among leaf
litter arthropods from contrasting communities. State
________________________________________________
your question here. ___________________________
Least common? ___________________________________
___________________________________________
________________________________________________ 5. Pose a testable hypothesis that will provide at least a
Is a Berlese funnel a good device to extract leaf litter arthro- partial answer to your question. State that hypothesis
pods? Would it work well for all microorganisms? Why or here. _______________________________________
why not? ________________________________________ ___________________________________________
________________________________________________ 6. Discuss with your lab group and instructor an
experimental design to test your hypothesis.
Procedure 9.4 7. Design your raw data sheet and analysis sheet
Design and test a hypothesis concerning factors affecting (similar to tables 9.1 and 9.2), and conduct your
the density and diversity of arthropods in a leaf litter experiment.
community. Questions 4
Did you accept or reject your hypothesis? ______________
1. Review Exercises 1 and 2.
2. After discussion with your lab group, list 5–10 What was the answer to your question? _______________
ecological variables likely to influence the density ________________________________________________
and diversity of arthropods in a leaf litter community.
List these factors in the following space. Was your experimental design adequate to answer your
1. _________________________________________ question fully? ___________________________________

2. _________________________________________ ________________________________________________

3. _________________________________________ How would you improve your experimental design? ______

4. _________________________________________ ________________________________________________

5. _________________________________________ ________________________________________________
3. Identify the ecological variable you want to Good research usually leads to further questions. How would
investigate. _________________________________ you expand your research to better answer your question?
___________________________________________ ________________________________________________
___________________________________________ ________________________________________________

9–8

88 Exercise 9
Questions for Further Thought and Study

1. What are the major ecological factors that influence the density of leaf litter arthropods?

2. What major ecological factors would influence the invertebrate community in a lichen?

3. For which other environments would lichen invertebrates be well adapted?

4. What other microcommunities occur in a forest ecosystem? Are microcommunities self-contained? How so?

5. What role did sample size play in testing your hypotheses?

9–9

Microcommunity Assessment 89
exercise ten

Sampling a Plant Community 10

Objectives
As you complete this lab exercise you will:
1. Survey plant species richness and habitat gradi-
ents in a local environment.
2. Assess herb, shrub, and tree densities with meth-
ods of quadrats, strip transects, line transects, and
line intercepts.
3. Calculate densities, frequencies, and relative
importances of herb, shrub, and tree communities.
4. Assess stratification of plant communities along a
gradient.

C ommunities would be much easier to study if plants were

regularly distributed in space and time. But they aren’t.
Plants don’t grow in uniform, spatial patterns. Put simply,
Figure 10.1
All of the plants in this 1-m2 quadrat can be counted in just minutes.
plant distributions vary because the environment is patchy, Counts from multiple, randomly placed quadrats of a known area
and the distribution of all organisms ultimately depends on represent samples used to estimate plant densities in a much larger area.
patchy resources, both biotic and abiotic. This patchiness
is so pervasive in natural communities that ecologists need each sample. Randomization ensures that your data will meet
rigorous sampling techniques and large sample sizes to over- assumptions of statistical analyses. Always remember that
come natural variation (fig. 10.1). these quantitative procedures are further enhanced by your
The fundamental goals of sampling a plant commu- initial observations of the species present and of variations
nity are to determine (1) which plants live there; (2) how in soil, moisture, disturbance, drainage, etc. Raw data is gath-
many plants occur per square meter; and (3) which species ered objectively, but the quality of the questions asked and
dominate the local environment. These aren’t easy to deter- the overall sampling design require walking, observing, and
mine. Sample size, shape, and number are all critical for good examining the environment through the eyes of an ecologist.
results. The more large and uniform samples you take, the
better the estimate, but the greater the cost and effort. The Procedure 10.1
best sampling designs balance sampling effort with accuracy. Assess by observation the species richness, habitat
Question 1 gradients, and boundaries of a plant community.
Would you expect animal distributions to be as patchy as
those of plants? Why or why not? _____________________ 1. Review Exercise 7. Decide which observations relate
to the environment under study.
________________________________________________ 2. Discuss with your instructor the general location and
In this lab exercise you will use a variety of sampling boundaries of a study site(s).
methods and learn the best applications and effort required 3. Divide into teams to qualitatively (by observation)
for each method. Most sampling designs involve randomly compile a list of tree species, shrub species, and herb
distributing as many sampling units (quadrats and transects) species. One team should also survey boundaries and
over the study area as possible and counting the plants in gradients of abiotic factors (soil, moisture, inclines, etc.).

10–1

91
 Table 10.1
Qualitative list of tree species, shrub species, and herb species

Trees Shrubs Herbs

Identifying Identifying Identifying
Species Characteristic Species Characteristic Species Characteristic

Observations of abiotic factors and gradients:

4. Each team should record in table 10.1 the species list sampling, the frequency of a species is the percent of all sam-
for its assigned taxon (trees, shrubs, herbs). pling units that have at least one individual. Because a quadrat
5. The team assessing abiotic factors should record its is a known area, the results for one species, or for all species,
observations in table 10.1 and prepare sketched maps can also be expressed as absolute density. Density is the num-
of each relevant abiotic factor. ber of organisms per unit area. Occurrence of a species can also
6. Teams should consult with each other and define the be expressed as relative density, which is the percentage of the
boundaries of major tree-, shrub-, and herb-dominated total number of individuals represented by that species.
areas of the broader environment. Sketch these Choosing the size of the quadrat depends on the den-
boundaries. sity of plants being sampled. Typical quadrats are squares of
1 square meter or more. They should be large enough to
Questions 2
frequently contain five or more individuals, but small
Practiced observation is a powerful technique. How would
enough for you to count all individuals in a reasonable time.
you improve your powers of observation and the accuracy of
Herbs, shrubs, and trees are sampled well by quadrats 1 m2,
your species list? __________________________________
4–10 m2, and 100–500 m2, respectively.
________________________________________________ The term quadrat specifically refers to a four-sided rect-
angle, but quadrats can be any shape, including a circle
What are the major drawbacks to relying solely on observa-
(fig. 10.2). Shape is usually determined by ease of layout.
tion to assess a plant community? ____________________
Some ecologists prefer to use a circular quadrat to minimize
________________________________________________ edge length and to eliminate problems in determining if
a plant is inside or outside the edge. Circular quadrats are
easily laid out by pushing a large metal pin (nail) into the
QUADRATS AS SAMPLING UNITS ground, tying a string to the pin so the string can slip around
FOR HERBS the pin, marking the string at 56.4 cm (radius) from the pin,
and rotating the string around the pin to delineate a 1-m2
Many methods measure numbers, densities, and distributions of circle. A radius of 79.8 cm encompasses a 2-m2 quadrat.
plants in terrestrial communities. Probably the most widely used The number of quadrat samples should be as large as
method is quadrat sampling. Uniform quadrats of a known area reasonable, and depends on the nature of the plants being
(sometimes called plots) are randomly distributed in a habitat studied and the effort needed to count each quadrat. Thirty
and the organisms in each quadrat are counted. For quadrat quadrats usually produce reliable results.

10–2

92 Exercise 10
 Procedure 10.2
Assess herb density using the quadrat method.
1. Refer to the results of Procedure 10.1, step 6 and
determine the boundaries of the area to sample
herbs. For sampling purposes, herbs are defined as
nonwoody plants less than 40 cm high.
2. Consult with your instructor and choose quadrat size
(1 m2 recommended), shape (square or circle), and
number of quadrats for sampling.
3. Quadrats must be placed randomly. Using the
following steps, establish an imaginary grid with
numbered positions over the area being sampled.
To establish the grid:
a. Establish and mark the ends of a baseline along
Figure 10.2 one edge of the area being sampled. Make the
Circular plots are good sampling units and can be easier to define baseline as long as the longest edge of the area.
than a square plot. A radial string is rotated around a center point b. Mark the baseline at 1-m intervals.
to define the perimeter of a circle of known area. The students are c. At one end of the baseline, establish a perpen-
counting plants inside the circle.
dicular line long enough for a grid of positions
defined by the two lines to overlay the entire area
being studied. Mark the perpendicular line at 1-m
intervals. Points (1-m intervals) along the baseline
represent columns. Points (1-m intervals) along the
perpendicular line represent rows (fig. 10.3).

10

9 9,10

8 8,4 8,7

7 7,5 7,13
Habitat
6

5 5,7

4 4,7 4,12

3 3,10

1 1,10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Baseline (m)

Figure 10.3
A baseline and perpendicular line position a grid over an irregular area of a habitat to be sampled. Mark the lines in 1-m intervals. Then
choose row and column numbers randomly to locate 10 replicate quadrats for sampling.

10–3

Sampling a Plant Community 93

 Table 10.2
The number of each herb species found in 20 randomly positioned quadrats

Quadrat One quadrat ______ m2

Species 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 # Quadrats With Total Number of
at Least One Individuals Speciesi
Individual

Total
herbs

Table 10.3
Densities of herbs sampled with quadrat sampling units

Species Frequency (%) Absolute Density (num m 2) Relative Density (%)

Total Herbs 100%

d. To position a quadrat in a random box of the grid, 7. For each speciesi and for total herbs, calculate and
use a random number table to select a column. record in table 10.2 the number of quadrats that
Then use a random number table to select a row. included at least one individual of that species.
The intersection of the row and column is the po- 8. Calculate and record in table 10.2 for each species
sition of the quadrat. Repeat random column and the total individuals counted in all quadrats.
row selections for each quadrat needed. Record 9. Calculate and record in table 10.3 the frequency,
the coordinates (column, row) for all expected absolute density, and relative density for each
quadrats before you begin sampling. speciesi.
4. Familiarize your team with the species list of herbs in
frequencyspecies (100 number of sampling quadrats
table 10.2. i
with one or more individuals of speciesi) / total
5. Place the pin (or quadrat square) at the first
number of sampling quadrats
coordinates. Count and record in table 10.2 the
number of each species of herb whose basal (ground- absolute densityspecies
i
total number
level) stems lie within or on the quadrat boundaries. of individualsspecies / total area sampled
i
Sum these numbers and record in table 10.2 the total
relative densityspecies (100 total number
herbs in the quadrat. i
of individualsspecies ) / total number of individuals
6. Repeat step 5 for each set of coordinates (i.e., each of all species
i

quadrat).

10–4

94 Exercise 10
Questions 3 LINE TRANSECTS AS SAMPLING UNITS
Did the random placement of your quadrats adequately sam- FOR HERBS
ple the community? Why or why not? _________________
________________________________________________ Another common sampling technique is the line intercept
method. In this method, transects, or lines, are established
The densities of herb species are rarely even. How many and randomly laid out within the community. Organisms
herb species include 90% of the total herb plants? _______ that touch or “intercept” the transect are counted and mea-
________________________________________________ sured (fig. 10.4). Calculations with these data reveal the
relative abundances, frequencies, and distributions of the
Consider absolute density and relative density. Is one more plant species that compose the community.
informative than the other? _________________________ A disadvantage of the line intercept method is that it
________________________________________________ does not measure ground surface area. Therefore, you can-
not calculate absolute density. You can, however, calcu-
Are the species with highest frequency also the ones with late measures of relative density among plant species. This
highest density? __________________________________ reveals which plants are more dense than others, but not
________________________________________________ their absolute densities per unit area. Oftentimes ecologists
are just as interested in relative density as they are in abso-
Which of the three parameters in table 10.3 requires the lute density.
least effort to gather the necessary data? Why? __________ Space is a precious resource for plants, especially the
________________________________________________ area that a plant occupies with sunlight. Coverage is a mea-
sure of aerial space a plant occupies. A species’ relative cov-
Which season would be best for sampling a plant commu- erage is the percent of total plant coverage represented by
nity? What difference might it make? _________________ that species.
________________________________________________

pla ne
Ver tical

Transect line

Figure 10.4
A transect includes a line (or tape measure) on the ground plus the imaginary vertical plane extending above the line. Plants rooted to the side of
the line but extending into the vertical plane are counted as well as plants touching the ground line are all part of the sample.

10–5

Sampling a Plant Community 95

Question 4 tion of the row and column is the starting point of
Is coverage directly related to density? Why or why not? the transect.
f. Select a random number (0–360) to determine
________________________________________________
the compass direction to extend the 5-m transect
________________________________________________ from the starting point.
Experience with plant sampling quickly reveals that no 5. Familiarize your team with the species list of herbs in
single parameter fully describes a plant community. No sin- table 10.4.
gle value measures “success.” Therefore, ecologists usually 6. For your first transect, stretch the 5-m measuring
measure a variety of parameters to understand the commu- tape on the ground from your random set of
nity. One combination of relevant parameters is an impor- coordinates (step 4).
tance value. This value combines the data for two or more 7. Divide the transect into five 1-m intervals.
variables such as density, coverage, frequency and others. In 8. Along the first interval, count plants that touch or
the next procedure you will calculate importance value as a underlie the line. For each individual plant, record in
combination of relative density and relative coverage. table 10.4 the species and the length (cm) of the line
that the plant intercepts. Similarly, for plants that
Procedure 10.3 overhang the line, record in table 10.4 the length of
Assess the frequencies, relative densities, and importance the imaginary vertical plane from the line that the
values of herb species by using line transect and line plant intercepts. If necessary, also record as Species
intercept methods. Bare any uncovered (bare) lengths of the interval.
Table 10.4 accommodates one transect with data for
1. Refer to the results of Procedure 10.1 and determine
seven individuals of each of three species in each
the boundaries of a suitable area to sample herbs. For
1-m interval.
sampling purposes, herbs are defined as nonwoody
plants less than 40 cm high. 9. When all plants from all five 1-m intervals of the
5-m transect have been recorded, summarize your
2. Consult with your instructor and choose a suitable
raw data in table 10.5.
transect length (5 m recommended) and a suitable
number of transects (one per team recommended). 10. Check that each of the other teams has completed
data collection for its transect.
3. For your team’s transect, obtain a 5-m measuring
tape (or measured rope) and a notepad. 11. Use the data in table 10.5 to calculate the following
four parameters for each species. Record your results
4. Your transect must be randomly placed. Use the
in table 10.6.
following steps to select random starting and ending
points for your transects on an imaginary grid with frequencyspecies
i
(100 number of sampling intervals
numbered positions encompassing the area being with one or more individuals of speciesi) / total
sampled. number of sampling intervals
a. Establish and mark the ends of a baseline along
one edge of the area being sampled. Make the relative densityspecies
i
(100 total number of
baseline as long as the longest edge of the area. individuals of speciesi) / total number of individuals
b. Mark the baseline at 1-m intervals. of all species
c. At one end of the baseline, establish a perpen-
dicular line long enough for a grid of positions relative coveragespecies
i
(100 total intercept length
defined by the two lines to overlay the entire area by speciesi ) / total length intercepted by all plants
being studied.
d. Mark the perpendicular line at 1-m intervals. importance valuespecies
i
(relative densityspecies
i
Points (1-m intervals) along the perpendicular relative coveragespecies ) / 2
i
line represent rows. Points (1-m intervals) along
the baseline represent columns. 12. Your instructor may ask you to compare or combine
e. To randomly position a transect, use a random your data with those from other transects.
number table to select a column. Then use a ran-
dom number table to select a row. The intersec-

10–6

96 Exercise 10
Table 10.4
Raw data for herbs intercepting a transect at each interval

Extend this table if necessary to accommodate more species.

Interval 1 Interval 2 Interval 3 Interval 4 Interval 5

Intercept Intercept Intercept Intercept Intercept
Individual (m) Individual (m) Individual (m) Individual (m) Individual (m)
Species: 1st 1st 1st 1st 1st
nd nd nd nd
2 2 2 2 2nd
3rd 3rd 3rd 3rd 3rd
4th 4th 4th 4th 4th
th th th th
5 5 5 5 5th
6th 6th 6th 6th 6th
th th th th
7 7 7 7 7th
Species: 1st 1st 1st 1st 1st
2nd 2nd 2nd 2nd 2nd
rd rd rd rd
3 3 3 3 3rd
4th 4th 4th 4th 4th
th th th th
5 5 5 5 5th
6th 6th 6th 6th 6th
th th th th
7 7 7 7 7th
Species: 1st 1st 1st 1st 1st
2nd 2nd 2nd 2nd 2nd
rd rd rd rd
3 3 3 3 3rd
4th 4th 4th 4th 4th
th th th th
5 5 5 5 5th
6th 6th 6th 6th 6th
th th th th
7 7 7 7 7th
Species: 1st 1st 1st 1st 1st
bare
2nd 2nd 2nd 2nd 2nd
rd rd rd rd
3 3 3 3 3rd
4th 4th 4th 4th 4th
th th th th
5 5 5 5 5th
6th 6th 6th 6th 6th
th th th th
7 7 7 7 7th

10–7

Sampling a Plant Community 97

 Table 10.5
Summary of raw data for herb species intercepting a transect

Transect length __________ Number of intervals __________

Number of Intervals with at Least Total Number of Individuals
Speciesi One Individual of Speciesi Total Intercept Length by Speciesi

Total number individuals Total intercept length

of all species by all plants

Table 10.6
Relative values of each herb species in a selected community using parameters of the
line-intercept method

Species Frequency Relative Density Relative Coverage Importance Value

Questions 5 Did your line intercept sampling of herbs reveal any species
Are there any “wrong” places to locate a transect? Why or you missed with your observations in Procedure 10.1?
why not? ________________________________________
________________________________________________
________________________________________________
________________________________________________
What herb species had the highest frequency? _______
Relative density? _______ Relative coverage? _______
Importance? _______
STRIP TRANSECTS AS A SAMPLING
What is the meaning of an importance value? __________ UNIT FOR SHRUBS
________________________________________________
Ecologists frequently extend a transect to include a strip of
Why would we calculate this in addition to density, cover- area on each side of the transect line. In this way a strip
age, and frequency? _______________________________ transect can be treated as a long, narrow, rectangular quad-
rat. Strip transects are especially good for measuring density
________________________________________________
of plants such as shrubs that are farther apart than herbs. For

10–8

98 Exercise 10
sampling purposes, shrubs are defined as woody-stemmed 9. For each species and for total shrubs, calculate and
plants over 40 cm high with stems < 2.5 cm diameter at record in table 10.7 the number of intervals with at
1.5 m above the ground. Stem diameter at 1.5 m above the least one individual of that species.
ground is called diameter at breast height (DBH). For sam- 10. Calculate and record for each species in table 10.7 the
pling purposes, trees are woody-stemmed plants typically total number of individuals counted in all intervals.
having a single main stem and a DBH > 2.5 cm. 11. Use the data in table 10.7 to calculate the following
three parameters for each species. Record your results
Procedure 10.4 in table 10.8.
Use the strip transect method to measure the density of
frequencyspecies (100 number of sampling intervals
shrubs. i
with one or more speciesi ) / total number of
1. Familiarize your team with the species list of shrubs sampling intervals
in table 10.1.
absolute densityspecies (total number of
i
2. Refer to Procedure 10.1 and determine the individualsspecies ) / total area sampled
i
boundaries of a suitable area to sample shrubs.
relative densityspecies (100 total number of
3. Consult with your instructor and choose a suitable strip i
individualsspecies ) / total number of individuals
transect length and width (20 m 2 m recommended) i
of all species
and a suitable number of transects (one per team
recommended). 12. Your instructor may ask you to compare or combine
4. For your team’s transect, obtain a 20-m measuring your data with those from other transects.
tape (or measured rope), meter stick, and a notepad. Questions 6
5. Your transect must be randomly placed. Follow What shrub species had the highest density? ___________
step 4 of Procedure 10.3 to establish the coordinates ________________________________________________
of randomly placed strip transects.
6. For your transect, stretch the 20-m measuring Did all three measures of occurrence in table 10.8 portray
tape on the ground to establish a transect at your the same species composition of the shrub community?
coordinates. Divide the transect length into 1-m ________________________________________________
intervals.
________________________________________________
7. As you move long the transect, use the meter stick
perpendicularly on each side of the line to judge Which parameter in table 10.8 reveals the most evenly dis-
which shrubs are included in the strip transect. tributed shrubs? ___________________________________
8. Count and record in table 10.7 the number of each ________________________________________________
species of shrub whose basal (ground-level) stems lie
at least partially within the area of each interval of Most unevenly distributed? _________________________
the strip transect boundaries. ________________________________________________

Table 10.7
The number of each shrub species found in a randomly positioned strip transect

Interval One Interval ______ m2

Species 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 # Intervals Total Number
with at of Individuals
Least One in All
Individual Intervals

Total
shrubs

10–9

Sampling a Plant Community 99

 Table 10.8
Densities of shrubs sampled with strip transect sampling units

Species Frequency (%) Absolute Density (num m 2) Relative Density (%)

Total Shrubs 100%

LINE TRANSECTS AS SAMPLING UNITS in table 10.9 the species and the length (m) of the
FOR TREES line that the tree intercepts. Similarly, for trees that
overhang the line, record in table 10.9 the length
Trees are large and spaced relatively far apart. This distri- of the imaginary vertical plane from the line that
bution calls for a method that covers as much territory as the tree intercepts. Also record as Species Bare any
possible without extraordinary effort to count all the trees. uncovered (bare) lengths of the interval. Table 10.9
A line intercept procedure works well, especially if the tran- accommodates data for seven individuals of each of
sect is long and only trees are counted. The protocol for three species.
assessing trees is the same as for herbs described in Proce- 8. When all trees from all five 10-m intervals have been
dure 10.3, except the transects are longer. recorded, summarize your raw data in table 10.10.
9. Sum the data in both columns and record the totals
Procedure 10.5 in table 10.10.
Assess tree coverage and importance using the line 10. Check that each of the other teams collected the
intercept method. data for its transect.
1. Familiarize your team with the species list of trees in 11. Use the data in table 10.10 to calculate the following
table 10.1. four parameters for each tree species within the
2. Refer to Procedure 10.1 and determine the community. Record your results in table 10.11.
boundaries of a suitable area to sample trees. Trees frequencyspecies (100 number of sampling intervals
i
include single-stemmed plants with DBH > 2.5 cm. with one or more individuals of speciesi) / total
3. Consult with your instructor and choose a suitable number of sampling intervals
transect length (30–100 m recommended) and
a suitable number of transects (one per team relative densityspecies
i
(100 total number of
recommended). individuals of speciesi) / total number of individuals
4. For your transect, obtain a measured 50-m string or of all species
rope marked in 10-m intervals.
5. Your transect must be randomly placed. Follow the relative coveragespecies (100 total intercept length
i

procedure in step 4 of Procedure 10.3 to randomly by speciesi) / total length intercepted by all plants
place your transect on an imaginary grid with
numbered positions encompassing the area being importance valuespecies (relative densityspecies
i i

sampled. relative coveragespecies ) / 2

i

6. Stretch the 50-m measured rope on the ground from 12. Your instructor may ask you to compare or combine
your random set of coordinates. your data with those from other transects.
7. Along the first 10-m interval, count trees that touch,
overlie, or underlie the line. For each tree, record

10–10

100 Exercise 10
Table 10.9
Raw data for trees intercepting a transect at each interval

Extend this table if necessary to accommodate more species.

Interval 1 Interval 2 Interval 3 Interval 4 Interval 5

Intercept Intercept Intercept Intercept Intercept
Individual (m) Individual (m) Individual (m) Individual (m) Individual (m)
Species: 1st 1st 1st 1st 1st
nd nd nd nd
2 2 2 2 2nd
3rd 3rd 3rd 3rd 3rd
4th 4th 4th 4th 4th
th th th th
5 5 5 5 5th
6th 6th 6th 6th 6th
th th th th
7 7 7 7 7th
Species: 1st 1st 1st 1st 1st
2nd 2nd 2nd 2nd 2nd
rd rd rd rd
3 3 3 3 3rd
4th 4th 4th 4th 4th
th th th th
5 5 5 5 5th
6th 6th 6th 6th 6th
th th th th
7 7 7 7 7th
Species: 1st 1st 1st 1st 1st
2nd 2nd 2nd 2nd 2nd
rd rd rd rd
3 3 3 3 3rd
4th 4th 4th 4th 4th
th th th th
5 5 5 5 5th
6th 6th 6th 6th 6th
th th th th
7 7 7 7 7th
Species: 1st 1st 1st 1st 1st
bare
2nd 2nd 2nd 2nd 2nd
rd rd rd rd
3 3 3 3 3rd
4th 4th 4th 4th 4th
th th th th
5 5 5 5 5th
6th 6th 6th 6th 6th
th th th th
7 7 7 7 7th

10–11

Sampling a Plant Community 101

 Table 10.10
Summary of raw data for tree species intercepting a transect
Transect length __________ Number of intervals __________

Speciesi Total Number of Individuals Speciesi Total Transect Length Intercepted by Speciesi

Total number individuals of all species Total intercept length by all plants

Table 10.11
Relative values of each tree species in a selected community using parameters of the line
intercept method

Species Frequency (%) Relative Density (%) Relative Coverage (%) Importance Value

Questions 7 STRATIFED RANDOM SAMPLING

Which tree species were most evenly distributed among all ALONG A GRADIENT
transect intervals? ________________________________
________________________________________________ Assessing a plant community, such as herbs in an open field,
is often complicated by gradients of resources or habitat fea-
Did your sampling of trees reveal any tree species that were tures. For example, moisture, soil type, distance from a river,
not recorded during Procedure 10.1? __________________ elevation, or distance from a disturbance often vary from
________________________________________________ one end of a study site to the next. These gradients affect
plants and their distributions. The previous procedures have
Which plant group (herbs, shrubs, trees) was best sampled dealt with this variation by randomizing the placement of
by quadrats? _____________________________________ sampling units throughout the environment. Another sam-
________________________________________________ pling procedure that recognizes the realities of the environ-
ment and maximizes the information from the samples is
Is “best sampled” and “most easily sampled” the same thing? stratified random sampling (fig. 10.5).
Why or why not? _________________________________ In a stratified random sampling design, the habitat
________________________________________________ being studied is stratified into meaningful units along a

10–12

102 Exercise 10
 Grassland

River

Soil Soil Soil

moisture moisture moisture
level 1 level 2 level 3

Figure 10.5
For random stratified sampling, a habitat is stratified (divided) into subhabitats along a known environmental gradient such as moisture from an
adjacent stream. Samples are randomly distributed within each subhabitat. Stratified sampling accounts for variation of a known environmental
gradient.

known gradient. For example, a grassland bordered on one 5. Divide the habitat into biologically meaningful strata
side by a river may have a moisture gradient. If so, the grass- along the gradient.
land can be stratified into two or more contiguous areas 6. Design the tables needed for recording raw data,
(strata), each a different distance from the stream. Each of summarizing raw data, and calculation of the
the areas is sampled independently and the strata are later variables needed to describe the community.
compared to determine if the gradient has an effect. Each 7. Divide into teams (one team for each stratum) and
of the three strata (sub-environments) along the moisture sample the designated plant community according to
gradient is sampled with its own randomly placed sampling the steps in the previous procedures.
units. Each stratum (defined area along the gradient) is sam-
8. Calculate and compare your results for the strata.
pled as described in the previous procedures, and the results
provide accurate information for areas along the gradient. Questions 8
After completing your sampling, do your original divisions
Procedure 10.6 of the habitat into strata still appear biologically meaning-
ful? ____________________________________________
Compare plant communities stratified along a gradient.
________________________________________________
1. Review the observations of the team that surveyed
boundaries and gradients of abiotic factors in What did you conclude about the effect of the gradient on
Procedure 10.1. Consult with your instructors and herbs? __________________________________________
review the results of the previous four procedures.
________________________________________________
2. Identify a gradient suitable for analysis.
3. Determine which plant type (herb, shrub, tree) to On shrubs? ______________________________________
survey. ________________________________________________
4. Choose the most appropriate sampling design
On trees? _______________________________________
(quadrat, line transect, strip transect) and
randomization procedure. ________________________________________________

10–13

Sampling a Plant Community 103

Questions for Further Thought and Study

1. Evaluate the advantages and disadvantages of

quadrat sampling:

strip transect sampling:

line transect sampling:

2. Would a greater number of quadrats used in a sampling design narrow or widen the confidence intervals around a
mean density? Why? How might you investigate this by subdividing your data set from Procedure 10.2?

3. How would you design an experiment to test the relative effectiveness of circular versus square versus rectangular
quadrats?

4. The importance of a plant species to a community is difficult to define. What would be five or more meaningful
parameters to measure?

10–14

104 Exercise 10
exercise eleven

Sampling Animal Communities 11

Objectives
As you complete this lab exercise you will:
1. Estimate the size of a simulated population using
the Lincoln-Petersen calculations of mark-
recapture data.
2. Use mark-recapture to determine insect popula-
tion size.
3. Use variable circular plots to determine the den-
sity of a bird population.
4. Trap small mammals with Sherman live traps to
determine the number of species and their rela-
tive abundance in a small mammal community.
5. Use animal sampling data to calculate Jacquard’s
coefficient of community similarity and investi-
gate minimum and maximum similarity values.
6. Choose and conduct a data-collection technique
(sweep net, mammal trapping, etc.) to determine
Figure 11.1
similarity between two communities.
A sweep net is a standard collecting device that is ruggedly built with
canvas and netting. Sampling effort with a sweep net is quantified
with a consistent number of sweeps at a consistent speed.

P opulations of animals are hard to sample because they

move (fig. 11.1). However, the same principles of good
design used in plant sampling also apply to animal sam-
MEASURING POPULATION DENSITY
WITH MARK-RECAPTURE
pling. Careful planning, familiarity with the area, and care- Sampling population size of highly mobile species is diffi-
ful observation are crucial. The best procedures include cult at best. Fortunately, an ingenious procedure to over-
taking enough repeated samples to estimate population come the movement problem has been around for over 200
values accurately. Most procedures to sample animal popu- years—a procedure called mark-recapture. For this proce-
lations gather data to answer questions about population dure, a sample of animals is captured by whatever means.
size, density, diversity, similarities among communities, and All of the individuals are marked with a number, a notch,
changes in these parameters through time. or a tag, and then released. A second sample is taken some
This lab exercise presents three common procedures days later. Some of the individuals captured in the second
for sampling populations of invertebrates and small ver- sample were marked from the first sample. The size of the
tebrates: mark recapture, circular plots, and mammal live two samples and the portion of the second sample that is
trapping. Data analyses for these procedures focus on calcu- marked allow calculation of total population size.
lating population size and community similarity. However, The most common calculations for mark-recapture fol-
data from these techniques also allow calculation of a vari- low the Lincoln-Petersen method:
ety of parameters and indices. The best sampling procedures
provide enough information to broaden your research. P (M p)/m

11–1

105
where:
P total population size

M total number of marked individuals

( size of the first sample)

p size of the second sample

m number of marked individuals in the second

sample.

Procedure 11.1
Estimate population size using the Lincoln-Petersen
calculations of mark-recapture data.
1. Examine the following simulated mark-recapture
data. These data include counts of fish that were
marked, released, and recaptured.
The first sample of fish contained 250 individu-
als; all were marked and released.
Figure 11.2
The second sample of fish contained 300 indi-
This grasshopper is being marked for recapture. Mark-recapture
viduals, 25 of which were marked. methods determine population size, but are also part of behavioral
2. Calculate the total population size: studies.

P (M p)/m

P total population size is _______ individuals. 2. Walk the habitat and take note of significant
variations that might influence the distribution and
3. Verify your calculations with your instructor. abundance of grasshoppers and the placement of your
Question 1 sampling sites.
If you used mark-recapture to estimate population size for 3. Use an insect sweep net to collect a sample of
a grasshopper population and for a fish population, could grasshoppers from the area. After examining the
you relate population size to population density for either of sample, consult with your instructor about marking
these populations? How so? _________________________ all species of grasshopper, or a single abundant
________________________________________________ species. Determine the smallest grasshopper (L 1 cm)
that can be marked on top of the thorax with a blue
Mark-recapture sampling applies to a variety of species and or red dot from a felt-tip marker.
habitats. Like all procedures, however, certain assumptions
4. Designate sampling sites about 10–20 m apart
must be met for valid results. Mark-recapture assumes that:
throughout the habitat. Record the sampling site IDs
• Animals retain their marks. in table 11.1. Establish teams of two to four students
and designate one sampling site for each team.
• Marks do not alter natural behavior or survival rates.
5. Obtain an insect sweep net to sample grasshoppers at
• Marked animals have the same probability of being your team’s site.
recaptured as do unmarked animals. 6. To collect grasshoppers, “sweep” the net back and
• Marked animals distribute themselves randomly among forth so the hoop brushes through the grass rather
unmarked animals. than above it. Sweep the net five to seven times.
Pass each sweep through a new patch of grass. Be
• Emigration and immigration are equal for marked and consistent with speed and movement. If populations
unmarked individuals. are low, more sweeps might be necessary.
7. Concentrate the captured insects into the bottom of
Procedure 11.2
the net by shaking it, but don’t let them escape.
Use the mark-recapture procedure to determine
8. Grab and constrict the net just above the
population size of grasshoppers.
concentrated insects to prevent them from escaping
1. Consult with your instructor to locate a tall-grass and invert the end of the net into a wide-mouth jar.
habitat with an abundant grasshopper population Replace the lid. This takes practice. Watch your
(fig. 11.2). instructor do this.

11–2

106 Exercise 11
9. Examine the captured insects for grasshoppers What assumptions are most suspect for your procedure?
suitable for marking.
________________________________________________
10. Patiently take each grasshopper out of the jar, mark
it on top of the thorax, and put it in a holding ________________________________________________
container. On a raw data sheet, place one tally mark Does sweep netting bias your samples? How so? _________
for each marked and released grasshopper.
11. Repeat steps 6–10 until you have marked 50–100 ________________________________________________
grasshoppers. Should marks be as easy to see as possible to make sure you
12. Release all of the marked grasshoppers. don’t miss one? Why? ______________________________
13. Sum the tally marks and record for your sample site ________________________________________________
in table 11.1 your Number of Marked Grasshoppers for
the first sample. Also record the totals from the other What weaknesses in mark-recapture would you try to mini-
teams’ sampling sites. mize? ___________________________________________
14. Leave the sampled habitat undisturbed for 1 or 2 ________________________________________________
days. Then return for a second sample.
15. Repeat steps 4–8 until you have captured 50–100
grasshoppers. Accumulate your second sample in a
holding container. VARIABLE CIRCULAR PLOTS TO SAMPLE
16. Count and record in table 11.1 the Total Number of
BIRD POPULATIONS
Grasshoppers Captured in your second sample.
The variable circular plot method works well for census-
17. Count and record in table 11.1 the Total Number of ing animals by sight and sound. In this method, a stationary
Marked Grasshoppers in the second sample. observer records reliable sightings of animals appearing in
18. Release the second sample of grasshoppers. any direction from a central point. This makes the plot cir-
19. Record in table 11.1 the totals for the second samples cular (fig. 11.3). The radius of the plot ultimately depends
by the other teams. on the farthest reliable sighting, which varies with density
20. Use the Lincoln-Petersen method (Procedure 11.1) of the vegetation. Distances to sighted animals are recorded
to calculate the Total Population Size Estimate and and may be any length. Determining the distance of reliable
record it in table 11.1. detection (i.e., the radius of the circle and the area of the
Questions 2 plot) requires a graph of the raw data showing the distance
Was the grasshopper population smaller than you at which detection drops off.
expected? _______________________________________
Procedure 11.3
________________________________________________ Circular plot procedure to determine the density of a
Are you confident that your procedure to measure grasshop- bird population.
per population size met all the assumptions for successful 1. Choose with your instructor a consistent, typically
mark-recapture? __________________________________ wooded habitat with an abundant bird population.
________________________________________________

Table 11.1
Mark-recapture data for determining the size of a grasshopper population

Number of Marked Grasshoppers Total Number of Grasshoppers Number of Marked Grasshoppers

Sampling Site ID (first sample) Captured (second sample) (second sample)

Total: Total: Total:

Total population size estimate:

11–3

Sampling Animal Communities 107

 2. Establish sampling stations (one for each student or
pair of students) along a transect covering the entire
x
habitat. Place the sampling stations 50–200 m apart
Band
depending on the habitat available and the potential
x 6 sight-line from each observer. The distance should
x x xx x Band
x 5 prevent two observers from seeing the same bird
x Band during the same time interval.
x x
4
x x
Band 3. At one or all sampling stations, mark 10-m distance
x 3
x bands (intervals) to help the observer learn to gage
x Band
x 2
x
x distances.
x
Band
x 4. Position yourself as a stationary observer at the
60 m 50 m 40 m x 30 m 20 m 10 m 1 x
x center of a 360° circular sampling station. As you
x x Observer
x x walk to the center, count and record the distance
x x from the station center of any bird seen flying away.
x 5. As an observer, sample for 15 min and record in a
x field notebook each bird you have seen or heard and
x
x its estimated distance away.
x 6. There is no maximum distance. Record all birds
except those flying far overhead that are not using
the general habitat.
7. Transcribe your field notebook data into table 11.2.
8. Record in table 11.2 the other teams’ data from their
Figure 11.3 sampling stations.
Variable circular plot. The plot for this sample site includes six 9. For each band, calculate and record the Total number
bands. Each X marks an observation of an organism. The number of of birds by summing the bird counts from all stations.
observations is sharply lower in the two outermost bands. Therefore,
10. To determine the area sampled reliably, first
they are not considered reliable detection bands.
calculate for each band the Mean number of birds per
station by dividing the total number of birds by the
number of stations. Record these values in table 11.2.
11. Calculate and record in table 11.2 the Density of birds
for each band as:
density of birds in bandi (mean birds per station
for bandi) / (area m2 of bandi)

12. To determine the visual area that is most probably

sampled accurately, plot in figure 11.4 the Density of
birds for each band versus Band number.
13. Examine your graph in figure 11.4 and determine the
Density of birds

band at which bird density drops off sharply. This is

the inflection point. The concentric bands before the
inflection point represent the area with reliable bird
counts. They are the reliable detection bands. Indicate
the Reliable detection bands with a Yes or No in
table 11.2.
14. Determine the area of the circular plot with a radius
that includes all reliable detection bands by summing
the Area of band values for all columns marked as
Reliable detection bands. Record this value as Total
area (km2) of reliable detection bands.
1 2 3 4 5 6 7 8 9 15. Sum for all sampling stations the number of birds
Band number sighted or heard within each reliable detection band.
Figure 11.4 Divide this value by the number of stations and
Graph for density of birds versus band number for student-collected record it as Mean number of birds within all reliable
data. detection bands per station.

11–4

108 Exercise 11
 Table 11.2
Data and calculations for using the variable circular plot method to determine a bird
population density

Number of birds seen or heard in 15 minutes

Sampling Station Band 1 Band 2 Band 3 Band 4 Band 5 Band 6 Band 7 Band 8 Band 9
0–10 m > 10–20 m > 20–30 m > 30–40 m > 40–50 m > 50–60 m > 60–70 m > 70–80 m > 80–90 m

Total number of birds

Mean number of birds per
station
Density of birds for each
band
Reliable detection band
(Y/N)
Area of band 314 m2 942 m2 1571 m2 2199 m2 2827 m2 3456 m2 4084 m2 4713 m2 5340 m2
Total area (km2) of reliable detection Mean number of birds within all reliable detection Total bird population
2
bands _______ bands per station _______ density _______ km

16. Calculate and record in table 11.2 the Total bird 2. Watch your instructor operate a Sherman live trap.
population density by dividing the Mean number of 3. Consult with your instructor and locate a community
birds within all reliable detection bands per station by the for sampling with a grid of Sherman traps.
Total area of reliable detection bands. 4. Use a 100-m tape to lay out a grid to cover the
Questions 3 sampling site. The grid does not have to be a square.
Why shouldn’t the observation time be as long as possible It should cover all contiguous areas with consistent
to see as many birds as possible? ______________________ habitat. The lines of the grid should intersect at
________________________________________________ 10-m intervals.
5. Sketch the grid and number the intersections.
What weaknesses in variable circular plot procedures would A 50-m 50-m grid has 36 intersections.
you try to minimize? _______________________________
6. The traps should be set during late afternoon and
________________________________________________ checked the next morning. A grid of 36 traps set and
checked after each of two nights constitutes 72 trap
nights.
SAMPLING A POPULATION OF SMALL
MAMMALS 7. Place one trap at each intersection. For each trap:
a. Bait the trap with dry oats or with a small amount
Sherman live traps distributed in the field can effectively of peanut butter mixed with dry oats. Do not use
sample a population of small mammals. These traps are typi- peanut butter if experience has shown that ants
cally baited and distributed over a grid large enough to cover are a problem in the sampling area. Ants can kill
the study area. Traps are typically set to capture animals at a trapped animal.
night when they are most active (fig. 11.5). b. Set the trap with the door open and in the shade
of leaves or a rock if possible. Covering the top
Procedure 11.4 of the trap with leaves will help insulate the trap.
Use Sherman live trapping to determine the number of Your instructor may advise you to include cotton
species and their relative abundances in a small-mammal nestlets to help insulate a small mammal on a cold
community. night.
c. If a grid intersection lies on an incline, face the
1. Discuss with your instructor the ethical treatment
trap door uphill.
of animals and the safety risks involved in small-
mammal trapping. 8. Let the traps remain undisturbed overnight.

11–5

Sampling Animal Communities 109

 12. Close the bag, identify the animal, and record
the species in table 11.3. Also record in column
Number of Individuals one tally mark for each
trapped individual of that species. Consult with your
instructor about gathering weight and gender data.
13. Gently release the animal, return the trap to its grid
position, and close the door.
14. Be sure to check ALL the traps. Never let an animal
remained trapped for more than 12–15 h.
15. After checking all the traps and closing the doors,
leave them in position until you return that
afternoon to begin another trapping night.
16. For the next trapping night, return to the grid in the
afternoon. Follow steps 7–15.
17. Each morning for three days, gather and record your
data in table 11.3.
18. Calculate and record in table 11.3 the Cumulative
Figure 11.5 number of species for all dates and the Number of
A Sherman trap is easily set by pushing the spring-loaded door down species trapped for each date.
to hook against a latch plate. An animal enters and must walk across 19. For each species, sum the number caught for all
the plate to get the food inside. Its weight releases the plate and the
door swings up for live capture. nights and record that number in the column for
Total abundance of speciesi .
20. Record in the Species Rank column of table 11.3 the
names of the species in order from most abundant to
least abundant. Record next to the ranked species
names Total Abundance of Each Ranked Species. These
9. In the morning, check the traps for sprung doors. If are simply reordered values from a previous column.
the door is still open, close it. Don’t leave traps open
during the day. 21. Your instructor may ask you to graph the abundances
of each species as a rank abundance curve (see
10. If a trap door is sprung closed, gently detect if a small Procedure 12.3).
animal is inside.
Questions 4
11. If an animal is in the trap, open a pre-weighed dark Are the species equally abundant? How many species com-
cloth bag, put the mouth of the bag over the sprung prise 90% of the individuals? ________________________
door, and open the door facing up to coax the animal
to crawl out. ________________________________________________

Table 11.3
Data from trapping small mammals to determine the number of species in a small-mammal
population

Date: Date: Date:

Total Abundance Total Abundance
Number of Number of Number of of Speciesi for All of Each Ranked
Species List Individuals Individuals Individuals Dates Species Rank Species

Cumulative number Number of Number of Number of

of species for all dates species trapped species trapped species trapped
_______ _______ _______ _______

11–6

110 Exercise 11
Small mammals sometimes become “trap happy” and are cies. A common measure of similarity is Jacquard’s coef-
caught over and over again. Why would this be so com- ficient of community similarity, defined as:
mon? ___________________________________________
CSJ c / (s1 s2 c)
________________________________________________
where:
How would you minimize the bias that trap-happy mammals
CSJ Jacquard’s community similarity coefficient
introduce to your data? _____________________________
c the number of species common to both communities
________________________________________________
s1 number of species in community 1 but not in
What factors limit this procedure’s sensitivity to all mam-
community 2
mals present in the community? _____________________
s2 number of species in community 2 but not in
________________________________________________
community 1
What weaknesses in small-mammal trapping procedures
Jacquard’s coefficient uses presence/absence data. Abun-
would you try to minimize? _________________________
dance data isn’t needed.
________________________________________________
Procedure 11.5
Could small-mammal trapping be part of a mark-recapture
procedure? What would be the greatest concerns? _______ Calculate Jacquard’s coefficient of community similarity
and investigate minimum and maximum values.
________________________________________________
1. Examine the columns of Simulated Data for
Calculation of CSJ in table 11.4 for tree species in two
communities.
JACQUARD’S COEFFICIENT OF 2. Calculate and record in table 11.4 the number of tree
COMMUNITY SIMILARITY species unique to each community (s1 and s2), the
number of species common to both communities (c),
One powerful way to learn about a community is to com- and Jacquard’s coefficient (CSJ).
pare its species composition to that of another community. 3. Verify your calculations with your instructor.
Ecologists compare diversity, tree dominance, food chains,
4. Propose and record in table 11.4 two simulation data
species lists, and a myriad of other parameters to find pat-
sets—one that will result in the minimum possible
terns that tell us how a community is put together and how
coefficient of similarity, and one that will result in
it “ticks.” One group of metrics, called community similar-
the maximum coefficient.
ity indices, compares communities based on the number of
species in each community and the number of shared spe- 5. Calculate CSJ for both proposed data sets and record
them in table 11.4.

Table 11.4
Simulated data for practice calculations of CSJ and two blank forms for simulation of
communities with minimum similarities and with maximum similarities

Proposed Data to Calculate Minimum Proposed Data to Calculate Maximum Possible

Simulated Data for Calculation of CSJ Possible Similarity Similarity
Tree species in Tree species in
community 1 community 2 Community 1 Community 2 Community 1 Community 2
Species A Species G
Species B Species B
Species C Species C
Species D Species F
Species E

s1 s2 s1 s2 s1 s2
Number of species in common c Number of species in common c Number of species in common c
Similarity coefficient CSJ Minimum similarity coefficient CSJ Maximum similarity coefficient CSJ

11–7

Sampling Animal Communities 111

Questions 5 Procedure 11.6
What is the maximum possible value for Jacquard’s coef- Choose and conduct a data-collection technique (sweep
ficient of similarity? Minimum? ______________________ net, mammal trapping, etc.) to determine similarity
________________________________________________ between two communities.

Does the use of a single taxon such as trees, birds, or fish 1. Consult with your instructor and choose two
rather than all species bias your conclusions about the simi- communities to compare.
larity of two communities? If so, how? ________________ 2. Choose a taxon as your basis for comparison of the
communities.
________________________________________________
3. Choose a sampling technique and design your
What taxon would you suggest is the most indicative of a experiment.
community’s structure? Why? _______________________ 4. Record your data in a field notebook and transcribe
________________________________________________ the information into table 11.5.
5. Calculate the number of species in each community,
the number of species common to both communities,
and Jacquard’s coefficient of community similarity.

Table 11.5
Data for calculation of similarity between two communities

Raw Data for Calculation of CSJ

Species in Community 1 Species in Community 2

Number of species s1 Number of species s2

Species in common, c
Jacquard’s coefficient of community similarity CSJ

11–8

112 Exercise 11
Questions for Further Thought and Study

1. Each technique to sample animal populations and communities has inherent error and bias. How would you minimize
that error in most cases?

2. Which of the techniques presented in this lab exercise offers the most information about the population? Is the
amount of information a function of the procedure?

11–9

Sampling Animal Communities 113

exercise twelve

Species Diversity 12
Objectives
As you complete this lab exercise you will:
1. Calculate species richness, Shannon-Wiener
diversity, Hmax, evenness, and Simpson’s diversity
index.
2. Calculate and graph a rank-abundance curve.
3. Compare insect diversity in two contrasting ter-
restrial environments.
4. Compare fish diversity in two contrasting aquatic
environments.

S pecies diversity is among the most calculated and cited

of all ecological variables. No other measure provides
more insight into complex communities. Diversity reflects
stability, aesthetics, and health of ecosystems (fig. 12.1).
Ecologists define and calculate species diversity based on
two characteristics: (1) the number of species in the com-
munity, which ecologists call richness; and (2) the relative
abundance of species, called evenness.
In this lab exercise you will calculate a variety of diver-
sity indices and graph relative abundance to visualize the
diversity of organisms in a community or ecosystem. You’ll
apply diversity calculations to samples of insects collected
with a sweep net and to fish collected with a seine net.
Figure 12.1
Tropical forests are among the most diverse ecosystems in the world.
This upland forest in the Galápagos Islands lies on the equator and
SPECIES RICHNESS includes an unusual variety of ferns, trees, herbs, and epiphytes.

Richness is the easiest measure of diversity because it’s sim-

ply a count of the species present. Everyone agrees that the
most diverse communities have the most species. But, using
SPECIES EVENNESS
richness to measure diversity assumes that one can record all
Richness clearly relates to community diversity—a commu-
species in a community, even the rare ones. That’s difficult
nity with 80 species is obviously more diverse than one with
to do. Therefore, richness can be misleadingly correlated
20 species. But evenness is also meaningful. For example, the
with sample size—the greater the sample size, the more
data in table 12.1 contrast two forest communities with equal
likely rare species are found. Nevertheless, recording rich-
richness. Examine the Species and Counts columns. Both con-
ness is a good first step to measure community diversity.
tain five tree species. However, community b is more diverse
richness number of species s than community a because the relative abundance of species

12–1

115
 ln pi the natural logarithm of pi
Table 12.1
Example data for richness, evenness, s the number of species in the community
and diversity
To calculate H , determine the proportions (pi) of each spe-
Community a Community b cies in the community, then the ln of each pi. Then multi-
Species Count Species Count
ply each pi times ln pi and sum the results for all species from
species 1 to species s, where s the number of species in
1 21 1 5
the community. This sum is negative, so take the absolute
2 1 2 5 value to complete the calculations. Examine table 12.2 for
3 1 3 5 example calculations.
4 1 4 5 The minimum value of H is 0, which is the value for a
5 1 5 5
community with a single species. Values range to ∞, but 7
denotes an extremely rich community. Communities with a
Total abundance 25 Total abundance 25
Shannon-Wiener diversity of 1.7 or higher are considered
relatively diverse. The maximum value increases as species
richness and species evenness increase. H max is defined as:
H max
ln s

is more even for community b. In community b, all five spe-

cies are equally abundant, each comprising 20% of the tree AN INDEX OF SPECIES EVENNESS
community. In contrast, 84% of the trees in community a
belong to one species, whereas each of the remaining species An index of evenness (J ) can be derived from the Shannon-
is only 4% of the community. On a walk through the two Wiener index. This index of evenness ranges from 0–1, and
forests, you would certainly sense more diversity in commu- the value is most meaningful for comparisons between com-
nity b, despite equal species richness in the two forests. For munities rather than as a stand-alone measure. An index of
this reason, most indices of diversity combine information evenness reveals the extent that a community is dominated
of evenness (relative abundance) with richness. by only a few of its species (uneven) or has all of its species in
Questions 1 similar numbers (even). Evenness can be defined as:
In your experience, which is more common—a community
J H /H
dominated by a few species or a community with all species max

represented equally? _______________________________ where:

________________________________________________ H Shannon-Wiener diversity index

Species richness may sometimes correlate with sampling H max

ln s
effort. Why is this a problem for accurately assessing a com-
s the number of species in the community
munity? _________________________________________
________________________________________________
SIMPSON’S INDEX OF SPECIES
DIVERSITY
SHANNON-WIENER INDEX OF SPECIES Simpson’s index is another common measure of diversity.
DIVERSITY As in the Shannon-Wiener index, the calculations are
based on the proportion of each species in the total sample.
Ecologists have developed many species-diversity indices Simpson’s index weighs dominant species somewhat more
with components of species richness and of evenness. One than rare species in its measure of diversity than does the
of the most commonly used indices is the Shannon-Wiener Shannon-Wiener index. Simpson’s index tends to vary
index, which states: the higher the index value, the greater less between samples, and it ranges from 0 to ∞. Values
the diversity. are commonly a few points higher than calculations of the
s Shannon-Wiener index. Simpson’s index is defined as:
Hr 5 2 a pi ln pi
i51
D 1 / a (pi2)
where:
where:
H Shannon-Wiener diversity index
D Simpson’s index
pi the proportion of the ith species abundance of
speciesi / total abundance of all species pi the proportion of the ith species

12–2

116 Exercise 12
 Table 12.2
Example data for richness, evenness, and Shannon-Wiener diversity index (H )
calculations for two hypothetical forest communities
Different values of H for the two communities reflect different species evenness. H for community b, the community with higher species evenness,
is 1.610; H for community a is 0.662.

Community a Community b
Proportion Proportion
Species Count (pi) ln pi pi ln pi Species Count (pi) ln pi pi ln pi
1 21 0.84 0.174 0.146 1 5 0.20 1.609 0.322
2 1 .04 3.219 0.129 2 5 0.20 1.609 0.322
3 1 .04 3.219 0.129 3 5 0.20 1.609 0.322
4 1 .04 3.219 0.129 4 5 0.20 1.609 0.322
5 1 .04 3.219 0.129 5 5 0.20 1.609 0.322
Total Total
abundance 25 1.00 H 0.662 abundance 25 1.00 H 1.610

Question 2 nance by only a few species. Environmental stress usually

Could a community of 15 species have the same diversity as steepens the curve for a community.
measured by Shannon-Wiener or Simpson’s index as a com- Questions 3
munity with 30 species? Why or why not? _____________ Why would stress steepen a relative-abundance curve?
________________________________________________ ________________________________________________
________________________________________________ ________________________________________________
Rank-abundance curves are constructed with data from
RANK-ABUNDANCE CURVES sampling a community. How would more samples likely
change the curve? ________________________________
A rank-abundance curve (fig. 12.2) makes informa- ________________________________________________
tion about diversity, richness, and evenness accessible at
a glance. In this graph, the rank of each species is plotted
along the x axis. The most abundant species is ranked 1, the Procedure 12.1
second most abundant species is ranked 2, and so forth. The Calculate species richness, Shannon-Wiener diversity
abundance of each species is plotted on the log scale of the index, H max, and evenness from simulation data.
y axis. The shape of the curve reveals subtleties about com- 1. Examine the example data and calculations in
munity structure. High, flat curves indicate high diversity. table 12.2.
Low and steep curves reveal low diversity and greater domi-
2. Examine the raw data in table 12.3.
Table 12.3
Raw data set for calculation of richness, Shannon-Wiener diversity, and species evenness

Species Abundance Rank Proportion (pi) ln pi pi ln pi

A 40
B 28
C 254
D 14
E 225
F 150
G 75
H 110
I 101
J 95
Richness Shannon-Wiener index H max
Evenness

12–3

Species Diversity 117

 3.0

2.5
Log10 Abundance (number per square meter)

2.0

1.5

1.0

0.5

0
1 2 3 4 5 6 7 8 9
Species rank of polluted stream arthropods

4.0

3.5
Log10 Abundance (number per square meter)

3.0

2.5

2.0

1.5

1.0

0.5

0
1 2 3 4 5 6 7 8 9 10 11 12
Species rank of mountain stream arthropods

Figure 12.2
Two contrasting rank-abundance curves for (a) benthic (sediment dwelling) arthropods in a polluted, silt-bottom stream;
(b) benthic arthropods in a rocky-bottom, mountain stream.

12–4

118 Exercise 12
3. Calculate richness, Shannon-Wiener diversity, H max
, Procedure 12.3
and species evenness for the data in table 12.3. Calculate a rank-abundance curve from practice data.
4. Record your results in table 12.3 and verify your
1. Examine the raw data in table 12.4 and the ranking
results with your instructor.
of each species.
Procedure 12.2 2. Plot in figure 12.3 the log10 of the abundances versus
rank for each species.
Calculate Simpson from a raw data set.
Questions 5
1. Examine the raw data table 12.4. Do any steep parts of the curve indicate dominance by just
2. Indicate the rank of each species in the Rank a few species? ____________________________________
column. The most abundant species is ranked 1, etc.
________________________________________________
3. Calculate and record in table 12.4 the proportion
(pi) of the total for each species, then calculate the Some communities have many rare species. How would the
squared proportions of each species (pi2). curve be shaped for such a community? _______________
4. Sum the squared proportions. ________________________________________________
5. Calculate and record Simpson’s diversity for these
data. Verify your calculations with your instructor.
INSECT DIVERSITY AS MEASURED
Question 4
FROM SWEEP NET SAMPLES
Simpson’s index is 1.4 for community a in table 12.1. What
is it for community b? ______________________________
Measuring total diversity of a community would, in theory,
________________________________________________ require counting all species present. This is a monumental

Table 12.4
Data for calculating Simpson’s diversity index and for ranks used in producing
a rank-abundance curve

Species Abundance Rank Proportion (pi) pi2

A 50
B 5
C 342
D 7
E 798
F 5
G 503
H 100
I 200
J 327
K 11
L 350
M 1112
N 339
O 300
P 375
Q 42
Richness Total (pi2)
Simpson’s index _____

12–5

Species Diversity 119

 3.0

2.5

2.0
Log10 Abundance

1.5

1.0

0.5

0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Species rank

Figure 12.3
Rank-abundance curve for the simulation data in table 12.4.

task, especially if you include the myriad of microorgan- c. Replace the lid immediately and tightly. The
isms. In practice, ecologists usually sample a narrower fumes are lethal to insects.
taxon—such as trees, plants, vertebrates, or insects—either 3. Obtain an insect sweep net.
according to their interests or as a surrogate index of over- 4. To collect insects, “sweep” the net back and forth so
all community diversity. For a selected taxon to realistically the hoop brushes across the tops of the vegetation.
represent the entire community, the taxon should have Sweep the net 25 times, with each sweep passing
high inherent diversity. Certainly, the most diverse group across a new patch of vegetation. Be consistent in
of organisms is arthropods, especially insects (fig. 12.4). speed and movement.
Therefore, insect species are often used to reflect overall
5. The insects accumulated in the net represent one
community diversity.
sample from one “unit effort.” The unit effort
includes 25 sweeps (or the number of sweeps advised
Procedure 12.4
by your instructor).
Compare insect diversity in two contrasting
6. Concentrate the caught insects down to the bottom
environments.
of the net by shaking it, but don’t let them escape.
1. Consult with your instructor and choose two 7. Grab and constrict the net just above the
contrasting habitats to compare insect diversity. concentrated insects to prevent them from escaping.
Choose a site in each habitat to sample. 8. Open the killing jar and invert the end of the net
2. Prepare an insect killing jar. with the trapped insects into the jar and quickly
a. Pack 2 cm of cotton in the bottom of a large replace the lid. This takes practice. Watch your
(L 1 qt) jar with a tight-fitting lid. Pour a 2–3-cm instructor do this.
deep layer of plaster of Paris to cover the cotton 9. After 20 min, the insects should be dead. Open
completely. Let it harden. the jar and shake them out into another container
b. Activate the killing jar by pouring 10 ml of ethyl labeled for site and sample number.
acetate into the jar. It will penetrate the porous
10. Take at least three replicate samples from each site.
plaster of Paris and saturate the cotton.

12–6

120 Exercise 12
 Hymenoptera Ephemeroptera Odonata Diptera

Lepidoptera Coleoptera Dermaptera Hemiptera (plant bugs)

Thysanoptera Neuroptera Isoptera

Trichoptera Homoptera (leaf hopper) Orthoptera

Figure 12.4
The most common insect orders.

12–7

Species Diversity 121

11. Before classifying and counting the insects, You chose environments to sample based on some contrast-
familiarize yourself with representatives of each ing features. How might those features account for the dif-
Order. Use a practice collection provided by your ferences in diversity? _______________________________
instructor.
________________________________________________
12. For each sample, pour the specimens into a tray and
sort them into groups of similar specimens. Did all measures of diversity lead you to the same conclu-
13. Examine the specimens with a dissecting microscope, sions? ___________________________________________
and use the key in the Boxed Reading on page 123 to ________________________________________________
classify them.
Consider the variation among your sweep-net samples.
14. Decide with your instructor if you will classify
Would more samples likely lead you to different conclu-
specimens to Order only, or if you will try to
sions? Why? _____________________________________
group specimens into apparent species based on
morphological similarities. ________________________________________________
15. Count and record in table 12.5 the insects of each Ecologists sometimes express the number of organisms as
Order (or species). “catch-per-unit-effort” rather than absolute density. What
16. Calculate and record in table 12.5 richness, was the mean catch-per-unit-effort for your sweep-net sam-
Shannon-Wiener diversity, Hmax, evenness, and ples for each sampling site? _________________________
Simpson’s index for each of the two sampling sites.
________________________________________________
Questions 6
What was the most abundant order of insects in your
samples? ________________________________________ Procedure 12.5
________________________________________________ Compare fish diversity in two contrasting environments.

Were the differences in the two environments reflected in 1. Consult with your instructor and choose two
their insect diversities? _____________________________ contrasting aquatic sites to compare fish diversity.
They may be a stream pool versus riffle, two streams,
________________________________________________ or a stream and a lake shore.
Which sampling site was the most diverse? _____________ 2. Obtain a 12-ft (or longer) fish seine net.
________________________________________________

Table 12.5
Insect counts, classification, and diversity from sweep net samples of two
contrasting sites

Site ______________________ Site ______________________

Taxon Sample 1 Sample 2 Sample 3 Sample 4 Totals Taxon Sample 1 Sample 2 Sample 3 Sample 4 Totals

Shannon- Shannon-
Richness Wiener Hmax Evenness Simpson’s Richness Wiener Hmax Evenness Simpson’s

_______ _______ _______ _______ _______ _______ _______ _______ _______ _______

12–8

122 Exercise 12
 A Dichotomous Key to Common Orders of Insects
A common tool to identify organisms is a dichotomous key. 3. Forewings thick and leatherlike at base, tips much
Dichotomous keys list and describe pairs of opposing traits, thinner and may be transparent; mouthparts
each of which leads to another pair of traits until a level of clas- pointed and beaklike to puncture prey and suck
sification of the specimen being identified is reached. By using body fluids ........................................ (bugs) Hemiptera
a key, you’ll learn the characteristics that distinguish each of Forewings same texture throughout, biting mouthparts
the groups identified by the key. with opposing mandibles ............................................4
Insects are classified into 26 Orders distinguished mainly 4. Forewings leathery and with veins ........... (grasshoppers,
by the structure of wings, mouthparts, and antennae. The key crickets) Orthoptera
below will help you identify the Order of the insects collected Forewings hard, without veins .........(beetles) Coleoptera
in your sweep net samples. To use the key: 5. Wings of same length, antennae usually shorter than
1. Select a specimen and read the first pair of head ..............................................................................6
characteristics. Wings not of same length, antennae long or enlarged
2. Choose the one that best describes your specimen. toward end ...................................................................7
3. Proceed according to the number at the end of your 6. Large insects (usually > 3 cm), wings long, transparent
choice to the next pair of characteristics. and with many strong veins; abdomen long and
slender ...................................... (dragonflies) Odonata
DICHOTOMOUS KEY TO COMMON ORDERS
Smaller insects, wing venation faint, wings extending
OF INSECTS
posterior to the abdomen ..............(termites) Isoptera
1. Insects with 2 wings .....................................(flies) Diptera
7. Wings covered with fine, opaque scales; tubular,
Insects with 4 wings, a pair of forewings, and a pair of
coiled, sucking mouthparts................ (butterflies, moths)
hindwings .....................................................................2
Lepidoptera
2. Fore- and hindwings are not alike in texture and color
Wings thin, transparent, and not covered with scales;
One pair may be hard and dense while the other
mandibles well developed.............(ants, bees, wasps)
may be light and transparent .....................................3
Hymenoptera
Fore- and hindwings similar, usually clear, thin, and
transparent ...................................................................5

3. Your instructor will demonstrate how to effectively g. Handle the fish as little as possible. Return all fish
use a fish seine (see fig. 8.9). to the water alive.
4. Remember these tips for seining: 5. Have a team of students waiting to quickly count the
a. Sweep the seine net upstream rather than down- fish as soon as the net is brought to the shore.
stream. 6. Take two or three seine hauls (totaling > 200 fish)
b. Hold the seine poles so that you push the bottom for each sampling site.
end in front of you rather than walking backwards 7. Count and record in table 12.6 the number of each
and pulling the seine. type (species) of fish for each seine haul.
c. Be safe. Move quickly but do not lose control or
8. Sum the counts for each species across all seine hauls
overwhelm your seining partner.
and record as Totals in table 12.6.
d. While moving with your partner, separate the two
poles only about one-half to two-thirds the total 9. Calculate and record in table 12.6 richness,
length of the seine. For example, the poles of a Shannon-Wiener diversity, Hmax, evenness, and
12-ft seine should be keep 6–9 ft apart, no more. Simpson’s index for each of the two sampling sites.
e. Keep the lower, weighted edge of the seine against Questions 7
the sediment. As you move, bump the ends of the What was the most abundant species of fish in your
poles along the bottom of the sediment. This will samples? ________________________________________
keep the seine low in the water column. ________________________________________________
f. To finish a seine haul, sweep the net toward and
onto the shore rather than lifting the net out of Which sampling site was the most diverse? _____________
the water while you stand in the stream. ________________________________________________

12–9

Species Diversity 123

Did all measures of diversity lead you to the same conclu- Ecologists sometimes express the number of organisms as
sions? __________________________________________ “catch per-unit-effort” rather than as absolute density.
What was the mean catch per-unit-effort for your seine net
________________________________________________
samples for each sampling site? ______________________
Consider the variation among your seine net samples.
________________________________________________
Would more samples likely lead you to different conclu-
sions? Why? _____________________________________
________________________________________________

Table 12.6
Fish abundances, classification, and diversity from samples from two contrasting
aquatic sites

Site ________ Site ________

Number Number Number Number Number Number Number Number
Species in Seine 1 in Seine 2 in Seine 3 in Seine 4 Totals Species in Seine 1 in Seine 2 in Seine 3 in Seine 4 Totals

Shannon- Shannon-
Richness Wiener Hmax Evenness Simpson’s Richness Wiener Hmax Evenness Simpson’s

_______ _______ _______ _______ _______ _______ _______ _______ _______ _______

12–10

124 Exercise 12
Questions for Further Thought and Study

1. Which common diversity index would a good ecologist use to characterize a community? How do you justify your
answer?

2. How would you design artificial data sets to determine the theoretical maximum and minimum values for Simpson’s
index?

3. Re-examine the terrestrial habitat that had the highest insect diversity, and the aquatic habitat that had the highest
fish diversity. What characteristics of those habitats likely promote diversity?

12–11

Species Diversity 125

exercise thirteen

Primary Productivity
in an Aquatic Community
13
Objectives gross primary production respiration
net primary production
As you complete this lab exercise you will:
1. Collect lake water samples for analysis of dis- NPP is either consumed by herbivores or decomposed.
solved oxygen, chlorophyll content, and primary Question 1
production. Could gross primary production be less than or equal to net
2. Measure dissolved oxygen content using the Win- primary production? In other words, could all of GPP go
kler titration method. towards growth? Why or why not? ____________________
3. Extract chlorophyll from a lake water sample and
________________________________________________
measure its concentration.
4. Use the light bottle-dark bottle oxygen method to The first step to understand ecosystem energetics is
measure primary productivity in a lake. to measure photosynthesis by the community’s producers
(plants and autotrophic microorganisms). A review of the
summary equation for photosynthesis reveals that measur-
ing production can be based on the rate of CO2 uptake, the
C ommunities are alive. They grow, reproduce, and
respond to their environment, and it all requires energy.
This energy enters the ecosystem as sunlight captured by
increase in weight of tissue (synthesized organic molecules),
or the rate of oxygen liberation.
photosynthesis, which converts inorganic carbon (CO2) to Photosynthesis Summary Equation
organic sugars (C6H12O6). These organic molecules store
the energy of sunlight in their carbon bonds. Food chains light
process the organic molecules and pass the energy from one 6 CO2 12 H2O ¡ C6H12O6 6 H2O 6 O2
trophic level to the next (fig. 13.1). chlorophyll
The total organic material synthesized by autotrophs
and heterotrophs is called production (grams) or productiv- In this lab exercise you will develop techniques to mea-
ity (grams per unit time), and the organisms that do it are sure O2 production and assess primary productivity. You’ll
producers. Autotrophic organisms are primary producers also measure chlorophyll content as an index of productiv-
and use sunlight, water (H2O), and inorganic carbon (CO2) ity. Specifically, you will assess primary productivity by mea-
to synthesize organic molecules. Primary producers include suring (1) dissolved oxygen (DO) using Winkler titration;
plants and some microorganisms (primarily algae). Het- (2) chlorophyll concentrations by filtering lake water and
erotrophic organisms are consumers and they also produce extracting with acetone; and (3) dissolved oxygen changes
tissue that is available to the next trophic level. They eat due to respiration and photosynthesis in a sample of a fresh-
organic molecules and use their energy and building blocks water plankton community incubating in sunlight.
to synthesize their own tissue. This tissue is secondary pro- Questions 2
duction and consumers may also be considered secondary How does chlorophyll content generally relate to biomass of
producers. primary producers? ________________________________
The total energy fixed in organic molecules by auto-
________________________________________________
trophs is gross primary production (GPP). Part of GPP is
metabolized in respiration and part is used for production How does chlorophyll content relate to primary productivity?
of tissue (growth) available to the next trophic level (her- ________________________________________________
bivores). The portion of GPP available to herbivores is net
________________________________________________
primary production (NPP).

13–1

127
 Primary consumers – Primary producers –
Energy vascular plants
plant–eating insects

Primary producers –
microscopic plants and algae (phytoplankton)
Primary consumers –
microscopic animals
(zooplankton)

Decomposers
(bacteria and fungi) Secondary consumer

Tertiary consumer

Figure 13.1
Microscopic autotrophs (photosynthetic plants and algae) are primary producers. Their productivity (organic molecules with stored energy) serves
as food for primary, secondary, and tertiary consumers. As the food (production) passes through the trophic levels, some is lost to respiration. A
portion of the production from each trophic level moves to decomposers.

13–2

128 Exercise 13
MEASURING DISSOLVED OXYGEN

Dissolved oxygen (DO) in aquatic ecosystems comes from

diffusion of oxygen (O2) from the atmosphere and O2
released as a byproduct of photosynthesis. Tracking daily
changes in DO is a common technique to assess the health
and primary productivity of lakes and streams. DO levels are
particularly critical because aquatic life depends on oxygen,
and oxygen doesn’t dissolve well in water. Atmospheric oxy-
gen (O2) occurs as 200,000 parts per million (ppm) in air. In
contrast, oxygen dissolved in water is commonly 8–10 ppm.
This is low. DO of 3 ppm ( 3 mg L 1) generally stresses
aquatic organisms. Ecologists often monitor DO concentra-
tions because poor solubility in water means that acquiring
oxygen is limiting for aquatic organisms.
Question 3
An accidental discharge of organic sewage into a lake or
stream will cause the system’s dissolved oxygen concentra-
tion to plummet. How so? __________________________
________________________________________________
A common method to measure DO in a water sample
is the Winkler titration method. Although specialized elec-
tronic meters also can measure dissolved oxygen, the titra-
tion method is simple and accurate. Winkler titration is an Figure 13.2
This dissolved oxygen bottle will hold a water sample taken from
important part of procedures to assess primary productivity.
the surface of a lake. A Winkler titration procedure will measure the
dissolved oxygen concentration. (See fig. 4.5.)
Procedure 13.1
Practice the Winkler titration method for measuring
dissolved oxygen.
1. Examine a DO bottle (fig. 13.2). When it is full, Question 4
its shape and narrow neck minimizes the surface How did boiling affect the dissolved oxygen concentration
area exposed to air. Notice that the glass stopper is of the water sample? _______________________________
precisely ground to enclose exactly 300 mL.
________________________________________________
2. Fill a 1-L flask with cool tap water, and swirl it
vigorously to aerate the water.
3. Fill a DO bottle from the flask and stopper the bottle Procedure 13.2
so no bubbles are trapped. Record in table 13.1 the Practice collecting an undisturbed lake water sample for
DO bottle number. dissolved oxygen analysis.
4. Boil the remaining water in the flask for 5 min. This 1. Obtain a DO bottle and ground-glass stopper. Rinse
reduces the dissolved gas content. the inside of the DO bottle with some lake water.
5. Let the boiled water cool for 10–15 min and use it 2. Examine a van Dorn water sampler (see fig. 4.6) and
to fill a second DO bottle. Pour the water gently to its drain tube. Close the drain valve at the base of
minimize mixing with air. Record in table 13.1 the the flexible tube.
DO bottle number of the boiled sample. 3. Your instructor will show you how to CAREFULLY
6. Assemble the chemicals, ring stand, and burette set the spring-loaded suction cups, and how to trigger
provided by your instructor for a Winkler titration. the release mechanism with a weighted messenger
7. Follow the steps listed in the boxed insert “Winkler (fig. 13.3).
Titration: Chemistry and Procedure” in Exercise 4 to 4. Cock the van Dorn sampler. Slowly submerge the
determine the DO concentrations in the two water cocked van Dorn completely in lake water (or a sink
samples. The volume of each DO bottle allows three full of water if you are practicing). Lower the van
replicate 100-mL titrations. Dorn to the necessary depth (1 m for practice) as
8. Record your results in table 13.1. marked on the line.

13–3

Primary Productivity in an Aquatic Community 129

 Table 13.1
Titration volumes and dissolved oxygen concentrations for aerated and boiled
water samples

Aerated Water Sample DO Bottle Number _____ Boiled Water Sample DO Bottle Number _____
mL of titrant DO (mg L 1) mL of titrant DO (mg L 1)
Replicate 1 ________ ________ ________ ________
Replicate 2 ________ ________ ________ ________
Replicate 3 ________ ________ ________ ________
x ________ x ________

13. Empty the van Dorn sampler back into the lake.
Question 5
Why should the volume be overflowed three times while
filling a DO sample bottle? _________________________
________________________________________________

CHOLORPHYLL CONTENT AS A
MEASURE OF POTENTIAL PRIMARY
PRODUCTIVITY
Chlorophyll captures the energy of sunlight during pho-
tosynthesis, and chlorophyll content (mg L 1) is a useful
measure of phytoplankton biomass. In turn, phytoplank-
Figure 13.3 ton biomass is directly proportional to primary productiv-
This van Dorn water sampler is cocked and ready to be lowered to
ity in aquatic ecosystems. Although chlorophyll content of
the desired depth to capture a water sample. The drain tube will be
attached after the sampler is brought to the surface. (See Fig. 4.6.) the water is not a direct measure of the grams of carbon
fixed during photosynthesis, it is a reliable and proportional
indicator of primary productivity. Chlorophylls a and b
occur in plants, and a variety of slightly different chloro-
5. Move the van Dorn about 1 meter horizontally phylls also occur in algae. Chlorophyll a has the broadest
to displace any previously disturbed water in the occurrence in plants and algae. In the following procedure,
cylinder. phytoplankton is filtered from a water sample and its chlo-
6. Drop the messenger to close the cylinder and enclose rophyll is extracted with acetone. A spectrophotometer set
the water sample. to chlorophyll’s peak absorption wavelength measures the
7. Raise the cylinder out of the water and rest it chlorophyll content.
vertically on the edge of a solid surface so the drain
valve is at the lower end. Attach the drain tube. Procedure 13.3
8. Push the drain tube into the DO bottle so the end of Collect a lake water sample and measure its chlorophyll
the tube touches the bottom of the bottle. content.
9. Open the drain valve. If water doesn’t flow freely 1. Collect water samples from appropriate depths
into the bottle, lift the edge of the upper suction cup (surface, 2 m, 4 m, 8 m) using a van Dorn sampler
to break the seal and allow air flow. (see Procedure 13.2). Put the samples in labeled,
10. Allow the water to overflow until the volume of the opaque, clean 1-L bottles, and keep them cool until
bottle has been displaced three times. you are ready to filter the water.
11. As the water continues to flow, slowly pull the tube 2. Assemble a vacuum filtration apparatus (fig. 13.4)
out of the bottle. compatible with 47-mm diameter membrane filters.
12. Insert the ground-glass stopper into the bottle to seal 3. Place a 0.8-μm pore size membrane filter on the
the 300-mL volume with no bubbles. The sample is apparatus and moisten it slightly with a few drops of
now ready for Winkler titration. distilled water.

13–4

130 Exercise 13
 8. Fold the filter with the phytoplankton on the inside
and place it in a labeled 1-in diameter test tube
compatible with a spectrophotometer.
9. Add 20 mL of 90% alkalized acetone to the tube and
stopper the tube. Shake the tube until the membrane
filter dissolves.
10. If the acetone is not slightly green, filter another
liter of water and add this second filter to the tube.
Record the total volume of water filtered.
11. Repeat steps 3–10 for replicate samples from each
depth.
12. Store the tubes overnight in a dark refrigerator to
extract all the chlorophyll from the cells.
13. If the solutions are turbid with undissolved material,
centrifuge the solutions in labeled, stoppered tubes.
14. Review with your instructor how to use a spectro-
photometer. Calibrate the spectrophotometer with
an acetone blank.
15. Record each sample and replicate ID in table 13.2.
Measure and record in table 13.2 the absorbance of
each sample at 750 nm and 663 nm.
Figure 13.4 16. Calculate the chlorophyll a concentration in the
In this vacuum filtration apparatus, a small-pore filter has been
extract as:
inserted between the top funnel and bottom collecting flask. A
vacuum tube is attached and pulls air from the flask and draws water chlorophyll a (mg L 1
extract) (abs @ 663 nm)
through the filter to capture suspended phytoplankton.
(abs @ 750 nm) 7.5

(If the tube light path is 1 in., use 7.5. If the light path
4. For samples from each depth, shake the bottle to is 1 cm, use 13.4.)
mix any settled algae. Measure 1 L of water from the
collected lake sample with a graduated cylinder. 17. Calculate the chlorophyll a concentration in the
lake water sample as:
5. Start the vacuum suction and add water from the
lake water sample to the filtration receiving funnel. chlorophyll a (mg L 1 lake water) [chlorophyll a (mg
6. Filter 1 L of water. Continue suction for a few L 1 extract) acetone extract volume (mL)] /
seconds after the last few milliliters have passed filtrate volume (L)
through until the filter is damp-dried. If the
18. Your instructor may ask you to graph your results as
filter clogs before a full liter is filtered, you can
chlorophyll content per liter of lake water versus depth.
measure the volume of the filtrate and correct the
calculations later. Question 6
Does the chlorophyll content per liter of lake water vary
7. Use forceps to carefully remove the filter from the
with depth? How so? ______________________________
apparatus. Do not touch the filter’s upper surface
with your fingers. ________________________________________________

Table 13.2
Data for measurement of chlorophyll content of lake water samples

Absorbance Absorbance mg Chlorophyll a mg Chlorophyll a

Sample Number Depth mL Filtered @ 663 nm @ 750 nm L 1 Extract L 1 Lake Water

13–5

Primary Productivity in an Aquatic Community 131

MEASURING PRIMARY PRODUCTIVITY
BY THE LIGHT BOTTLE-DARK BOTTLE
OXYGEN METHOD

Primary productivity can be measured by measuring oxygen

release during photosynthesis versus oxygen uptake during
respiration. For aquatic environments, a common proce-
dure for this is the light bottle-dark bottle oxygen method.
This method compares changes in DO in lake water samples
exposed to light versus samples kept in the dark. Samples of
lake water and its phytoplankton are simultaneously incu-
bated in light bottles (clear glass) and dark bottles (opaque)
at various depths of the water column.
Both photosynthesis and respiration occur in a light
bottle. Only respiration occurs in a dark bottle. During
incubation, the initial concentration of dissolved oxygen
(DOinit) in the dark bottles decreases to DOdark due to res-
piration. The initial concentration of dissolved oxygen
(DOinit) in the light bottles increases to DOlight as the differ-
ence between photosynthetic O2 release and respiratory O2
uptake. This assumes that photosynthesis releases more O2
than respiration consumes. Figure 13.5
This light-dark pair of dissolved oxygen bottles is tied to an
• (DOinit DOdark) measures respiration per unit volume incubation line to suspend them in the lake. These and other pairs
during incubation. are tied along the line so the suspended string of bottles will have
a pair at each 1-meter depth interval. Productivity decreases as the
• (DOlight DOinit) measures net photosynthetic activity. deeper water decreases light penetration. (See fig. 4.7.)
• The sum of respiration plus net photosynthetic activity
[(DOinit DOdark) (DOlight DOinit)] measures the 6. Set aside one filled light bottle from each depth for
gross photosynthetic activity. DOinit measurement. Attach the other light bottle
and the dark bottle to its appropriate depth level
Procedure 13.4 on the line so it can later be returned to the water’s
Prepare light and dark bottles and measure original depth and incubated there.
photosynthesis and primary production. 7. Record in table 13.3 the identification numbers of
Assemble the incubation line. the bottles filled from each depth.
1. Discuss with your instructor where to locate a 8. Suspend the line and the attached pairs of bottles
sampling and incubation station in a nearby lake to from a flotation device so the line trails into the
adequately represent its productivity. Also discuss the water and holds each pair of light-dark bottles at the
number of depths for measuring primary production. appropriate depth.
One-meter depth intervals are common. 9. While in the field, fix the contents of the retained
2. Assemble three 300-mL DO bottles for each light bottles (DOinit) according to the STEPS FOR
sampling depth. Cover one of each triplet of bottles SAMPLE FIXATION in the boxed reading: Winkler
completely (light-tight) with tinfoil (fig. 13.5). Titration Chemistry and Procedure, Exercise 4.
3. Assemble some heavy line and a method to attach the 10. Incubate the samples in the lake for 24 h.
bottles to the line so they can be suspended at various Determine the initial concentration of dissolved oxygen.
depths. Two bottles will be suspended at each depth. 11. Return to the lab and titrate the retained light
Collect water samples and prepare samples for incubation. bottles (DOinit bottles for each depth) according to
4. Use a van Dorn sampler to sample water from each the STEPS FOR SAMPLE TITRATION in the
depth according to Procedure 13.2. boxed reading: Winkler Titration Chemistry and
5. For each depth, fill two light bottles and one dark Procedure.
bottle with water from that depth. Use a small square 12. Record the titration results for the three 100-mL
of tinfoil to cover the stopper and neck of the dark aliquots from each DOinit bottle in table 13.3 as mL
bottle. They must be absolutely light tight. of titrant.

13–6

132 Exercise 13
 Table 13.3
Data and calculations for measurement of primary productivity in lake water by the
light bottle-dark bottle oxygen method.

Depth Bottle ID Number mL Titrant for 100-mL Aliquot Dissolved Oxygen

1
Surface DOinit bottle _____ ____ mL ____ mL ____ mL x ____ ___ mg L DOinit
1
Surface DOdark bottle _____ ____ mL ____ mL ____ mL x ____ ___ mg L DOdark
1
Surface DOlight bottle _____ ____ mL ____ mL ____ mL x ____ ___ mg L DOlight

Net photosynthetic Gross photosynthetic Net primary Gross primary

Community respiration activity activity productivity productivity
______ mg O2 L 1 d 1 ______ mg O2 L 1 d 1
______ mg O2 L 1 d 1 ______ mg C L 1 d 1
______ mg C L 1 d 1

1
1m DOinit bottle _____ ____ mL ____ mL ____ mL x ____ ___ mg L DOinit
1
1m DOdark bottle _____ ____ mL ____ mL ____ mL x ____ ___ mg L DOdark
1
1m DOlight bottle _____ ____ mL ____ mL ____ mL x ____ ___ mg L DOlight
Net photosynthetic Gross photosynthetic Net primary Gross primary
Community respiration activity activity productivity productivity
______ mg O2 L 1 d 1 ______ mg O2 L 1 d 1
______ mg O2 L 1 d 1 ______ mg C L 1 d 1
______ mg C L 1 d 1

1
2m DOinit Bottle _____ ____ mL ____ mL ____ mL x ____ ___ mg L DOinit
1
2m DOdark Bottle _____ ____ mL ____ mL ____ mL x ____ ___ mg L DOdark
1
2m DOlight Bottle _____ ____ mL ____ mL ____ mL x ____ ___ mg L DOlight

Net photosynthetic Gross photosynthetic Net primary Gross primary

Community respiration activity activity productivity productivity
______ mg O2 L 1 d 1 ______ mg O2 L 1 d 1
______ mg O2 L 1 d 1 ______ mg C L 1 d 1
______ mg C L 1 d 1

1
3m DO bottleinit _____ ____ mL ____ mL ____ mL x ____ ___ mg L DOinit
1
3m DO bottledark _____ ____ mL ____ mL ____ mL x ____ ___ mg L DOdark
1
3m DO bottlelight _____ ____ mL ____ mL ____ mL x ____ ___ mg L DOlight

Net photosynthetic Gross photosynthetic Net primary Gross primary

Community respiration activity activity productivity productivity
______ mg O2 L 1 d 1 ______ mg O2 L 1 d 1
______ mg O2 L 1 d 1 ______ mg C L 1 d 1
______ mg C L 1 d 1

Stop the incubation and determine final light and dark Calculate photosynthetic activity, respiration,
dissolved oxygen concentrations. and primary production.
13. After 24 h incubation, retrieve the light and dark 16. Calculate and record in table 13.3 the mean (x)
bottles. While in the field, fix the contents of the milliliters of titrant per 100 mL for each bottle. This
retrieved bottles according to the STEPS FOR mean value equals the DO concentration in mg L 1.
SAMPLE FIXATION in the boxed reading: Winkler 17. Calculate and record community respiration per day
Titration Chemistry and Procedure. for each depth in table 13.3.
14. Return to the lab and titrate the samples according 1
community respiration (mg O2 L d 1) DOinit
to the STEPS FOR SAMPLE TITRATION in the
DOdark
boxed reading: Winkler Titration Chemistry and
Procedure. 18. Calculate and record net and gross photosynthetic
15. Record the titration results for the three 100-mL activity for each depth in table 13.3.
aliquots from each DOlight and DOdark bottle in table 1
net photosynthetic activity (mg O2 L d 1)
13.3 as mL of titrant.
DOlight DOinit

gross photosynthetic activity (mg O2 L 1 d 1)

community respiration net photosynthetic activity

13–7

Primary Productivity in an Aquatic Community 133

19. The release of 1 mg O2 during photosynthesis is Questions 7
equivalent to synthesis of approximately 0.375 mg of Does primary productivity differ among the depths
carbon in organic molecules. Calculate and record sampled? ________________________________________
net and gross primary production for each depth in
________________________________________________
table 13.3.
Does the depth profile of productivity parallel that of chlo-
net primary production (mg C L 1 d 1)
rophyll content? Why might they differ? _______________
net photosynthetic activity 0.375
1
________________________________________________
gross primary production (mg C L d 1)
gross photosynthetic activity 0.375

20. Your instructor may ask you to graph your results of

net and gross primary productivity versus depth.

13–8

134 Exercise 13
Questions for Further Thought and Study

1. Fish kills often occur in small, nutrient-rich ponds due to oxygen depletion. Why are the dead fish almost always
discovered in the morning?

2. How might suspended sediments (turbidity) impact primary productivity?

3. How does primary productivity relate to the number of trophic levels present in an ecosystem?

4. Chlorophyll content is a good indicator of potential primary productivity. However, some algal species compensate for
low light conditions by producing more chlorophyll. How would this influence our use of chlorophyll concentration as
a predictor of primary productivity?

13–9

Primary Productivity in an Aquatic Community 135

exercise fourteen

Competition 14
Objectives
As you complete this lab exercise you will:
1. Investigate the effects of intraspecific competition
on individual plant growth, population growth,
and age structure of an animal.
2. Experiment with inter- and intraspecific competi-
tion pressures on two species of plants.
3. Investigate the effects of plant allelopathic chemi-
cals on germination and success of potentially
competing plant species.

C harles Darwin realized that competition for limited

resources was central to survival, fitness, and evolution-
ary change. Competition is an interaction among individu-
als seeking a common resource that is scarce (fig. 14.1)—it
negatively impacts the competing organisms. The inten-
sity of competition for a resource depends on the amount
of the resource, the number of individuals competing, and
the needs of each individual for that resource. Remember
that sharing resources such as light, food, water, space, and
nutrients is not necessarily competition. There must be a
negative effect on the competitors’ success. This negative
effect usually reduces fitness (reproductive capacity). Com-
petition selects for individuals best adapted to growth and Figure 14.1
survival in their environment and for those with the great- Light is a valuable resource in a crowded forest. This community’s
dense growth of plants competes for light and shows a few tall trees
est reproductive success. This immediate effect of compe-
forming an upper canopy. A small stream below is almost completely
tition may be negative, but long-term competition often obscured and receives little light to promote algal growth.
leads to partitioning of resources, specialization, and greater
community diversity. In this lab exercise you will measure the effects of
Over many generations, natural selection usually competition on growth (1) among sunflower seedlings;
dampens the intensity of competition. Competition shifts (2) between radish and wheat plants; and (3) among beetles
and separates the overlapping niches of highly competitive growing with limited resources. You will also demonstrate
species. This separation lessens the negative effects of com- how plants can ease competition for resources by inhibit-
petition, and it often makes competition difficult to detect ing the growth of nearby competitors. Competition among
among natural populations. For this reason, we often use members of the same species is intraspecific competition,
laboratory populations to demonstrate the negative effects whereas competition among members of different species is
of competition, especially on growth. interspecific competition.

14–1

137
INTRASPECFIC PLANT COMPETITION
Procedure 14.1
Examine competition among sunflower seedlings.
1. Obtain 15 pots containing potting soil.
2. Follow your lab instructor’s directions to plant 4, 6,
12, 20, and 40 sunflower seeds with three replicate
pots of each treatment. Label each pot with the
number of seeds, the date, and your name (fig. 14.2).
3. Water the pots gently with a consistent amount of
water.
4. Place the pots randomly in trays in a greenhouse or
well-lit area so each pot has the same environmental
conditions of light, temperature, etc.
5. After one growth interval (week), remove excess
seedlings so the treatments will have 2, 4, 8, 16, and
32 sunflower seedlings.
6. Examine the pots after each of three 1-week intervals.
At each interval record general observations and
measurements of the parameters called for in
table 14.1.
7. After 4 weeks (or a time recommended by your
instructor) count and record in table 14.2 the
number of plants surviving. Then cut and remove the Figure 14.2
aboveground tissues of all plants in each pot and place Small pots of germinating seeds are replicate sample units for
them on a pre-weighed paper towel for each pot. competition experiments. These pots contain sunflower seedlings.
Each pot should be marked with a sample ID number.
8. Weigh the paper with plants, subtract the weight
of the paper, and record the Mean fresh weight of
aboveground biomass for the appropriate treatment in 4. Fresh weight (g seed⫺1) with standard error bars vs.
table 14.2. Express your results as grams of tissue per days of growth
pot and as grams of tissue per seed. Conclusions: _____________________________
9. If you have worked in groups, follow your instructor’s ________________________________________
directions to either combine and record in table 14.2 Questions 1
the mean and standard error of data from all groups Was competition greater in the more-crowded pots?
or from your group only.
________________________________________________
10. Prepare the following four graphs and draw con-
clusions. Each graph will have five curves, one for ________________________________________________
each density of competitors. For each graph draw
Which parameters showed the effects of competition?
conclusions about how intraspecific competition
affected the variable. ________________________________________________
1. Mean height with standard error bars vs. days of
________________________________________________
growth
Conclusions: _____________________________ What other characteristics of competing plants might you
measure for an extended experiment? _________________
________________________________________
2. Mean leaf width with standard error bars vs. days ________________________________________________
of growth
Did competition more noticeably affect the number of indi-
Conclusions: _____________________________
viduals or the biomass of each individual? _____________
________________________________________
________________________________________________
3. Fresh weight (g pot⫺1) with standard error bars vs.
days of growth What kind of environments would likely intensify competi-
Conclusions: _____________________________ tion among sunflowers? ____________________________
________________________________________ ________________________________________________

14–2

138 Exercise 14
 Table 14.1
Effects of competition on sunflower seedlings

Treatment (plants per pot)

2 4 8 16 32
Rep Rep Rep Rep Rep Rep Rep Rep Rep Rep Rep Rep Rep Rep Rep
1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
First interval General
Total growth observations
days _____
Second interval General
Total growth observations
days _____
Third interval General
Total growth observations
days _____

First interval Mean height of

individuals (cm)
Second interval Mean height of
individuals (cm)
Third interval Mean height of
individuals (cm)

First interval Range of height

of individuals
(cm)
Second interval Range of height
of individuals
(cm)
Third interval Range of height
of individuals
(cm)

First interval Mean width

10 widest leaves
(cm)
Second interval Mean width
10 widest leaves
(cm)
Third interval Mean width
10 widest leaves
(cm)

Would you expect different results if different potting soil What competitive effect of plant density was not tested
was used? Why? __________________________________ because you over-planted and then thinned the seedlings to
a precise treatment number? ________________________
________________________________________________
________________________________________________
What information might the range of heights provide
that the mean height does not provide (review Exercises 1
and 2)? _________________________________________ INTRASPECIFIC ANIMAL COMPETITION
________________________________________________
Intraspecific competition reduces growth and fitness, and is
Are general observations valuable to your experiment even best studied in species with short life cycles. Flour beetles
if they are not quantified? How so? ___________________ (Tribolium spp.) (fig. 14.3) are good for laboratory studies
________________________________________________ of competition because they culture easily and vary in com-
petitive abilities. Be sure to review the egg-larva-pupa-adult
life cycle of beetles.
14–3

Competition 139
 Table 14.2
Data summary of the effects of competition on sunflower seedlings harvested after four
1-week growth intervals

Total Days Treatment (plants per pot)

of Growth
2 4 8 16 32
Rep 1 Rep 2 Rep 3 Rep 1 Rep 2 Rep 3 Rep 1 Rep 2 Rep 3 Rep 1 Rep 2 Rep 3 Rep 1 Rep 2 Rep 3
General
observations
Number
plants
Mean Mean Mean Mean Mean
surviving at
harvest Std. err. Std. err. Std. err. Std. err. Std. err.
Class mean Class mean Class mean Class mean Class mean

Mean
height of
Mean Mean Mean Mean Mean
individuals
Std. err. Std. err. Std. err. Std. err. Std. err.
Class mean Class mean Class mean Class mean Class mean

Mean width
10 widest
Mean Mean _ Mean Mean Mean
leaves
Std. err. Std. err. Std. err. Std. err. Std. err.
Class mean Class mean Class mean Class mean Class mean

Number
plants
Mean Mean Mean Mean Mean
surviving at
harvest Std. err. Std. err. Std. err. Std. err. Std. err.
Class mean Class mean Class mean Class mean Class mean

Mean fresh
weight of
Mean Mean Mean Mean Mean
aboveground
biomass Std. err. Std. err. Std. err. Std. err. Std. err.
(g pot 1)
Class mean Class mean Class mean Class mean Class mean

Mean fresh
weight of
Mean Mean Mean Mean Mean
aboveground
biomass Std. err. Std. err. Std. err. Std. err. Std. err.
(g seed 1)
Class mean Class mean Class mean Class mean Class mean

Procedure 14.2 4. Using a fine-mesh screen, separate, count, and record

Examine intraspecific competition among Tribolium. in table 14.3 the number or weight for the total
individuals of each life stage. Your instructor will direct
1. For each lab group, obtain two pairs (male-female) of you to measure either number or weight, or both.
Tribolium confusum, and a 1-L jar containing 50 g of
5. Reintroduce all counted eggs, larvae, pupae, and
well-sifted, enriched whole-wheat flour mixed with
adults to a jar of fresh culture media.
1 g of dried yeast.
6. Repeat steps 4 and 5 at 2-week intervals for
2. Put the two pairs of beetles in the jar of culture
2 months. Record your counts in table 14.3.
media and cover the jar with a loose, fine-mesh
cover. 7. At the conclusion of your experiment, your
instructor will direct you to graph your data or the
3. Incubate the culture for two weeks at 25–30°C.

14–4

140 Exercise 14
 Did you see any signs of cannibalism? Signs that cannibal-
ism could be a selective force? How so? ________________
________________________________________________
Did competition for limited resources affect the age struc-
ture of the population? How so? ______________________
________________________________________________
Could intraspecific competition be intense enough to elimi-
nate the species? How so? __________________________
________________________________________________

INTRA- AND INTERPSECIFIC PLANT

COMPETITION
Procedure 14.3
Examine intraspecific and interspecific competition
between plant species.
1. Obtain a supply of radish seeds, wheat seeds, plant
labels, and Jiffy® pots. Jiffy pots are dehydrated bags
of potting soil commonly sold at garden centers
Figure 14.3
(fig. 14.4).
Flour beetles (Tribolium sp.) are ideal for experimentation, and they grow
well on wheat flour with a minimum of water. Beetles have a distinctive 2. Rehydrate 36 pots by soaking them in water in a
life cycle including larva (top), pupa (middle), and adult (bottom). large beaker or sink.
3. In separate pots, plant 2, 10, 20, and 40 radish seeds.
combined data from all groups with time on the x axis
Make two more replicate treatment sets for a total of
and number of individuals on the y axis.
12 pots. Label the pots with IDs as listed in table 14.4.
4. In separate pots, plant 2, 10, 20, and 40 wheat seeds.
Questions 2 Make two more replicate treatment sets for a total of
Did high densities of Tribolium confusum have competitive 12 pots. Label the pots as listed in table 14.5.
consequences? What was your evidence? ______________
5. In separate pots, plant 1 wheat 1 radish seed,
________________________________________________ 5 wheat 5 radish seeds, 10 wheat 10 radish
seeds, and 20 wheat 20 radish seeds. Make two
In what ways other than eating food could T. confusum
more replicate treatment sets for a total of 12 pots.
interact competitively? ____________________________
Label the pots as listed in table 14.6.
________________________________________________

Table 14.3
Effects of intraspecific competition on life stages of the flour beetle TRIBOLIUM CONFUSUM

Number or weight of individuals in each life stage

Eggs Larvae Pupae Adults Total Individuals
Days of population Group: Group: Group: Group: Group:
growth: Class: Class: Class: Class: Class:
Days of population Group: Group: Group: Group: Group:
growth: Class: Class: Class: Class: Class:
Days of population Group: Group: Group: Group: Group:
growth: Class: Class: Class: Class: Class:
Days of population Group: Group: Group: Group: Group:
growth: Class: Class: Class: Class: Class:
Days of population Group: Group: Group: Group: Group:
growth: Class: Class: Class: Class: Class:
Days of population Group: Group: Group: Group: Group:
growth: Class: Class: Class: Class: Class:

14–5

Competition 141
 6. Place all 36 pots in trays. Put the pots in random
positions. Speak to your instructor about how to
randomize the pot positions.
7. Allow the seeds to germinate and grow for 10–14
days in a greenhouse.
8. After 10–14 days, count and record in tables 14.4,
14.5, and 14.6 the success of germination (number of
plants in each pot).
9. For each pot, harvest the plants and gently shake the
soil loose from the roots. Then submerge the roots
of the plants in a beaker of water and gently massage
away any remaining soil.
10. Blot the plants dry on a paper towel, and place all
of the plants from the pot on a pre-dried and pre-
weighed paper towel.
11. Weigh the paper with plants, subtract the weight of
the paper, and record the net weight (fresh biomass
as grams of fresh weight) of the plants in the tables
for the appropriate treatments.
12. Air dry the plants and paper towel for 24 h. Reweigh
them, and record the dried biomass as grams of air-
dried plant in the appropriate tables.
Figure 14.4 13. Calculate and record in tables 14.4, 14.5, and 14.6
Dry Jiffy pots and pots with radish and wheat seedlings. the means of the germination numbers and the fresh
and dried weights of each set of replicates.
14. Your instructor may direct you to combine the data
from all groups.

Table 14.4
Germination rates and biomass production by competing radish seedlings

Germination Fresh Biomass Dried Biomass

Treatment Pot ID (number of viable plants) (g fresh wt) (g air-dry wt)
2 radish seeds
Rep 1 Rad2-1
Rep 2 Rad2-2
Rep 3 Rad2-3
Mean Mean Mean
10 radish seeds
Rep 1 Rad10-1
Rep 2 Rad10-2
Rep 3 Rad10-3
Mean Mean Mean
20 radish seeds
Rep 1 Rad20-1
Rep 2 Rad20-2
Rep 3 Rad20-3
Mean Mean Mean
40 radish seeds
Rep 1 Rad40-1
Rep 2 Rad40-2
Rep 3 Rad40-3
Mean Mean Mean

14–6

142 Exercise 14
 Table 14.5
Germination rates and biomass production by competing wheat seedlings

Germination Fresh Biomass Dried Biomass

Treatment Pot ID (number of viable plants) (g fresh wt) (g air-dry wt)
2 wheat seeds
Rep 1 Whe2-1
Rep 2 Whe2-2
Rep 3 Whe2-3
Mean Mean Mean
10 wheat seeds
Rep 1 Whe10-1
Rep 2 Whe10-2
Rep 3 Whe10-3
Mean Mean Mean
20 wheat seeds
Rep 1 Whe20-1
Rep 2 Whe20-2
Rep 3 Whe20-3
Mean Mean Mean
40 wheat seeds
Rep 1 Whe40-1
Rep 2 Whe40-2
Rep 3 Whe40-3
Mean Mean Mean

Table 14.6
Germination rates and biomass production by competing radish and wheat seedlings

Germination Fresh Biomass Dried Biomass

Treatment Pot ID (number of viable plants) (g fresh wt) (g air-dry wt)
1 radish seed 1 wheat seed
Rep 1 RadWhe2-1
Rep 2 RadWhe2-2
Rep 3 RadWhe2-3
Mean Mean Mean
5 radish seeds 5 wheat seeds
Rep 1 RadWhe10-1
Rep 2 RadWhe10-2
Rep 3 RadWhe10-3
Mean Mean Mean
10 radish seeds 10 wheat seeds
Rep 1 RadWhe20-1
Rep 2 RadWhe20-2
Rep 3 RadWhe20-3
Mean Mean Mean
20 radish seeds 20 wheat seeds
Rep 1 RadWhe40-1
Rep 2 RadWhe40-2
Rep 3 RadWhe40-3
Mean Mean Mean

14–7

Competition 143
Questions 3 compounds inhibit germination, growth, or reproduction of
For radish seedlings, which are more significant competi- potential competitors (fig. 14.5).
tors—wheat seedlings or other radishes? What is your evi- Questions 4
dence? __________________________________________ What are adaptive advantages of producing allelopathic
________________________________________________ compounds? _____________________________________

For wheat seedlings, which are more significant competi- ________________________________________________

tors—radish seedlings or other wheat? What is your evi- What are possible disadvantages of producing allelopathic
dence? __________________________________________ compounds? _____________________________________
________________________________________________ ________________________________________________
Did interspecific competition reduce the biomass per indi-
vidual wheat competitor? Radish competitor? __________ Procedure 14.4
Demonstrate allelopathy.
________________________________________________
1. Determine with your instructor the overall
Did intraspecific competition reduce the biomass per indi- experimental design for the class. Determine how
vidual wheat competitor? Radish competitor? ___________ many plant extracts your group will test. This
________________________________________________ procedure assumes three replicates. Copy table 14.7
for each extract being tested.
Do your results allow conclusions about fitness of the com-
2. Obtain tissue (stems and leaves) from the variety of
petitors? Why or why not? __________________________
plants provided by your instructor. Some of these
________________________________________________ plants may produce allelopathic compounds.
Did competition more noticeably affect the number of indi- 3. For each plant, homogenize 10 g of tissue with
viduals or the biomass of each individual? _____________ 100 mL of water in a blender or mortar and pestle.
Let the slurry soak for 5–10 min to leach chemicals
________________________________________________ from the disrupted tissue.
What is your conclusion about the relative intensity of
interspecific versus intraspecific competition for radishes
and for wheat? ___________________________________
________________________________________________
Would you expect different results if different potting soil
was used? Why? ___________________________________
________________________________________________
How would you design an experiment to test if radish and
wheat compete for space versus nutrients? _____________
________________________________________________

Allelopathy
The “struggle for existence” at the heart of Darwin’s model
of evolution conjures visions of violent battles among ani-
mals vying for scarce resources. But more subtle forms of
“combat” are common in animals and plants. One such
mechanism of competition is allelopathy. Some plants pro-
duce chemicals that inhibit the growth of nearby plants.
Allelopathy is the inhibition of a plant’s germination or
growth by exposure to compounds produced by another
Figure 14.5
plant. Allelopathic compounds can be airborne or leach
Comparison of germination success versus failure of seedlings in an
from various plant parts into the soil. Rainfall, runoff, and extract with allelopathic chemicals. The lettuce seeds on the right are
diffusion distribute inhibitory compounds in the immediate on a paper towel soaked with an allelopathic chemical. Seeds on the
area of the producing plant. In the nearby area, allelopathic left are controls.

14–8

144 Exercise 14
4. Filter or strain the slurry to remove large particulates. 8. After 72 h (or the time specified by your instructor),
Collect the filtrate in a beaker. measure the length of 10 radicles randomly
5. For each plant extract: subsampled from each dish. Record the lengths in
a. Obtain six petri dishes (three treatment, three table 14.7.
control) and line the bottoms with circular pieces 9. Compare each control mean with the appropriate
of filter paper. treatment mean to determine if the extract
b. Label each petri dish with the plant extract name significantly retarded, enhanced, or had no effect
and replicate ID number for that dish. on germination or growth.
c. Record in table 14.7 the plant extract name and Questions 5
replicate IDs. Was allelopathy apparent from the tested plant species?
d. Saturate the filter paper in three of the dishes
with 5 mL of the extract. Saturate the filter paper ________________________________________________
of three dishes with the same amount of water ________________________________________________
from the same source used to prepare the extract.
e. Obtain seeds of radish, lettuce, or oat. Distribute Which plant species has the most intense allelopathy?
50 seeds uniformly on the filter paper in each ________________________________________________
dish.
________________________________________________
6. Your instructor may extend the experimental design
by asking you to set up treatments of multiple Why was water used as a comparable treatment? ________
extracts and to test the effects on different kinds of
________________________________________________
seeds. Follow their directions.
7. Incubate the covered dishes at room temperature How would you detect allelopathy in the field? _________
in the laboratory or in a greenhouse. After 24 and ________________________________________________
48 h, count and record in table 14.7 the number
of germinated seeds and calculate the percent
germination for each replicate and control dish.

Table 14.7
Data for germination and radicle growth by seeds exposed to potentially allelopathic
plant extracts

Plant extract _______

24-h Germination 48-h Germination 72-h Radicle Lengths
Petri dish ID (number of germinated seeds) (number of germinated seeds) (mm)
Rep1-____ ____ ____ ____ ____ ____

____ ____ ____ ____ ____

Rep2-____ ____ ____ ____ ____ ____

____ ____ ____ ____ ____

Rep3-____ ____ ____ ____ ____ ____

____ ____ ____ ____ ____

Treatment mean Treatment mean Treatment mean
Rep1-water ____ ____ ____ ____ ____

____ ____ ____ ____ ____

Rep2-water ____ ____ ____ ____ ____

____ ____ ____ ____ ____

Rep3-water ____ ____ ____ ____ ____

____ ____ ____ ____ ____

Control mean Control mean Control mean
Control treatment mean Control treatment mean Control treatment mean

14–9

Competition 145
AN INVESTIGATION: COMPARE 3. Review Exercises 1 and 2, and form a testable
ALLELOPATHIC CHEMICAL hypothesis about the comparison by your experiment.
PRODUCTION IN ROOTS, STEMS Write your hypothesis here: ____________________
AND LEAVES ___________________________________________
4. Describe your experimental design here: __________
Not all organs (i.e., roots, stems, leaves) of allelopathic
plants produce equal amounts of allelopathic chemicals. ___________________________________________
___________________________________________
Procedure 14.5
5. Do your experiment.
Compare allelopathy from various plant tissues.
Questions 6
1. Use Procedure 14.4 to document the allelopathic Do you accept or reject your hypothesis? ______________
chemical production by a readily available plant
species of your choice. ________________________________________________
2. Design an experiment to compare allelopathic What do you conclude about variation in allelopathic
chemical production by roots, stems, leaves, and chemical production in different plant organs? __________
flowers of the selected plant.
________________________________________________

14–10

146 Exercise 14
Questions for Further Thought and Study

1. How does competition influence natural selection? Is the presence of competitors a selective force?

2. What characteristics indicate that a community has been undisturbed for a few years? Is there a link between
disturbance and the outcome of competition between two species?

3. Why would we expect natural selection to dampen the intensity of competition over many generations?

4. Would you expect inter- or intraspecific competition to be the most intense? Why?

5. Would plants and animals compete for the same resources? How so?

14–11

Competition 147
exercise fifteen

Natural Selection 15
Objectives
As you complete this lab exercise you will:
1. Examine working definitions of evolution, fitness,
selection pressure, and natural selection.
2. Determine the genotypic and phenotypic frequen-
cies within a population and apply the terms allele,
dominant, recessive, homozygous, and heterozygous.
3. Use the Hardy-Weinberg Principle to dem-
onstrate negative selection pressures on a
population.

S pecies and their environments change with time—with-

out a doubt. To ecologists, the most profound changes
are genetic. The theory of evolution broadly describes
genetic change in populations. Many mechanisms can
change the genetic makeup of populations, and our under-
standing of the relative importance of each mechanism is
Figure 15.1
constantly being refined. Nevertheless, genetic change and,
Darwin greets his “monkey ancestor.” In his time, Darwin was
therefore, evolution, are universally accepted by ecologists. often portrayed unsympathetically, as in this drawing from an 1874
Events such as mutations (changes in the genetic message publication.
of a cell) and catastrophes (e.g., meteor showers, ice ages) © Mary Evans Picture Library/Photo Researchers, Inc.
all lead to some degree of genetic change. However, all
modern evidence points to natural selection as the major Review in your textbook the theories of evolution and the
force behind genetic change and evolution. mechanism of natural selection.
Charles Darwin first described the mechanics of natu- Natural selection in living populations over many gen-
ral selection (fig. 15.1). Darwin postulated that organisms erations is difficult to demonstrate in the lab. Therefore, in
that survive and reproduce successfully in a competitive this exercise you will simulate reproducing populations with
environment must have traits better adapted for their envi- nonliving, colored beads representing organisms and their
ronment than those of their competitors. In other words, gametes. This artificial population quickly reveals genetic
adaptive traits increase organisms’ fitness, and these traits change over many generations. Before you begin, review
are passed more frequently to the next generation. If traits the terms gene, allele, dominant alleles, recessive alleles,
of the most fit individuals are transmitted to the next gen- homozygous, and heterozygous.
eration through increased reproduction, then the frequency You will begin your experiments with a “stock popula-
of these traits will, after many generations, increase in the tion” of organisms consisting of a container of beads. Each
population. Subsequently, the population and its charac- bead represents a haploid gamete (having one set of chro-
teristics will gradually change. Darwin called this overall mosomes). Its color represents the allele it is carrying. An
process natural selection and proposed it as a major force organism from this population is diploid (has two sets of
guiding genetic change and the formation of new species. chromosomes per nucleus) and is represented by two beads.

15–1

149
UNDERSTANDING ALLELIC AND 6. Calculate the total number of individuals and the
GENOTYPIC FREQUENCIES total number of alleles in your newly established
parental population. Use this information to
Frequency refers to the proportion of alleles, genotypes, or calculate and record in table 15.1 the correct
phenotypes of a certain type relative to the total number genotypic frequencies for your parental population.
considered. Frequency is a decimal proportion of the total 7. Complete table 15.1 with the number and frequency
alleles or genotypes in a population. For example, if 1/4 of of each of the two alleles.
the individuals of a population are genotype Bb, the geno- Questions 1
typic frequency of Bb is 0.25. If 3/4 of all alleles in a popula- How many of the total beads are colored? ______________
tion are B, then the frequency of B is 0.75. Remember, by
definition the frequencies of all possible alleles or genotypes How many are white? ______________________________
or phenotypes will always total 1.0. What color of fur do Bb individuals have? _____________
In the following procedures you will simulate evolu-
tionary changes in allelic and genotypic frequencies in an ________________________________________________
artificial population. How many beads represent the population of 100
• The trait is fur color. organisms? _______________________________________

• A colored bead is a gamete with a dominant allele ________________________________________________

(complete dominance) for black fur (B)
• A white bead is a gamete with a recessive allele for THE HARDY-WEINBERG PRINCIPLE
white fur (b).
• An individual is represented by two gametes (beads). The Hardy-Weinberg Principle enables us to calculate and
predict allelic and genotypic frequencies. We can compare
• Individuals with genotypes BB and Bb have black fur these predictions with actual changes that we observe in
and those with bb have white fur. natural populations and learn about factors that influence
gene frequencies.
Procedure 15.1 This predictive model includes two simple equations
Establish a parental population. first described for stable populations by G. H. Hardy and
1. Obtain a “stock population” of organisms consisting W. Weinberg. Hardy-Weinberg equations (1) predict allelic
of a container of colored and white beads. and genotypic frequencies based on data for only one or two
frequencies; and (2) establish theoretical gene frequencies
2. Obtain an empty container marked “Parental
that we can compare to frequencies from natural popula-
Population.”
tions. For example, if we know the frequencies of B and BB,
3. From the stock population select nine homozygous we can use the Hardy-Weinberg equations to calculate the
dominant individuals (BB) and place them in the frequencies of b, Bb, and bb. Then we can compare these
container marked “Parental Population.” Each frequencies with those of a natural population that we
individual is represented by two colored beads. might be studying. If we find variation from our predictions,
4. From the stock population select 42 heterozygous we can study the reasons for this genetic change.
individuals (Bb) and put them in the container For the Hardy-Weinberg equations, the frequency of the
marked “Parental Population.” Each individual is dominant allele of a pair is represented by the letter p, and
represented by a colored and a white bead. that of the recessive allele by the letter q. Also, the geno-
5. From the stock population select 49 homozygous typic frequencies of BB (homozygous dominant), Bb (het-
recessive individuals (bb) and put them in the erozygous), and bb (homozygous recessive) are represented
container marked “Parental Population.” Each by p2, 2 pq, and q2, respectively. Examine the frequencies in
individual is represented by two white beads.

Table 15.1
Frequencies of genotypes and alleles of the parental population

Genotypes Frequency Alleles Frequency Phenotype Frequency

BB ●● B● Black fur
Bb ●❍ b❍ White fur
bb ❍❍

15–2

150 Exercise 15
table 15.1 and verify calculations of the Hardy-Weinberg Procedure 15.2
equations: Verify the Hardy-Weinberg Principle.
p⫹q⫽1
1. Examine figure 15.2 for an overview of the steps of
2 2
p ⫹ 2 pq ⫹ q ⫽ 1 this procedure.
2. Establish the parental population described in
The Hardy-Weinberg Principle and its equations predict that fre-
Procedure 15.1 (fig. 15.2a, 15.2b).
quencies of alleles and genotypes remain constant from generation
to generation in stable populations. Therefore, these equations 3. Simulate random mating of individuals by mixing
can be used to predict genetic frequencies through time. the population (fig. 15.2c).
However, the Hardy-Weinberg prediction assumes that: 4. Reach into the parental container (without looking)
and randomly select two gametes. Determine their
• The population is large enough to overcome random
genotype (fig. 15.2d).
events.
5. Record the occurrence of the offspring’s genotype in
• Choice of mates is random. figure 15.2e as a mark under the heading “Number,”
or temporarily on a second sheet of paper and return
• Mutations do not occur.
the beads to the container.
• Individuals do not migrate into or out of the population. 6. Repeat steps 4 and 5 (100 times) to simulate the
• Natural or artificial selection pressures are not acting on production of 100 offspring.
the population. 7. Calculate the frequency of each genotype and allele,
Questions 2 and record the frequencies in figure 15.2e. Beside
Consider the Hardy-Weinberg equations. If the frequency each of these new-generation frequencies write (in
of a recessive allele is 0.3, what is the frequency of the domi- parentheses) the original frequency of that specific
nant allele? ______________________________________ genotype or allele from table 15.1.
Questions 3
________________________________________________
The Hardy-Weinberg Principle predicts that genotypic fre-
If the frequency of the homozygous dominant genotype is quencies of offspring will be the same as those of the paren-
0.49, what is the frequency of the dominant allele? ______ tal generation. Were they the same in your simulation?
________________________________________________ ________________________________________________
If the frequency of the homozygous dominant genotype is ________________________________________________
0.49, what is the frequency of the homozygous recessive
If the frequencies were different, then one of the assump-
genotype? _______________________________________
tions of the Hardy-Weinberg Principle was probably vio-
________________________________________________ lated. Which one? ________________________________
Which Hardy-Weinberg equation relates the frequencies of ________________________________________________
the alleles at a particular gene locus? __________________
________________________________________________
EFFECT OF A SELECTION PRESSURE
Which Hardy-Weinberg equation relates the frequencies of
the genotypes for a particular gene locus? ______________ Selection is the differential reproduction of phenotypes—
that is, some phenotypes (and their associated genes) are
________________________________________________
passed to the next generation more often than others. In
Which Hardy-Weinberg equation relates the frequencies of positive selection, genotypes representing adaptive traits in
the phenotypes for a gene? __________________________ an environment increase in frequency because their bearers
survive and reproduce more. In negative selection, geno-
________________________________________________
types representing nonadaptive traits in an environment
To verify the predictions of the Hardy-Weinberg Prin- decrease in frequency because their bearers are less likely to
ciple, use the following procedure to produce a generation survive and reproduce.
of offspring from the parental population you created in the Selection pressures are factors such as temperature
previous procedure. Remember, the fact that the genetic and predation that result in selective reproduction of phe-
frequencies of various alleles, genotypes, and phenotypes notypes. Some pressures may elicit 100% negative selec-
total 1.0 is not a prediction of the Hardy-Weinberg Prin- tion against a characteristic and eliminate all successful
ciple. The total of 1.0 is a mathematical fact. The predic- reproduction by individuals having that characteristic. For
tion is that the relative frequencies will not change if all example, mice with white fur may be easy prey for a fox if
assumptions are met. they live on a black lava field. This dark environment is a

15–3

Natural Selection 151

 B

BB

Bb

bb 9 42 49

(a) Genotypes (b) Establish Parental population (c) Random mating

parental and allele
population recombination

?
? Genotypic Allelic
? Genotype Number frequency Allele frequency

BB B

Bb b

bb

Next generation
(d) Retrieve 75–100 (e) Record the genotype of
offspring each selected individual.

Figure 15.2
Steps in the verification of the Hardy-Weinberg Principle.

negative selection pressure against white fur. If survival and to produce subsequent generations. Record the
reproduction of mice with white fur were eliminated (i.e., occurrence of this genotype on a sheet of paper.
if there is 100% negative selection), would the frequency 5. Repeat steps 2–4 until the parental population is
of white mice in the population decrease with subsequent depleted, thus completing the first generation.
generations? To test this, use the following procedure to 6. Calculate the frequencies of each of the three
randomly mate members of the original parental population genotypes recorded on the separate sheet and record
to produce 100 offspring (fig. 15.3). these frequencies for the first generation in table
Procedure 15.3 15.2. Individuals in each generation will serve as the
Simulate 100% negative selection pressure. parental population for each subsequent generation.
7. Repeat steps 2–5 to produce second, third, fourth,
1. Establish the same parental population (Proce- and fifth generations. After the production of each
dure 15.1) you used to test the Hardy-Weinberg generation, record your results in table 15.2.
prediction.
8. Graph your data from table 15.2 using the graph
2. Simulate the production of an offspring from this paper at the end of this exercise. Generation is the
population by randomly withdrawing two gametes to independent variable on the x axis and Genotype is
represent an individual offspring (fig. 15.3). the dependent variable on the y axis. Graph three
3. If the offspring is BB or Bb, place it in a container for curves, one for each genotype.
the accumulation of the “Next Generation.” Record
the occurrence of this genotype on a separate sheet Because some members of each generation (i.e., the bb
of paper. that you removed) cannot reproduce, the number of off-
4. If the offspring is bb, place this individual in a spring from each successive generation of your population
container for those that “Cannot Reproduce.” will decrease. However, the frequency of each genotype, not
Individuals in this container should not be used the number of offspring, is the important value.

15–4

152 Exercise 15
 Homozygous
recessive

Heterozygous

Homozygous
dominant

Parental population

Cannot
First generation reproduce

Second generation

Third generation
Figure 15.3
Demonstrating the effect of 100% selection pressure on genotypic and phenotypic frequencies across three generations. Selection is against the
homozygous recessive genotype. Random mating within the parental population is simulated by mixing the gametes (beads), and the parental
population is sampled by removing two alleles (i.e., one individual) and placing them in the next generation. Homozygous recessive individuals
are removed (selected against) from the population. The genotypic and phenotypic frequencies are recorded after the production of each
generation. The production of each generation depletes the beads in the previous generation in this simulation.

Table 15.2
Genotypic frequencies for 100% negative selection

Generation
Genotype First Second Third Fourth Fifth
BB ●●
Bb ●❍
bb ❍❍
Total 1.0 1.0 1.0 1.0 1.0

Questions 4 the third generation? From the third to the fourth genera-
Did the frequency of white individuals decrease with succes- tion? Why or why not? _____________________________
sive generations? Explain your answer. ________________
________________________________________________
________________________________________________
How many generations would be necessary to eliminate the
Was the decrease of white individuals from the first to sec- allele for white fur? ________________________________
ond generation the same as the decrease from the second to
________________________________________________

15–5

Natural Selection 153

 Most natural selective pressures do not completely 6. Calculate the frequencies of each of the three
eliminate reproduction by the affected individuals. Instead, genotypes recorded on the separate sheet and
their reproductive capacity is reduced by a small proportion. record these frequencies for the first generation
To show this, use Procedure 15.4 to eliminate only 20% of in table 15.3.
the bb offspring from the reproducing population. 7. Repeat steps 2–5 to produce second, third, fourth,
and fifth generations. Individuals in the “Next
Procedure 15.4 Generation” serve as the parental population for
Simulate 20% negative selection pressure. each subsequent generation. After production of
each generation, record your results in table 15.3.
1. Establish the same parental population (Procedure
15.1) that you used to test the Hardy-Weinberg 8. Graph your data from table 15.3 using the graph
prediction. paper at the end of this exercise. Generation is the
independent variable on the x axis and Genotype
2. Simulate the production of an offspring from this
frequency is the dependent variable on the y axis.
population by randomly withdrawing two gametes to
Graph three curves, one for each genotype.
represent an individual offspring.
3. If the offspring is BB or Bb, place it in a container Because some members of each generation cannot
for production of the “Next Generation.” Record the reproduce, the number of offspring from each generation of
occurrence of this genotype on a separate sheet of your population will decrease. However, the frequency of
paper. each genotype, not the number of offspring, is the impor-
4. If the offspring is bb, place every fifth individual tant value.
(20%) in a separate container for those that “Cannot Questions 5
Reproduce.” Individuals in this container should not Did the frequency of white individuals decrease with succes-
be used to produce subsequent generations. Place sive generations? _________________________________
the other 80% of the homozygous recessives in the
container for the “Next Generation.” Record the ________________________________________________
occurrence of this genotype on a sheet of paper. Was the rate of decrease for 20% negative selection similar
5. Repeat steps 2–4 until the parental population is to the rate for 100% negative selection? If not, how did the
depleted, thus completing the first generation. rates differ? ______________________________________
________________________________________________

Table 15.3
Genotypic frequencies for 20% negative selection

Generation
Genotype First Second Third Fourth Fifth
BB ● ●
Bb ●❍
bb ❍❍
Total 1.0 1.0 1.0 1.0 1.0

15–6

154 Exercise 15
Questions for Further Thought and Study

1. Charles Darwin wasn’t the first person to suggest that populations evolve, but he was the first to describe a credible
mechanism for the process. That mechanism is natural selection. What is natural selection? How can natural selection
drive evolution?

2. How would selection against heterozygous individuals over many generations affect the frequencies of homozygous
individuals? Would the results of such selection depend on the initial frequencies of p and q? Could you test this
experimentally? How?

3. How are the frequencies of genes for nonreproductive activities such as feeding affected by natural selection?

4. Do you suspect that evolutionary change always leads to greater complexity? Why or why not?

5. Is natural selection the only mechanism of evolution? Explain.

6. What change in a population would you expect if a selection pressure was against the trait of the dominant allele?

15–7

Natural Selection 155

15–8

156 Exercise 15
 15–9

Natural Selection 157

exercise sixteen

Adaptations of Vertebrates
to Their Environment
16
Objectives Procedure 16.1
Examine skeletal adaptations of representatives from the
As you complete this lab exercise you will:
major classes of vertebrates.
1. Examine skeletal adaptations of vertebrates.
2. Recognize the functions of external adaptations of 1. Examine skeletons representing the major classes of
vertebrates that contribute to fitness. vertebrates.
3. Simulate competition and success among similar 2. Consider the environment of each organism.
morphological adaptations. The environment selects for efficient functional
morphology needed for locomotion.
Questions 1
A daptations are characteristics and structures of an
organism that facilitate vital processes such as homeo-
stasis, food getting, and reproduction. Not all characteristics
Undulation is efficient in water and requires a flexible axial
skeleton. What part of a fish’s skeleton provides for flexibil-
ity needed for undulatory swimming? _________________
of an organism necessarily promote fitness. For example, a
vertebrate’s chin or claw color may be a neutral structural ________________________________________________
necessity or a by-product of other features. However, charac- What percentage of a fish’s length includes flexible verte-
teristics that promote survival and reproduction and are sub- brae? ___________________________________________
ject to environmental selection are considered adaptations.
They promote fitness. They result from a species’ long-term Are there other ways of moving through an aquatic envi-
interaction with its environment and are shaped by natural ronment besides undulation? How so? _________________
selective pressures that promote or retard the passing of the ________________________________________________
genetic blueprints of an adaptation to the next generation.
Examining structural adaptations quickly reveals that many ________________________________________________
serve multiple functions and are best studied in the context
3. The ecology and life history of amphibians is
of their environment.
associated with an aquatic environment. Examine a
In this lab exercise you will examine morphological
skeleton of Necturus (mud puppy).
characteristics common to groups of animals well-adapted to
their environment. As you examine each adaptation, con- Questions 2
sider the kind of environment that promotes and selectively Is an amphibian such as a mud puppy adapted for swim-
hones the gene frequencies of the adaptation’s genetic blue- ming? Crawling? __________________________________
print. Adaptations relate directly to the ecology of a species. ________________________________________________
What does this tell you about the microenvironment occu-
pied by such amphibians? ___________________________
ADAPTIVE SKELETAL FEATURES
________________________________________________
Primary among the many functions of the skeletal system
How does length and flexibility of the vertebral column
is providing sites for muscle attachment for flexible move-
compare with pectoral and pelvic appendage development?
ment. Movement, especially locomotion, involves generat-
Which is more robustly developed—the vertebral column
ing force and overcoming gravity. This requires rigid bones
or appendages? ___________________________________
to resist powerful muscles, and flexile joints for coordinated
movement. Skeletal adaptations are remarkably varied. ________________________________________________

16–1

159
Which form of locomotion likely generates the most power Has natural selection produced a singular “best” morphol-
for a mud puppy? What is your evidence? ______________ ogy for locomotion? Why or why not? _________________
________________________________________________ ________________________________________________
Frogs are also amphibians associated with water. What per- Are there flying reptiles? Have there ever been? _________
centage of their body length includes flexible vertebrae?
________________________________________________
Why so small? ___________________________________
________________________________________________ 5. Examine a bird skeleton.
Questions 4
What do you conclude about the environment that has Birds share a recent and direct lineage to reptiles. How
shaped frogs’ adaptations for locomotion? ______________ much axial flexibility for locomotion does a bird skeleton
________________________________________________ have? __________________________________________

Which frog bones appear best adapted for attachment of ________________________________________________

powerful muscles? _________________________________ Fish and amphibians and some reptiles use extended digits
________________________________________________ to push against their environment as they move. Are the
digits of a bird’s wing well-developed? If not, what other
Frogs have long digits. Are they adaptive for a strong grip? If adaptations accomplish this same function? ____________
not, what is the adaptive value of long digits for frogs?
________________________________________________
________________________________________________
Land animals must compensate for gravity with strong mus-
________________________________________________ cles and appendages. Birds, however, must not only com-
4. Reptiles include transitional morphologies adapted pensate, but must overcome gravity. This takes powerful
to life on land. Examine skeletons of a turtle, a muscles. What adaptive skeletal feature provides for broad
snake, and a small alligator. attachment of powerful muscles? _____________________
Questions 3 ________________________________________________
Is undulation (versus crawling or walking) a viable form of
The rigors of land and air environments select for powerful
locomotion in a terrestrial environment? What is your evi-
and adaptive bones and muscles. Which bones of the bird
dence? __________________________________________
are the thickest and most robust? ____________________
________________________________________________
________________________________________________
Terrestrial environments lack the buoyancy of water. Pow-
erful muscles are needed to lift and move body mass. Are 6. Examine a cat skeleton.
appendages of reptiles more developed than those of fish Questions 5
and amphibians? __________________________________ Which are among the most vital organs of mammals and
birds? ___________________________________________
________________________________________________
________________________________________________
The axial skeleton includes the head and spinal column.
Which of the reptile skeletons available has the least axial What skeletal structures are adapted to protect these
flexibility? _______________________________________ organs? _________________________________________

________________________________________________ ________________________________________________

Does a lack of axial flexibility mean that the skeleton is not Which bones of the cat are the thickest and most robust?
well-adapted to its environment? ____________________ ________________________________________________
________________________________________________ ________________________________________________
How does the anatomy and ecology of the organism com-
Question 6
pensate for less undulation to power locomotion? _______
Are the bones of a fish as thick and robust as those of a cat?
________________________________________________ Why or why not? _________________________________
Which features of an alligator’s developed appendages and ________________________________________________
axial flexibility are adaptive for its methods of locomotion?
7. Compare the teeth of all the vertebrate skeletons
________________________________________________ available, including fish, frog, alligator, bird, cat, and
________________________________________________ other mammals.

16–2

160 Exercise 16
Questions 7 2. Examine the external features of each of the major
From your experience, are fish, amphibians, and reptiles classes of vertebrates.
“gulpers” or “chewers” when they eat? _________________ 3. Identify as many external features as possible for each
________________________________________________ specimen. Make a note in table 16.1 concerning
the function(s) for which each feature confers an
What is your evidence for gulping or chewing from their advantage.
skeletal morphology? ______________________________
________________________________________________
A SIMULATION AND TEST OF ADAPTIVE
Do the teeth of an alligator have much variation? Or are MORPHOLOGIES
they all about the same length and shape? What are they
adapted to do? ___________________________________ A widely studied example of subtle variation of an adap-
________________________________________________ tation involves the beaks and feeding ecology of Darwin’s
finches of the Galápagos Islands. Review this topic. When
Which of the vertebrates on display show marked variation the parent population of finches arrived on the Galápagos,
between front teeth and cheek teeth? _________________ the birds became isolated as subpopulations on the islands.
________________________________________________ With time, speciation occurred and subpopulations evolved
beaks adapted to particular food items in the varied island
Some mammals have cheek teeth adapted for grinding and environments. Food availability and competition were
some have cheek teeth for cutting. How are a cat’s cheek selective pressures that shaped beak morphologies, allowing
teeth adapted? A human’s cheek teeth? Horse? Cow? each species to exploit a particular food.
________________________________________________ In the following procedure each student in a team of
four has a different hand tool analogous to the beak of a
________________________________________________ feeding bird. That beak represents an adaptation to gather
food items of a particular size or shape. Some adaptations
(beaks) are more advantageous than others at gathering
ADAPTIVE EXTERNAL FEATURES food of a particular size. In a competitive environment, the
organism with the best adaptive morphologies will gather
Natural selection has shaped available genetic variation more food and will therefore be more fit. The four students
and the results are adaptations. Over many generations, will simultaneously feed from the same resource, and their
characteristics with no adaptive advantage for survival and success at gathering food will measure the effectiveness of
reproduction may decrease in frequency and those with sig- the “beak” adaptations.
nificant advantage become prominent and frequent. Adap-
tive external features are an organism’s interface with its Procedure 16.3
environment and are subject to strong selective pressures. Test the adaptive advantages of four feeding
External adaptations and their functions vary a great deal morphologies.
among the classes of vertebrates. Among the major func-
1. Divide into groups of four students each. Each of the
tions subject to selective pressures are:
four students must have a different feeding tool.
• Protection
2. Obtain one food supply for your group consisting of a
• Sensing the environment small container filled with food items.
• Locomotion 3. Examine the size of the food item (Food item A).
• Gas exchange Hypothesize which of the available tools is best
Question 8 adapted to gather the food available.
Do you expect some external features to serve more than 4. All four organisms (group members) will “feed” from
one adaptive function? For example? __________________ the same container placed in the middle of the table
equidistant from each organism. A feeding session
________________________________________________ will last 20 seconds. All organisms will feed at the
same time from the same food container.
Procedure 16.2 5. Obtain four small cups, one for each organism. Each
Examine adaptations of the external features of organism will feed into a “stomach” represented by
representatives from the major classes of vertebrates. the cup kept directly in front of the organism and at
the outer edge of the table at all times.
1. Examine table 16.1 and the four broad functions
6. Feed for one 20-sec session (Feeding session 1).
listed. Can you add to the list?

16–3

Adaptations of Vertebrates to Their Environment 161

 Table 16.1
Adaptations of external features of members of the major classes of vertebrates

Specimens
Fish Amphibian Reptile Bird Mammal
Protection ______________ ______________ ______________ ______________ ______________
______________ ______________ ______________ ______________ ______________
______________ ______________ ______________ ______________ ______________
______________ ______________ ______________ ______________ ______________
Sensory ______________ ______________ ______________ ______________ ______________
______________ ______________ ______________ ______________ ______________
______________ ______________ ______________ ______________ ______________
______________ ______________ ______________ ______________ ______________
Locomotion ______________ ______________ ______________ ______________ ______________
______________ ______________ ______________ ______________ ______________
______________ ______________ ______________ ______________ ______________
______________ ______________ ______________ ______________ ______________
Gas exchange ______________ ______________ ______________ ______________ ______________
______________ ______________ ______________ ______________ ______________
______________ ______________ ______________ ______________ ______________
______________ ______________ ______________ ______________ ______________
______________ ______________ ______________ ______________ ______________
______________ ______________ ______________ ______________ ______________
______________ ______________ ______________ ______________ ______________
______________ ______________ ______________ ______________ ______________
______________ ______________ ______________ ______________ ______________
______________ ______________ ______________ ______________ ______________
______________ ______________ ______________ ______________ ______________
______________ ______________ ______________ ______________ ______________

7. Count the number of food items obtained and record Would a mixture of food sizes be more realistic of a natural
the value in table 16.2 for each organism. Return the situation? _______________________________________
gathered food to the central container. ________________________________________________
8. Rotate feeding tools among the team members and
feed for a second 20-sec session. Record the results in Is competition a factor in the success of adaptations? Why
table 16.2. or why not? ______________________________________
9. Repeat steps 7–8 until all four organisms have used ________________________________________________
all four beaks (four sessions). Record the results of
Does the success (adaptive advantage) of a beak depend on
each session in table 16.2.
which organism wields that beak? What is your evidence?
10. Select a food supply with a different size food item
(Food item B). Repeat steps 6–9. ________________________________________________
11. Select a food supply with a different size food item ________________________________________________
(Food item C). Repeat steps 6–9.
Would the effectiveness of an adaptation for feeding
Questions 9
increase with experience by the organism? How so? ______
Which beak is best adaptive to gather Food item A? ______
________________________________________________
________________________________________________
Would a mixture of food sizes amplify or diminish the differ-
Food item B? _____________________________________
ence among success of adaptations? ___________________
Food item C? _____________________________________
________________________________________________

16–4

162 Exercise 16
 Table 16.2
Experimental data testing the effectiveness of four varied adaptations

Adaptations
Food Item A Beak 1 Beak 2 Beak 3 Beak 4
Feeding session 1 ______ food items ______ food items ______ food items ______ food items
Feeding session 2 ______ food items ______ food items ______ food items ______ food items
Feeding session 3 ______ food items ______ food items ______ food items ______ food items
Feeding session 4 ______ food items ______ food items ______ food items ______ food items
mean items mean items mean items mean items
per session ______ per session ______ per session ______ per session ______
Food Item B Beak 1 Beak 2 Beak 3 Beak 4
Feeding session 1 ______ food items ______ food items ______ food items ______ food items
Feeding session 2 ______ food items ______ food items ______ food items ______ food items
Feeding session 3 ______ food items ______ food items ______ food items ______ food items
Feeding session 4 ______ food items ______ food items ______ food items ______ food items
mean items mean items mean items mean items
per session ______ per session ______ per session ______ per session ______
Food Item C Beak 1 Beak 2 Beak 3 Beak 4
Feeding session 1 ______ food items ______ food items ______ food items ______ food items
Feeding session 2 ______ food items ______ food items ______ food items ______ food items
Feeding session 3 ______ food items ______ food items ______ food items ______ food items
Feeding session 4 ______ food items ______ food items ______ food items ______ food items
mean items mean items mean items mean items
per session ______ per session ______ per session ______ per session ______

Procedure 16.4 2. Design and execute a procedure to answer your

Conduct a self-designed test of adaptation effectiveness. question.
3. Copy and modify table 16.2 as needed for your
1. Choose one of the following two questions to answer
procedure.
using the general protocol in Procedure 16.3:
Question 10
Does the effectiveness of an adaptation for feeding
What was the answer to your question in Procedure 16.4?
increase with experience by the organism?
Does a mixture of food sizes amplify or diminish ________________________________________________
the difference among success of adaptations? ________________________________________________

16–5

Adaptations of Vertebrates to Their Environment 163

Questions for Further Thought and Study

1. What functions other than feeding might the shape of a bird’s beak serve?

2. Could every characteristic of an organism be considered an adaptation? How so?

3. What is wrong with the statement “This adaptation evolved to promote reproduction”?

4. What function is the hand of a chimpanzee adapted to perform?

16–6

164 Exercise 16
exercise seventeen

Adaptations of Plants to Their

Environment
17
Objectives
As you complete this lab exercise you will:
1. Understand plant adaptations shaped by natural
selection.
2. Examine morphological plant characteristics and
link their structure with an adaptive function.
3. Discover how one structural feature can confer
multiple adaptive advantages.

P lants interact with their environment. As a result, evo-

lution driven by natural selection shapes structures and
strategies that maximize a plant’s survival and reproduction Figure 17.1
Leaves of this acacia plant in Belize have numerous nectaries that pro-
and therefore shape their ecology (fig. 17.1).
duce a sugary liquid. These nectaries are adaptive because they attract
Plant adaptations are characteristics and structures of a ants that protect the plant from caterpillars and other leaf eaters.
species that facilitate survival and fitness by enhancing vital
processes such as homeostasis, nutrient and gas acquisition,
or reproduction. Although adaptations can be metabolic,
the most easily examined are structural features. In this lab exercise you will examine a range of plant
The number of adaptations occurring in plants is adaptations for gas exchange, water relations, and light
immense. It can be argued that most characteristics of a acquisition.
plant species are, at least in part, adaptations that play a
role in promoting a major life process. A diffuse root sys-
tem may be an adaptation that promotes stability in loose GAS EXCHANGE
soil. Flowering in the spring may be an adaptation to exploit All organisms exchange gas with their environment. Auto-
the increasing pollinator (insect) population and therefore trophic plants depend on CO2 as a carbon source and O2 as
enhance fitness (reproductive success). The list of adapta- an electron acceptor for respiration. They also release O2 as
tions is almost endless. a by-product of photosynthesis.
Morphological features are the most obvious adapta-
tions, but any characteristic whose form or function promotes Procedure 17.1
survival and reproduction in response to selective pressures
Examine adaptations for gas exchange.
of the environment is an adaptation A unique enzyme that
produces a compound toxic to caterpillars is an adaptation to 1. Examine a prepared slide of a privet (Ligustrum)
prevent herbivory. Spines of a cactus are adapted to fend off leaf cross section that shows common structural
grazers. The rapid closing of a leaf-trap of a Venus Flytrap is adaptations for gas exchange. Locate the stomates
an adaptation to catch fast-moving insects. The requirement (pores) that allow gas exchange at the leaf surface.
of fire to open the cone and release seeds of some species Questions 1
of pine is an adaptation that synchronizes seed release with Are the pores most abundant on the upper or on the lower
newly available nutrients, more available high-quality light, surface of the leaf? ________________________________
reduced competition, and open soil, all of which increase the
chances of germination and success. ________________________________________________
17–1

165
Open stomates are needed for gas exchange, but can also
allow loss of water vapor. On which surface would it be
more adaptive for the stomates to occur to minimize water
loss? Why? ______________________________________
________________________________________________
Notice that the leaf interior is not a solid mass of cells. What
percentage of the cross-sectional area is open space for gas
movement? Be sure to examine three or four prepared slides
to provide an accurate estimate. ______________________
________________________________________________
Figure 17.2
2. Examine a prepared slide of a leaf cross section from This cross section of a water lily leaf reveals extensive, gas-filled
the water lily Nymphaea (fig. 17.2). Aquatic plants chambers. These atmospheric chambers are adaptive because they
increase availability of oxygen. These pockets of air hold a higher
are well adapted with “air” pockets within their concentration of oxygen than will dissolve in water.
tissues to supplement gas exchange because the
concentration of O2 in air is 25,000⫻ greater than
O2 dissolved in water.
Questions 2
Hydrophytes are plants adapted to aquatic environments.
Are air pockets evident in the leaf cross section of water
Mesophytes are terrestrial plants adapted to moderate water
lily? ____________________________________________
availability. Xerophytes are adapted to low water availabil-
________________________________________________ ity. Terrestrial mesophytes and xerophytes must be well-
adapted to acquire, transport, and conserve water.
What percentage of the cross-sectional area of a water lily
leaf includes air spaces? Is this percentage greater than that
Procedure 17.2
for the terrestrial privet leaf? ________________________
Examine adaptations for water relations.
________________________________________________
1. Examine a prepared slide of a root cross section from
Are stomates apparent in water lily leaves? _____________ the mesophyte buttercup (Ranunculus). The star-
________________________________________________ shaped cluster of cells in the center of a buttercup
root is xylem cells.
Would you expect much direct recycling of gases between Questions 4
respiration and photosynthesis within the leaf of a water How big are the xylem cells of buttercup root relative to the
lily? How so? _____________________________________ other cells? ______________________________________
________________________________________________ ________________________________________________
3. Examine a prepared slide of an elderberry stem Lignin, a reinforcing molecule in the cell walls, typically
(Sambacus) lenticels. stains red. Are the xylem cells of buttercup root reinforced?
Questions 3 ________________________________________________
How does a lenticel appear adaptive for gas exchange?
________________________________________________
________________________________________________
How are the cell walls and size of the xylem cells adapted to
________________________________________________ transport water? __________________________________
Are the cortex cells just inside the lenticels loose with small ________________________________________________
air spaces? _______________________________________
Most functioning xylem cells are hollow. Do the xylem cells
________________________________________________ of buttercup root appear empty? _____________________
Not all stems have lenticels. Do they still require gas ________________________________________________
exchange? Through what path? ______________________
If water can diffuse from one cell to another, what is the
________________________________________________ advantage of having hollow conducting cells? __________

WATER RELATIONS ________________________________________________

Water availability, more than any other environmental 2. Examine a prepared slide of a corn (Zea) root and
factor, governs the distribution and abundance of plants. stem cross sections.

17–2

166 Exercise 17
Questions 5
What adaptive characteristics distinguish the xylem cells of
corn? ___________________________________________
________________________________________________
Vascular bundles are scattered across a corn stem. How were
they arranged in the root? __________________________
________________________________________________
What characteristic readily distinguishes the water-
conducting xylem cells in each vascular bundle? ________
________________________________________________
Are the vascular bundles rich with structurally reinforcing
lignin? How is that adaptive? ________________________
________________________________________________
3. Examine a prepared slide of a stem cross section of
the hydrophyte Elodea.
Question 6
How does the vascular bundle of Elodea compare to that of a
mesophyte? How would this be adaptive for Elodea?
________________________________________________
________________________________________________ Figure 17.3
The grass leaf on the left has received plenty of water to retain its
4. Wilting leaves is an adaptive response to low water. shape to capture sunlight. The leaf on the right has wilted. Notice
Wilting constricts the space into which evaporation that the wilted leaf curls rather than droops. The curl encloses surface
with the most stomates.
from stomates occurs. Examine two sunflower plants—
one well-watered and the other dry and wilted.
Questions 7
When most dicots wilt, they droop and thereby enclose the If the buliform cells lose their turgor pressure and shrink,
lower surface. What is the adaptive significance of enclos- how would the leaf shape change? ____________________
ing the lower surface? ______________________________
________________________________________________
________________________________________________
Does the orientation of the curling of grass leaves appear to
Drooping leaves of a mesophyte also temporarily crush or crush the xylem as occurred in the sunflower leaves? _____
kink the xylem. How is this adaptive? _________________
________________________________________________
________________________________________________
One adaptive response to low water is for the leaves to
5. Most grasses are xerophytes. Examine some well- droop, enclose the stomates, and effectively stop all water
watered grass leaves and some wilted grass (fig. 17.3). flow through the xylem. A contrasting strategy is to curl
Question 8 and enclose the stomates, but allow water to flow down to
Do wilted grass leaves droop? Or do they tend to curl and the last drop. Which appears to be the common mesophyte
enclose a surface? _________________________________ strategy? _________________________________________

________________________________________________ ________________________________________________
Which is the common xerophyte strategy? Is one strategy
6. Examine a prepared slide of a leaf cross section of
the “correct” one? _________________________________
Poa, a common grass. Locate the large buliform cells
on either side of the midvein and the stomates on ________________________________________________
the surface.
7. Succulents are xerophytes adapted for water storage.
Questions 9
Examine a prickly pear cactus (Opuntia) stem cross
On which surface do the stomates occur? ______________
section. In Opuntia, the stem grows in “pads” that
________________________________________________ function as leaves.

17–3

Adaptations of Plants to Their Environment 167

Questions 10 10. The xylem of cone-bearing gymnosperms includes
Are stomates abundant in Opuntia? ___________________ elongated cells called tracheids through which water
passes from cell to cell through pores (fig. 17.4).
________________________________________________
The evolution of angiosperms included adaptive
Are the inner cells large and thin walled, or small and thick vessel elements with open ends that form long tubes.
walled? _________________________________________ Examine a prepared slide of a stem cross section of
pine (Pinus), a gymnosperm, and of oak (Quercus),
________________________________________________
an angiosperm.
8. Most succulent xerophytes are adapted to hold their Questions 13
shape and not wilt. Examine a prepared slide of a leaf Wood is primarily older, lignin-filled xylem cells that no
cross section of Yucca, a succulent xerophyte. longer conduct water. Are the alternating rings of large-
Question 11 celled spring wood and dense, fibrous summerwood appar-
Rigid, supportive fibers stain dark red. Do you suspect that ent? ____________________________________________
leaves of Yucca resist wilting? ________________________ ________________________________________________
________________________________________________ During which season is the most growth and water transport
9. In privet leaves, wilting and drooping encloses the likely occurring for pine and oak?_____________________
air outside of the stomates. Some leaves also have ________________________________________________
grooved or in-folded surfaces. Examine a slide of an
oleander (Nerium) leaf cross section and a yucca During which season (spring or summer) would the largest
(Yucca) leaf cross section. xylem be produced? ________________________________
Question 12 ________________________________________________
Where are the stomates located on Oleander and Yucca
Which species has large, open-vessel elements? _________
leaves? How is this adaptive? ________________________
________________________________________________
________________________________________________
What is the relative cross-sectional area of vessels versus
tracheids? How is this adaptive? ______________________
________________________________________________
How are open-ended vessels forming long tubes more adap-
tive than porous tracheids? __________________________
Pits
________________________________________________

11. Line the bottom of a petri plate with a thoroughly

wet, doubled paper towel. Sprinkle a dozen lettuce
Vessel seeds onto the towel. Cover the plate and place it in
Pores element the dark for 48 h, or until the seeds have germinated.
Use a stereoscope to examine the root hairs near the
Tracheid tip of the root (fig. 17.5).

Questions 14
What is the adaptive advantage of having root hairs?
Vessel
element ________________________________________________
________________________________________________
Each root hair is an extension of a single epidermal cell.
How long are the longest of the root hairs? Use a clear ruler
marked in millimeters for comparison. _________________
(a) (b) (c)
________________________________________________
Figure 17.4
Comparison of tracheids and vessel elements. (a) In tracheids, water Estimate the number of root hairs on a root tip. _________
passes from cell to cell through pits. (b, c) In vessel elements, water
moves through pores, which may be simple or interrupted by bars. ________________________________________________

17–4

168 Exercise 17
 During a windy day, over which surface of the curved leaf
would the air move fastest? Slowest? __________________
________________________________________________
How is the curved profile of an oak leaf adaptive? _______
________________________________________________

14. Use a stereoscope to examine both surfaces of an oak

leaf. You may also prepare a wet mount for a compound
microscope if you need more magnification.
Questions 17
Which surface is glabrous (smooth) and which is pubescent
(hairy) with minute trichomes? ______________________
________________________________________________
What is the shape of an oak-leaf trichome? ____________
________________________________________________
What is adaptive significance of the density of these surface
trichomes? ______________________________________
________________________________________________

15. Use a stereoscope to examine the upper surface of an

oak leaf. A strong backlight shining from underneath
and through the leaf will make the network of veins
more apparent. Find the patches of photosynthetic
cells that appear as islands bordered by vascular tissue.
Questions 18
Figure 17.5 What is the maximum distance between a photosynthetic
Germinated lettuce seed with root hairs. cell and a nearby vascular bundle? You may need a small
metric ruler in the field of view while you examine the sur-
face. ____________________________________________
________________________________________________

12. Root surface area is important for water absorption. What is the adaptive advantage of this short distance?
The surface area of a root 1 cm long with no root ________________________________________________
hairs is about 0.3 cm2. Dense root hairs will increase
surface area of a root as much as 100-fold. ________________________________________________
Question 15
How long must a root with root hairs be to have the same
surface area as a petri plate (10 cm dia.)? ______________
________________________________________________ LIGHT ACQUISITION
13. Examine the general morphology of some freshly Procedure 17.3
picked leaves of live oak (Quercus). The stomates
Examine adaptations for light acquisition.
occur on the lower surface.
Questions 16 1. Examine a prepared slide of a privet (Ligustrum) leaf
Is the leaf perfectly flat or is it curved at the edges? ______ cross section.
Questions 19
________________________________________________
Against which surface are photosynthetic cells most tightly
Does the curve enclose the lower or upper surface? ______ packed? How is that adaptive? _______________________
________________________________________________ ________________________________________________

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Adaptations of Plants to Their Environment 169

How are the axes of the cells oriented? Vertical to the leaf INTEGRATION OF ADAPTATIONS
surface? Or horizontal? _____________________________
________________________________________________ Many morphological adaptations have an obvious and
single functional advantage, but it’s not always that simple.
Which orientation would require the light to go through Extensive root systems, for example, are obviously adaptive
the least cell wall material to penetrate the leaf and reach for absorbing water from the soil. But, tall trees gathering
the most chloroplasts? _____________________________ light at the top of a forest canopy also benefit from exten-
________________________________________________ sive root support. So, is an extensive root system an adapta-
tion for absorbing water or for maximum light gathering? Or
Is this adaptive? __________________________________ both? Indeed, major adaptations are best understood when
________________________________________________ considering the total success of the whole organism. One
adaptation can have multiple advantages.
2. The stems of trees are rarely photosynthetic. Exa-
mine a cross section of a pine (Pinus) or basswood Procedure 17.4
(Tilia) stem. Examine patterns of integrated adaptations.
Questions 20
1. Briefly describe the integrated roles of air spaces
For what function is a tree stem best adapted? What is your
within the leaves of hydrophytes for each of the
evidence from examining this cross section? ____________
following processes of aquatic plants:
________________________________________________ Gas exchange ______________________________
How might this function also be adaptive for the tree’s light Light acquisition ___________________________
gathering? _______________________________________
Water relations _____________________________
________________________________________________ 2. Briefly describe the integrated roles of thick-walled
xylem cells within plant stems for each of the
3. We might expect a leaf to respond to low light by
following processes:
producing more chlorophyll to capture as much
Gas exchange ______________________________
limited light as possible, or by producing less
chlorophyll because it is not needed. Examine leaves Light acquisition ___________________________
of a bean plant (Phaseolus) exposed to low light and a
Water relations _____________________________
plant exposed to intense light.
3. Briefly describe the integrated roles of stomates of
Questions 21
plant leaves for each of the following processes:
Is the ability to vary chlorophyll synthesis an adaptation?
Gas exchange ______________________________
How so? ________________________________________
Light acquisition ___________________________
________________________________________________
Water relations _____________________________
Which leaves have synthesized the most chlorophyll? How
might this response to low light be adaptive? ___________ 4. Briefly describe the integrated roles that wilting and
enclosure of the stomatal surface play for each of the
________________________________________________ following processes:
Gas exchange ______________________________
4. Closely examine the stems of bean plants grown in
low light. Light acquisition ___________________________
Question 22 Water relations _____________________________
Are stems longer in low light plants? How might this
response to low light be adaptive? ____________________
________________________________________________

17–6

170 Exercise 17
Questions for Further Thought and Study

1. Are any characteristics adaptive in some situations but maladaptive in others? How so?

2. If stomates are adaptive for gas exchange, why don’t plants evolve more and more stomates with each generation?

3. Reproduction is the most vital of all plant processes. What are some common plant adaptations that promote
successful reproduction?

17–7

Adaptations of Plants to Their Environment 171