UNIT 4

Ecosystems: Stability
and Change
Lesson 1: Population Dynamics . . . 182

Lesson 2: Ecosystem Dynamics . . . 196

Thing Explainer: How

Forests Come Back . . . . . . . . . . . . . . 212

Unit Connections. . . . . . . . . . . . . . . . 216

Unit Review . . . . . . . . . . . . . . . . . . . . . 217

Unit Performance Task . . . . . . . . . . . 219

Image Credits: ©National Geographic/Roy Toft/Getty Images

Populations are sensitive to changes

in their environments.

180 Unit 4 Ecosystems: Stability and Change

 FIGURE 1: Pigeon populations can grow out of control.

Pigeons were domesticated and bred by people thousands of years ago. Pigeons were
often used as message carriers because of their ability to find their way home even
over long distances. Today, however, the pigeon population has boomed and the
birds are damaging buildings, statues, and landmarks. Cities often use wire and netting
to keep the birds out and off of buildings. These devices cause the birds to move to a
new location but don’t reduce the population. Some cities are looking into different
forms of birth control to reduce the population to more manageable numbers.

Predict How do you think changes in population size can affect the composition and
stability of an ecosystem?

DRIVING QUESTIONS

As you move through the unit, gather evidence to help you answer the following
questions. In your Evidence Notebook, record what you already know about these
topics and any questions you have about them.
1. How do scientists measure population and changes in population?
2. What causes populations and ecosystems to remain stable or to change
over time?
3. What factors affect populations within an ecosystem?
4. How do modest or drastic changes in ecosystems affect ecosystem stability?

UNIT PROJECT

Go online to download
Wetlands at Your Service the Unit Project
Image Credits: ©Milkovasa/Shutterstock

Worksheet to help
plan your project.
Wetlands are ecosystems that have water at or near the soil surface at various times
of the year. Examples of wetlands include marshes and swamps. Wetlands often form
along the boundaries of water features, such as lakes or rivers. How do wetlands help
stabilize ecosystems and populations? Model your own wetland and investigate the
services wetlands provide for humans and the ecosystem. How might the destruction
of wetlands change the composition of populations and ecosystems?

Unit 4 Ecosystems: Stability and Change 181

 4.1

Population Dynamics

Many different species CAN YOU EXPLAIN IT?

inhabit the Greater
Yellowstone Ecosystem.
FIGURE 1: About 500 wolves live in the Greater Yellowstone Ecosystem today. When first
reintroduced to the ecosystem, there were only 31 wolves.

Gather Evidence
As you explore the lesson,
gather evidence for how
populations interact in ecosystems.

Image Credits: (t) ©Michael H Spivak/Moment/Getty Images; (b) ©SBTheGreenMan/iStock/Getty Images Plus
Yellowstone National Park, located primarily in northwest Wyoming, is at the heart
of the Greater Yellowstone Ecosystem. The rugged terrain and abundance of prey
make this temperate ecosystem an ideal environment for wolves. However, due to
aggressive eradication efforts in the 1800s and early 1900s, wolves were hunted to
the point that they were no longer present in the park. By 1926, the last wolf pack in
Yellowstone had been eliminated. The effect of such a change on an ecosystem was
not well understood by scientists at that time.
In 1995, a program began to reintroduce wolves into Yellowstone National Park.
During the first several years in which the wolf population was restored, observational
studies reported that the diversity of plant species increased, certain songbirds
returned, and aquatic ecosystems within the park changed.

Predict How might the reintroduction of wolves into Yellowstone National Park
have caused both direct and indirect changes in populations of so many other species
within their ecosystem?

182 Unit 4 Ecosystems: Stability and Change

 EXPLORATION 1

Population Density and Dispersion

If you have ever traveled from a rural area to a city, you may have noticed a change in
FIGURE 2: Cities have dense
population density. Cities have more dense populations, while rural areas have more
human populations.
widely dispersed, or scattered, populations. Species populations are measured in a
similar way. What can we learn from population data?

Population Density
You may be familiar with the term density in the context of matter. It is the amount
of matter in a given space. Population density is very similar: it is the number of
individuals living in a defined space. When scientists such as wildlife biologists observe
changes in population density over time, one of the things they study is whether the
causes are due to environmental changes or natural variations in the life history of the
species. The biologists use this information to decide whether it is necessary to make
changes to maintain a healthy population.
One tool that biologists can use to make this decision is to calculate the ratio of Collaborate With a
individuals living in an area to the size of that area. partner, discuss whether
Population density is calculated using the following formula: the area where you live has a dense
or dispersed population. Explain
number of individuals
__ = population density your reasoning.
area (units2)
To calculate this ratio for the deer herd shown in Figure 3, a biologist would first
determine the size of the herd’s territory. Then the scientist would count all of the
individuals in that population within the defined area.

FIGURE 3: Deer gather in a field to graze.

Image Credits: (t) ©Robert Harding/Wendy Connett/Getty Images; (b) ©Tim Graham/Alamy

Math Connection A scientist and her team counted 200 individual deer in an area
of 10 square kilometers.
1. What is the population density?
2. Ten years later, scientists return to the same area and find that the population
density has declined to 5 deer per square kilometer. What might a decrease in the
density of a deer population tell scientists about the habitat in the area?

Lesson 1 Population Dynamics 183

 Population Dispersion
You may have noticed that people tend to separate themselves in different ways—
some hang out in large groups, some gather in twos and threes, while others prefer to
be alone. There are also patterns in the way different populations of other organisms
separate themselves. Figure 4 shows three main patterns of population dispersal:
clumped, uniform, and random.
Clumped dispersion occurs when resources are spread unevenly within an ecosystem.
Individuals gather into groups where resources are available. Clumped dispersion
helps protect individuals from predators and makes finding a mate easier. Uniform
dispersion occurs when individuals of the same species must compete for limited
resources and territory. Random dispersion is the least common pattern of distribution.
It occurs when resources are evenly distributed within an ecosystem. In plants, this
type of dispersion often occurs when seeds are scattered by wind or water, resulting
in seeds being dropped randomly. The seeds will only sprout if conditions are right,
which increases the randomness of the distribution.

FIGURE 4: Population Dispersion Patterns

Analyze Why might a

population exhibit uniform
dispersion? Think about why having
a defined space might be beneficial.

a Clumped dispersion b Uniform dispersion c Random dispersion

Image Credits: (l) ©Rich Carey/Shutterstock; (c) ©Moment/Stoneography/Getty Images; (r) ©Vilainecrevette/Shutterstock
Model Draw a diagram showing an overhead view of a population with each type of
dispersion: clumped, uniform, and random.

Measuring Population Size

Measuring population size over a large area may seem like an impossible task.
Sometimes, a complete count of every individual can be done, particularly if the
species lives in an enclosed area. However, what if you needed to count a very large
population over many square kilometers? In this case, biologists can use a variety of
sampling techniques to estimate the size of a population.
One method scientists use to measure the size of a population of animals is the mark–
recapture technique. Biologists capture individuals within a population, tag them, and
then release them. After a period of time, a second sample is captured, and biologists
look for and count the tagged individuals as well as any newly-captured animals.
They may also fit animals with radio collars or GPS devices to track their movements.
Another method is called quadrat sampling, in which ecologists use quadrats—
typically square or rectangular grids of a known size—to collect data about population
numbers in an ecosystem. Quadrat sampling works best with species that do not
move, such as plants and corals.

184 Unit 4 Ecosystems: Stability and Change

 Hands-On Lab FIGURE 5: Quadrat sampling

Quadrat Sampling is most often used to survey

populations of plants.

Use a quadrat sampling method to collect data about population numbers.

Predict Does quadrat sampling provide an accurate estimated of a population size

within a defined area?

PROCEDURE
1. Obtain a quadrat frame. Measure, calculate, and record the area of the quadrat on a
piece of paper or in your notebook. MATERIALS
2. Stand at the edge of the area you will sample and randomly throw your quadrat. • calculator
Make sure your quadrat does not overlap with another. • meterstick
3. Count how many individuals of each species are in your quadrat. Record your data • quadrat
in a data table. Repeat this procedure three times.

ANALYZE
1. Combine your data with that of your classmates. Find the average number of each
species for all of the samples.
2. Obtain the area of the sampling plot from your teacher. Calculate how many
quadrats would fit in the area of the sampling plot. Multiply this value by the
average number of each species found in one quadrat to estimate the population
of each species.

Scale, Proportion, and Quantity

1. Calculate the density of each species. Which species had the highest density? Which
had the lowest? Why do you think that is? Compare your population estimates
to the actual population number that your teacher provides. Was your estimate
accurate? Why or why not?
2. How can you make sure that your estimate of population size will be as close to the
actual population size as possible?
3. Why do scientists only gather data for a part of the population, instead of the entire
population? How does this affect the accuracy of the final population count?
Image Credits: ©Martyn F Chillmaid/Science Source

Explain In Yellowstone National Park, scientists track and gather data on many
species to study population dynamics within the park, and to monitor the health of each
population. Describe the types of data that scientists would need to gather to study the
effects of reintroducing a population, such as wolves, on other populations in the park.

Lesson 1 Population Dynamics 185

 EXPLORATION 2

Population Growth Patterns

Predict What might Imagine you leave an apple in your locker over winter break. Upon your return to
happen to populations that school, you open your locker door to find a cloud of fruit flies. When you left school,
cannot get enough resources? the fly population in your locker was zero—now it’s at least 100! Your locker ecosystem
had a huge change in its fruit fly population. This, hopefully, is not a normal occurrence
in your locker, but changes in population sizes and densities in ecosystems are normal
responses to changes in resource availability.

Population Size
How might biologists track the population size of a species,
FIGURE 6: A population of elephants has both young and
old individuals. such as a group of elephants? To accurately track the
population over time, they would need to account for four
factors: immigration, emigration, births, and deaths.
Immigration and emigration have to do with individuals
entering and leaving a population. For example, if a
disturbance occurred in a nearby habitat, some elephants
might immigrate, or move into, a new population. Then,
competition could increase, causing some elephants to move
out of the population, or emigrate, to a new area.
Births and deaths also change a population size over time.
Individuals have offspring, which adds more members to the
population. Some individuals die each year, which reduces
the population.
Explain Which factors The growth rate of a population can be measured with an equation that takes into
lead to an increase in a account these four factors:
population, and which factors lead
r = (b + i) – (d + e)
to a decrease in a population?
In this equation, r = population growth rate, b = birth rate, i = immigration rate,
d = death rate, and e = emigration rate. We can apply these factors to our locker
ecosystem example. A small population of fruit flies immigrated into the locker in
search of food. The population increased due to the birth of a new group of fruit flies.
Those flies that did not die when you swatted them in surprise emigrated away from
the locker when you threw the apple away.
Image Credits: ©john michael evan potter/Shutterstock

Problem Solving As part of a long-term elephant study, biologists counted

individuals in a population of elephants each spring. In one year, there were 18 males
and 34 females. Over the following year, each female gave birth, from which 28
offspring survived. Predators killed 9 elephants. A construction project cleared 50 acres
of nearby forested land, causing 5 males and 19 females to immigrate into the study
area. Competition for females increased, resulting in the emigration of 10 males to a
new territory in search of mates. Calculate the growth rate of this population.

186 Unit 4 Ecosystems: Stability and Change

Survivorship Curves
Biologists are also interested in the reproductive strategy of a population. Reproductive
strategies include behaviors that can improve the chances of producing offspring or
behaviors that can increase the survivorship rate of offspring after birth. Parental care is
an example of a reproductive strategy. Parental care is especially important for species
that produce offspring that cannot take care of themselves. By protecting their young,
parents are better able to make sure their young stay alive until they can survive on
their own. A population’s reproductive strategies can be assessed using a survivorship
curve. Figure 7 shows the three types of survivorship curves.

Survivorship Curves
FIGURE 7: There are three main types of survivorship curves.

120 Type I
Type II
100 Type III
Number of survivors

80

60

40

20

0
0 10 20 30 40 50 60 70 80 90 100
Percentage of maximum life span

A survivorship curve is a simplified diagram that shows the number of surviving Collaborate With a
individuals over time from a measured set of births. By measuring the number of partner, discuss which type
offspring born in a year and following those offspring through until death, survivorship of survivorship humans exhibit.
curves give information about the life history of a species.
Some species have a small number of offspring, and many of the offspring live long
enough to reach old age. Mammals and other large animals generally exhibit this Type
1 survivorship curve. Other species have a large number of offspring, but many of
these offspring do not survive long enough to reproduce. Many invertebrates, fish, and
plants exhibit this Type III survivorship. A fish may lay hundreds or thousands of eggs,
but only a small percentage of its offspring will survive to adulthood.
Between these two extremes is a third type of survivorship, in which the survivorship
rate is roughly equal at all stages of an organism’s life. At all times, these species have
an equal chance of dying, whether from disease or as a result of predation. Organisms
such as birds, small mammals, and some reptiles exhibit this Type II survivorship.

Analyze Can a survivorship curve be used to determine whether or not a species

cares for their young? Explain your answer.

Lesson 1 Population Dynamics 187

 Exponential and Logistic Growth
Population growth depends on the environment and available resources. The rate of
growth for a population is directly determined by the amount of available resources. A
population may grow very rapidly, or it may grow slowly over time.

Analyze According to
FIGURE 8: Exponential Growth
Exponential Growth
the graph in Figure 8, Almost any species that lives
during which time period is 1000
in ideal conditions of available
population growth occurring at the 800 resources, space, and other

Population size
fastest rate? factors will rapidly increase in
600 population. This type of growth,
400 called exponential growth, occurs
when a population size increases
200 dramatically over a relatively short
amount of time. As shown in Figure
0
0 1 2 3 4 5 6 7 8, a graph of exponential growth
Year looks like a J-shaped curve.
Exponential growth may occur when a species moves into a previously uninhabited
area. A real-world example of exponential growth in a population occurred in 1859,
when an Australian landowner brought 24 rabbits into the country for sport hunting
and released them into the wild. With no predators, abundant space, and plentiful
resources, the rabbit population grew exponentially and spread across the country.
After many unsuccessful attempts to control the population, Australian officials
estimate today’s population to be between 100 and 200 million rabbits.

Analyze According to the

FIGURE 9: Logistic Growth
Logistic Growth
graph in Figure 9, when
4500 When a population is growing
would you expect competition exponentially, resources are
4000
Population size

among individuals to be the least? plentiful and there are no factors

3500
2500 to interfere with survivability.
2000 However, most populations face
1500 limited resources and thus show
1000 a logistic growth pattern. During
500 logistic growth, a population
0 begins with a period of slow
0 1 2 3 4 5 6 7 8 growth followed by a period of
Year
exponential growth before leveling
off at a stable size. A graph of logistic growth takes the form of an S-shaped curve,
as shown in Figure 9. During the initial growth period, resources are abundant, and
the population is able to grow at a quick rate. Over time, resources are reduced, and
growth starts to slow. As resources become even more limited, the population levels
off at a size the environment can support.

Explain When wolves were reintroduced into Yellowstone National Park, the
populations of many other species began to change.
1. Which factors would scientists want to measure in order to learn how each population
changed over time?
2. How would scientists know if populations were increasing or decreasing over time?
3. How might the introduction of wolves change the growth patterns of other species?

188 Unit 4 Ecosystems: Stability and Change

EXPLORATION 3

Factors That Limit Population Growth

Because natural conditions are neither ideal nor constant, populations cannot grow
forever. Instead, resources are used up or an ecosystem changes, causing deaths to
increase or births to decrease within a population.

Carrying Capacity
The carrying capacity of an environment is the maximum population size of a species Explore Online
that a particular environment can normally and consistently support in terms of
Hands-On Activity
resources. As shown in Figure 10, once a population hits this limit, certain factors then
keep it from continued growth. These factors include availability of resources such as Modeling Carrying Capacity
food, water, and space, as well as competition among individuals. Model predation and the effects
The carrying capacity of an environment can change at any given time. For example, of environmental changes on a
sudden and rapid flooding could reduce the availability of food or shelter in an population and the environment’s
ecosystem. This change would lower the environment’s carrying capacity. As a result, carrying capacity.
fewer individuals would be supported by the environment. When conditions improve,
however, the carrying capacity would increase, and the environment would again be
able to support a larger population of that particular species.

2500 FIGURE 10:

An environment’s
carrying capacity carrying capacity
2000 limits the growth
of a population.
Population size

1500

1000

500

0
0 5 10 15 20 25 30 35
Day

Predict How might this graph change if an ecosystem experienced drought conditions?

Limiting Factors
Many factors can affect the carrying capacity of an environment for a population
of organisms. A factor that has the greatest effect in keeping down the size of a
population is called a limiting factor. There are two categories of limiting factors—
density-dependent and density-independent.

Lesson 1 Population Dynamics 189

 Density–Dependent Limiting Factors
Density–dependent factors are factors that are affected by the number of individuals
in an area. The larger the population, the greater the effect. Density–dependent
limiting factors include the following:
Competition Both plants and animals compete among themselves for needed
resources. As a population becomes more dense, the resources are used up, limiting
how large the population can grow.
Predation The relationship between predator and prey in an environment is ongoing
and constantly changing. Predator populations can be limited by the number of
available prey, and the prey population can be limited by being caught for food.
Parasitism and disease Parasites are much like predators as they live off their hosts,
weakening them, and even sometimes killing them. Parasites and disease spread more
quickly through dense populations. The more crowded an area becomes, the easier it
is for parasites and disease to spread.

Data Analysis

Moose–Wolf Interactions on Isle Royale

For over 50 years, the wolf and moose populations on Isle Royale in Lake Superior
served as a classic example of how predator-prey interactions limit population
growth. As shown in Figure 11, changes in population size occur in an offset
manner. In other words, it takes some time for an increase or decrease in one
population to affect the other. Over time, the populations rise and fall in a pattern.

Density-Dependent Limiting Factors

FIGURE 11: Predator–Prey Interactions on Isle Royale

2500 50

2000 40
Moose population

1500 30 Wolf population

1000 20

500 10

0 0
1959 1967 1975 1983 1991 1999 2007 2015
Year
Moose Source: Vucetich, JA and Peterson, RO. 2015. Ecological Studies
Wolf of Wolves on Isle Royale, Annual Report 2014-2015.
Image Credits: (l) ©Rolf O. Peterson

Analyze Study the graph showing the moose–wolf interactions on Isle Royale.
1. Based on this graph, what is the most likely explanation for the increase in the
moose population between 1989 and 1995?
2. In 2016, the wolf population on Isle Royale declined to only two individuals. How
will the lack of wolves affect the moose population? Will the moose population
grow exponentially? Explain your answers.

190 Unit 4 Ecosystems: Stability and Change

 Density–Independent Limiting Factors
Density–independent factors are factors that can impact a population regardless of its
density. These factors include things such as:
Weather Any weather-related event such as a drought, flood, frost, or severe storm
can wipe out a population or destroy their sources of food, water, or shelter.
Natural disasters Volcanic eruptions, earthquakes, tsunamis, and fires usually result in
a sudden decrease in population size.
Human activity Habitats, and sometimes entire ecosystems, are degraded or even
completely destroyed by human activities such as forest clearing, draining of wetlands
for land development, and habitat fragmentation by roads and fences.

FIGURE 12: Forest fires kill plants and animals and force animal populations to flee.

Explore Online

Limiting Factors
Go online to view an
animation of limiting factors
in an ecosystem.

Explain Why is fire considered a density–independent limiting factor?

Human activities have had a significant effect on populations. For example, the
introduction of nonnative species has caused population crashes in many parts of the
world where biodiversity is an important part of ecosystem stability. Nonnative species
are species that are brought into ecosystems in which they do not normally live. In
some cases, the nonnative species may outcompete one or more native species for
resources. Because of the complex network of ecosystems, such effects could alter the
ecosystem food web. In some extreme cases, the extinction of a species may occur.
Image Credits: ©Patrick Orton/Getty Images

Gather Evidence When wolves were reintroduced into Yellowstone National Park,
scientists noticed that the populations of elk and coyotes decreased. They also noticed
that populations of beaver and willow trees increased. Describe the factors that might
have led to these changes in the different populations, and explain how these factors
would affect the carrying capacity of the environment for each species.

Lesson 1 Population Dynamics 191

 CONTINUE YOUR EXPLORATION

Careers in Science

Biogeographer
Biogeographers are often involved
FIGURE 13: Biogeographers use digital tools such as geographic information systems
with the protection, conservation, and
(GIS) to study the distribution of plant and animal species.
management of natural resources.
Where plant and animal species
live, how they got there, and how
future conditions might affect their
populations are just a few of the topics
that biogeographers study.
Technology is an important part of
a biogeographer’s toolset. They use
a digital tool called geographic
information systems, or GIS, to make
data-rich maps. GIS can use any data
that is related to location such as
population size, land type, and the
location of human infrastructure such
as roadways, power lines, and building
locations. Biogeographers use GIS along
with statistical models to map and study
populations, habitats, ecosystems, and
ecological processes. Biogeographers often discuss the results
Language Arts Connection
A variety of job titles and work of their research in written technical
reports or in presentations given within A state wildlife management agency
settings are connected with this
career. Someone with a degree in their agency or to the public. Therefore, is considering reintroducing bobcats back
biogeography might work as a city a career in biogeography also requires into a forested area where they once
or county planner, as a mapping excellent writing and communication flourished. Imagine you are the agency’s
technician, or as a GIS specialist. skills, so a strong background in biogeographer. Using your knowledge
Biogeographers work for city, state, language arts is particularly useful. of population dynamics and carrying
or federal government agencies, for As our knowledge of climate change capacity, what questions would you ask
nonprofit and private organizations, or continues to grow, biogeographers will and investigate to determine whether
they might work in an academic setting play an important role in determining or not the area they have selected is
as university professors or researchers. how environmental changes will appropriate for this reintroduction? What
Biogeography uses knowledge from impact the global geographical kind of data would you need to collect?
a wide range of subjects. Along with distribution of populations of different Develop and record a plan for investigation
general geography and cartography, or species. The information gathered by and determine what questions you would
Image Credits: ©RosaIreneBetancourt 1/Alamy

map making courses, students may also biogeographers could be used to come need answered before the reintroduction
take classes in economics, computer up with solutions to help solve these could proceed.
science, history, mathematics, ecology, problems and to prevent species from
and evolutionary biology. going extinct.

POPULATION DENSITY CONTROLLING THE EXPONENTIAL Go online to choose one of

AND CARRYING CAPACITY GROWTH OF NONNATIVE SPECIES these other paths.

192 Unit 4 Ecosystems: Stability and Change

 EVALUATE

Lesson Self-Check

CAN YOU EXPLAIN IT?

FIGURE 14: Wolf reintroduction in Yellowstone National Park had a complex impact on
the ecosystem as a whole.

Wolf removal was one of many factors that changed the ecology of Yellowstone
National Park from 1926 until the early 1990s. Eliminating a predator helped the
elk population rise. Elk and beaver competed for some of the same food resources,
including willow trees. As willow trees were reduced by larger elk herds, fewer
beavers were able to survive in the park. Fewer beaver dams meant fewer marshy
environments, which are ideal willow habitat.

Explain Refer to the notes in your Evidence Notebook to explain how the
reintroduction of wolves into the Greater Yellowstone Ecosystem might have caused
both direct and indirect changes in the populations of so many other species within
their ecosystem. Write a short explanatory text that cites specific evidence from this
lesson about population dynamics to support your answer.

Interactions within any ecosystem, whether it be large or small, are often very
complex. All species within an ecosystem are connected. The impact and causes of
change in a system can be difficult to determine. The removal and later reintroduction
of wolves in Yellowstone definitely had the potential to cause change. But recent
Image Credits: ©SBTheGreenMan/iStock/Getty Images Plus

research has called into question how significant their impact really was. While at first
there was evidence that aspen and willow growth was occurring immediately after the
wolves were reintroduced, long-term studies indicate that this wasn’t actually the case.
Research by scientists at Colorado State University that focused on Yellowstone’s
willows found that the complete removal of wolves from the ecosystem had actually
caused permanent changes to the region. When the wolves were removed from the
system, elks removed nearly all of the region’s willow trees. Without willows to eat,
the beaver population decreased. No beavers meant no beaver dams, which caused
the once slow-moving waters to now cut deeply into the terrain. As a result, the water
table dropped far below the level where willows can survive. Even if the elk population
were drastically reduced by the newly reintroduced wolves, willow populations would
not recover.

Lesson 1 Population Dynamics 193

 EVALUATE

CHECKPOINTS

Check Your Understanding Use the graph to answer questions 6–9.

1. Which of these abiotic factors would contribute to a FIGURE 15: Survivorship Curves
clumped dispersion pattern in an ecosystem? Select all
120 Type I
correct answers.
Type II
a. unlimited water 100 Type III

b. limited water

Number of survivors
80
c. high temperatures
d. limited sunlight 60

40
2. A population of antelope has a negative population
growth rate. Which of these conditions must also be true 20
for the population growth rate to be negative?
0
a. births + deaths < immigrations + emigrations 0 10 20 30 40 50 60 70 80 90 100
b. births + deaths > immigrations + emigrations Percentage of maximum life span

c. births + immigrations < deaths + emigrations

d. births + immigrations > deaths + emigrations 6. A female salamander lays hundreds of eggs at a time.
However, after hatching, few of the larvae survive to
3. A population of warblers, a type of songbird, experiences adulthood. According to the graph, which type of
a period of exponential growth. Which of these factors survivorship does the salamander exhibit?
would be a density–dependent limiting factor that could a. Type I
decrease the carrying capacity of the ecosystem for this
b. Type II
population of songbirds?
c. Type III
a. a competing species moves into the forest
b. a period of lower than normal rainfall
7. A songbird has an equal chance of surviving at all
c. a builder removes trees for an office park stages of its life. According to the graph, which type of
d. high winds knock down a quarter of the trees survivorship does the songbird exhibit?
a. Type I
4. A population of deer is displaced by a massive flood in b. Type II
their habitat following a severe rainstorm. The flood is an
c. Type III
example of
a. a density–dependent limiting factor.
8. Which survivorship type is associated with parental care?
b. carrying capacity.
c. a density–independent limiting factor. 9. What is the connection between survivorship curves and
d. survivorship. reproductive strategies?

5. A population of rodents is introduced on a remote island 10. A herd of zebras has 9 males and 62 females. During a
due to a shipwreck. Eventually, the population reaches one-year period, 22 foals that are born survive and 25
the island’s carrying capacity. At this point, the birth and adults die. Six females join the herd. Three males and
death rates are 11 females leave the herd. Has the ecosystem reached
a. relatively equal. carrying capacity for the herd? How do you know?
b. crashing.
c. density independent. 11. Draw a graph of logistic growth. Label the point at which
the resources for the population are no longer abundant
d. density dependent.
enough to support exponential growth. Explain your
reasoning.

194 Unit 4 Ecosystems: Stability and Change

 MAKE YOUR OWN STUDY GUIDE

12. A sourdough bread starter is a colony of yeast that bakers

keep alive, sometimes for years. The bread is made by In your Evidence Notebook, design a study guide that
removing a portion of the colony, which is replaced by supports the main idea from this lesson:
adding back an equal volume of a solution of water,
Populations grow in predictable patterns and are limited by
sugar, and flour. Is the starter a model for exponential
resource availability.
growth or logistic growth? Explain your reasoning.
Remember to include the following information in your
13. Describe three advantages an individual organism study guide:
might have by living in a population with a clumped • Use examples that model main ideas.
dispersal pattern. • Record explanations for the phenomena you investigated.
• Use evidence to support your explanations. Your support can
14. A population of algae that lives in a pond is limited in include drawings, data, graphs, laboratory conclusions, and
size by the amount of sunlight that strikes the pond’s other evidence recorded throughout the lesson.
surface. Is sunlight a density-dependent or density-
independent limiting factor for the algae population? Consider how ecological factors such as resource availability limit
Explain your answer. population growth.

15. What might cause exponential growth to occur only for

a short period when a new species is introduced to a
resource-filled environment?

Use the graph to answer Questions 16–18.

Density-Dependent Limiting Factors

FIGURE 16: Predator–Prey Interactions on Isle Royale

2500 50

2000 40
Moose population

Wolf population

1500 30

1000 20

500 10

0 0
1959 1967 1975 1983 1991 1999 2007 2015
Year
Moose Source: Vucetich, JA and Peterson, RO. 2015. Ecological Studies
Wolf of Wolves on Isle Royale, Annual Report 2014-2015.

16. How does the wolf population on Isle Royale affect the
carrying capacity of the moose population?

17. Is there evidence from the data to suggest that the

wolf population crashed? What might have caused this
population crash?

18. Is there evidence from the data to suggest that the

moose population crashed? What might have led to this
population crash?

Lesson 1 Population Dynamics 195

 4.2

Ecosystem Dynamics

Lava flow from a volcanic

eruption can change an
ecosystem dramatically.

CAN YOU EXPLAIN IT?

FIGURE 1: A young plant grows in a crack in a sheet of bare volcanic rock.

Gather Evidence
As you explore the lesson,
gather evidence for how
ecosystems maintain stability
over time.

Image Credits: (t) ©Westend61/Getty Images; (b) ©Design Pics/Getty Images

Molten lava flowing from a volcano burns everything in its path. When it cools, a
layer of solid rock is left behind. However, over time, a new ecosystem will become
established on this seemingly lifeless landscape.

Predict How do you think an ecosystem can be reestablished in an area after a

disturbance such as a volcano?

196 Unit 4 Ecosystems: Stability and Change

 EXPLORATION 1

Interactions in Ecosystems

The ways in which flamingos interact with other organisms and their environment
FIGURE 2: Flamingos live and
are only a small part of the ecology of a tropical lagoon ecosystem. To understand
feed in large groups.
what individuals, populations, and communities need to survive, ecologists study the
interactions among and between species and their environment.

Habitat and Niche

Flamingos live in tropical and subtropical regions of the world. They prefer
environments that have alkaline lakes or saltwater lagoons and large mud flats. These
environmental features are examples of a flamingo’s habitat. A habitat includes all
of the biotic and abiotic factors in the environment where an organism lives. For a
flamingo, these factors include things such as the shrimp and other small invertebrates
that it eats, the water salinity, and the air temperature.

Model Draw a model of your habitat. Think of all the places that you regularly visit
and the people you interact with, and include those in your model.

Many species live in the same habitat, but each species occupies a different ecological
niche. An ecological niche contains all the physical, chemical, and biological factors
that a species needs to survive and reproduce.
The factors that make up a species’ niche include the following:
Explain How is a niche
Food sources The type of food a species eats, how a species competes with others for different from a habitat?
food, and where it fits in the food web are all part of a species’ niche.
Abiotic conditions A niche includes the range of conditions such as air temperature
and amount of water that a species can tolerate.
Behavior The time of day a species is active and where and when it feeds and
reproduces also are factors in the niche of a species.
An ecosystem is a collection of habitats. The organisms that occupy these habitats
have separate niches, but the niches have certain abiotic and biotic factors in common.
Think of a habitat as where a species lives and a niche as how the species lives within
its habitat.

Relationships in Ecosystems
Image Credits: ©Fabio Lamanna/Shutterstock

Each organism in an ecosystem interacts with other organisms as it goes about its
daily activities. The flamingos and other animals prey on the lagoon’s plankton,
invertebrate, and fish populations for food, and they in turn are food for larger
carnivores. Plants compete with one another for space, water, and nutrients. Still other
organisms form interspecies relationships to provide or gain shelter, get protection, or
find food. These interspecies interactions often benefit only one of the organisms in
the relationship, but sometimes both organisms benefit.

Lesson 2 Ecosystem Dynamics 197

 FIGURE 3: The frog is the
Predation and Competition
predator in this relationship. Predation is the process by which one organism, the predator, captures and feeds
upon another organism, the prey. The frog in Figure 3 is the predator, and the insect
is its prey. However, if a snake slithered by, the frog might become its prey. Predation
is not limited to carnivores—herbivores that seek out and eat parts of living plants are
considered predators, too. The relationship between predator and prey is important
for energy transfer in food chains.

Analyze Are humans predators in their ecosystem? Explain your answer.

Competition occurs when two organisms compete for the same limited resource, be
it food, shelter, water, space, or any other biotic or abiotic factor that both organisms
need to survive. Whenever two organisms need the same resource in a habitat, they
must compete for it. Competition can occur between members of different species or
FIGURE 4: Two blue jays between members of the same species, such as the blue jays that are fighting over a
compete for a food source. peanut in Figure 4.

Collaborate With a partner, think of at least two reasons why an organism might
compete with another organism of the same species for a limited resource. Explain why
two organisms would compete for these limited resources rather than share them.

Competition for limited resources in an ecosystem can be like a game of musical

chairs—not enough chairs are available for everyone and each chair seats only one
person. When the music stops, one person will be competitively blocked from the
chairs by the remaining players.

Stability and Change

In ecology, the principle of competitive exclusion states

FIGURE 5: These songbirds eat insects commonly found in
that when two species compete for the same resources,
spruce, pine, and fir trees.
one species will be better able to get the resources

Image Credits: (t) ©Cathy Keifer/Shutterstock; (c) ©RooM/10kPhotography/Getty Images

in the niche. The unsuccessful species will be pushed
into another niche or become extinct. The result is that
both species end up in distinctive niches so they do not
compete for the same limited resource.
Cape May
Blackburnian warbler
warbler
Analyze Use the diagram in Figure 5 to answer the
following questions.
1. What does this diagram show, and how does it relate
black-throated to the competitive exclusion principle?
green warbler
2. Suppose the tip of the spruce tree was broken off
myrtle
warbler during a wind storm. How might the birds be affected
bay-breasted
warbler by the loss of the uppermost niche in the tree?

198 Unit 4 Ecosystems: Stability and Change

 Symbiosis
Symbiosis is a close ecological relationship between two or more organisms of
different species that live in direct contact with one another. There are three major
types of symbiosis: mutualism, commensalism, and parasitism.

FIGURE 6: Symbiotic relationships

Mutualism
Mutualism occurs when both species Gather Evidence
benefit from the relationship. Pollination, What do the shrimp and the
in which an insect pollinates a plant, is fish each gain from this ecological
a common example of mutualism. Other relationship? Why doesn’t the fish
examples of mutualism include species eat the shrimp?
providing food or shelter, aiding in
reproduction, or providing protection
for one another. A shrimp cleaning
the mouth of a fish, shown at left, is an
example of mutualism.

Commensalism
Commensalism is a relationship
between two organisms in which one
organism receives an ecological benefit
from the other, while the other neither
benefits nor is harmed. A commensal
relationship between two species
might involve one organism providing
transportation or a home for the other
without harm or benefit to itself. As
shown at left, an egret eating the insects
Image Credits: (t) ©John A. Anderson/Shutterstock; (c) ©Ernie Janes/Alamy; (b) ©iStock/leonikonst/Getty Images Plus

stirred up by a cow as it moves and feeds

on grass is an example of commensalism.
The cow neither benefits nor is harmed
by the actions of the egret.

Parasitism
Parasitism is a relationship in which one
of the organisms benefits while the other
one is harmed. Unlike a predator, which
kills and eats its prey, a parasite benefits
by keeping its host alive for days or even
years. The needs of the parasite are met
by the victim of the parasite, called the
host. The host’s health often suffers due
to blood or nutrient loss. Galls made by
insects on the leaves of plants are an
example of parasitism, as shown here.

Explain How might symbiosis help the stability of an ecosystem? How might it hurt
ecosystem stability?

Lesson 2 Ecosystem Dynamics 199

 EXPLORATION 2

Biodiversity in Ecosystems

Coral reefs make up a small percentage of marine habitats, but contain most of the
FIGURE 7: Coral reefs are marine
oceans’ species diversity. The more diverse an ecosystem is, the more likely it is to
ecosystems where many
different species live. remain stable over the long term. If a disturbance, such as pollution or a fire, affects an
ecosystem, recovery can happen more quickly if that ecosystem has more biodiversity.

Biodiversity
The complexity of an ecosystem indicates its biodiversity level. Biodiversity refers
to the variety of species within an ecosystem. Note that biodiversity measures the
number of different species, not the number of individual organisms living in an area.
An area with a high level of biodiversity, such as a coral reef, has a large assortment
of species living near one another. Biodiversity depends on many factors, such as
moisture and temperature. The complex relationships in ecosystems mean that a
change in a single biotic or abiotic factor can have a variety of effects, both small and
large, on many different species.

Data Analysis

FIGURE 8: Ecologists analyze species richness (left) and species evenness (right) to evaluate ecosystem biodiversity.

Area 1 Area 2
40
Number of species

Area 1 Amphibians Amphibians

30 Reptiles Reptiles
Area 2
20 Birds Birds
Mammals Mammals
10

0
Amphibians Reptiles Birds Mammals Total

a Species richness b Species evenness

Image Credits: (t) ©Photographer’s Choice/Georgette Douwma/Getty Images

There are many different ways to measure biodiversity in an area. Two factors that
ecologists often use are species richness and species evenness. Species richness
is the number of species per sample of an area. Areas with a high number of
different species have high species richness and therefore high biodiversity. Species
evenness measures the abundance of different species that make up the species
richness. Species evenness considers the relative distribution of the numbers of
species in an ecosystem.

Explain Use the graphs in Figure 8 to answer the following questions:

1. What might happen if a new bird species arrived in Area 1?
2. How could this affect the species richness and species evenness in Area 1?
3. What conclusions can you draw about species richness and species evenness
between the two areas?

200 Unit 4 Ecosystems: Stability and Change

 Explore Online
FIGURE 9: Scientists have identified over 30 biodiversity hot spots around the world.

California Mediterranean Mountains of

Floristic Province Basin Southwest China

Tropical Andes Succulent Karoo Madagascar and

Sundaland New Zealand
Indian Ocean islands

A biodiversity hot spot is an area with a particularly high level of biodiversity. Figure
Analyze Biodiversity hot
9 shows a global map of biodiversity hot spots. These locations often contain species
spots are found around the
that are found nowhere else in the world. One hot spot located in North America is the
California Floristic Province, an area with a Mediterranean-like climate that is home to
world. Why can scientists not come
giant sequoia and coastal redwood trees. up with a single solution to protect
all of these areas?
Scientists are currently working to protect several biodiversity hot spots. Preserving
these areas helps to prevent species from going extinct and protects the ecosystem
as a whole. Maintaining as much biodiversity as possible makes the entire biosphere
healthier and provides a more stable habitat for plants, animals, and other species.
These areas also are important, because they may hold clues to new medicines and
new resources and may further our understanding of the biosphere.

Engineering

Building Artificial Coral Reefs FIGURE 10: Artificial reef

Earth’s coral reefs are critical for the stability of marine ecosystems. Unfortunately,
many are classified as threatened because of the effects of human activity. Living
Image Credits: (b) ©Zoonar GmbH/Dieter Möbus/Alamy

corals depend on the limestone deposited by their predecessors to get the minerals
necessary to build their own bodies. However, the limestone is being dissolved
from existing reefs due to increased ocean acidity caused by climate change. Marine
ecologists are now combating this destruction by sinking artificial reefs, such as
the one shown in Figure 10, which uses electrical currents to attract the limestone
deposits needed by growing coral.

Gather Evidence What is the relationship between biodiversity and ecosystem

stability? How do artificial reefs affect a marine ecosystem’s stability?

Lesson 2 Ecosystem Dynamics 201

 Keystone Species
Sometimes a single species has an especially strong effect on an entire ecosystem.
This species is called a keystone species. Whatever happens to this species affects all
the other species in that ecosystem. For example, when the beavers shown in Figure
11 built their dam across a stream, it turned a terrestrial ecosystem into a freshwater
ecosystem. This killed existing plants and forced land animals to move to new
territories. The new pond’s inhabitants rely on the beavers to maintain the dam. If the
beavers are removed, the dam will eventually fail. The pond will drain and over time
the land will return to a terrestrial ecosystem, such as a meadow.

FIGURE 11: Beavers are a keystone species that make and maintain pond ecosystems.

Collaborate With a
partner, discuss why
protecting a keystone species can
formation of
protect a habitat as a whole. wetland
ecosystem
increased
waterfowl
population

keystone species

increased
fish
population
nesting sites
for birds

FIGURE 12: Many farmers use Factors That Affect Biodiversity

pesticides to control insects and
weeds to increase the amount Many factors can reduce biodiversity. Human activities can reduce it very quickly.
of harvested crops. Humans need food, and much of that food comes from plants, which requires large
areas of land to be cleared to make fields to grow crops. Developing agricultural land
removes most of the native plant and animal species in a region and replaces them
with one or only a few species that are managed as crops. In addition, pesticide use
can negatively affect any remaining native organisms. Biodiversity also is lost when
land is cleared for human housing and industrial sites.
Image Credits: (l) ©Federico Rostagno/Shutterstock

Introduction of new plants and animals into ecosystems is another serious issue. These
species can reduce biodiversity by preying on native species or outcompeting native
species for resources, such as food or shelter.

Explain How does a natural phenomenon, such as the eruption of a volcano on an

oceanic island, affect biodiversity? Will the biodiversity that returns to the island be the
same as it was before?

202 Unit 4 Ecosystems: Stability and Change

 EXPLORATION 3

Disturbances in Ecosystems

An ecosystem is a complex web of relationships and interactions among organisms

FIGURE 13: A tsunami causes
in their environment. In general, an ecosystem can remain relatively constant over a
devastating flooding.
long time under stable conditions. However, a change in one or more of the biotic or
abiotic factors can disrupt the ecosystem and cause change. A change brought about
by a physical, chemical, or biological agent that impacts population size or community
structure is called a disturbance. Disturbances can occur over short or long time frames.
The type and size of the disturbance can affect how the ecosystem changes. For
example, a tsunami wave rapidly disrupts a coastal ecosystem by flooding habitats and
saturating soil with salt.

Analyze How might the carrying capacity of a coastal ecosystem change as the result
of a tsunami? Explain using one or more examples.

Natural Disturbances
Natural disturbances refer to the damage or destruction to ecosystems caused
by nature. Tornadoes, volcanic eruptions, and lightning-caused forest fires are all
examples of natural disturbances. These disturbances may affect only a small area. For
example, a tornado causes a natural disturbance in a relatively narrow path where it
touches down, while a forest fire or flood can cause natural disturbances that cover
many square miles.

Human-Caused Disturbances
People live in the environment, and many of our actions affect ecosystems.
Human-caused disturbances include human settlements, agriculture, air and water
Image Credits: (t) ©Agencja Fotograficzna Caro/Alamy; (b) ©Karin Hildebrand Lau/Shutterstock

pollution, clear-cutting forests, and mining. Like natural disturbances, human-caused

disturbances can affect both small and large areas. They destroy habitats, wipe out
producers, and contribute to a loss of biodiversity. However, some disturbances are
unique to humans, because the changes are more or less permanent. For example,
roads and highways can permanently fragment an ecosystem, changing the way
populations of species interact with their habitat and altering the way abiotic factors
cycle through an ecosystem.

FIGURE 14: Clear-cutting a forest means removing all the trees.

Collaborate With a
partner, discuss why
foresters might choose to clear-cut
a forest rather than use another
method to get wood for human
needs. What are the pros and cons
of clear-cutting?

Lesson 2 Ecosystem Dynamics 203

 Ecosystem Stability
Disturbances alter ecosystems, but if an ecosystem is relatively stable over time, it
can usually recover from a disturbance at a faster rate, adapting to or reversing any
changes. How well an ecosystem rebounds, however, is determined by two factors: its
resilience and its resistance.

Explore Online FIGURE 15: This old-growth forest has been stable for many years.
Language Arts
Connection
The Key(stone) to Ecosystem
Stability Prepare a presentation
describing the effects that your
chosen keystone species has on
ecosystem stability.

Analyze Old-growth forests have remained undisturbed for hundreds of years or more.
From what you see in Figure 15, what are some characteristics of a stable ecosystem?

Ecosystem Resilience

Explore Online Ecologists define ecosystem resilience as the ability of an ecosystem to recover after
it has undergone a disturbance. This means that even though the structure of the
Hands-On Activity
ecosystem is affected in some way, the ecosystem can recover quickly and return to
Simulating Fire in a Forest functioning as it did before the disturbance. For example, a grassland that has regular
Ecosystem Develop or use an fires is considered resilient, because the grasses quickly regrow and the animals return
already-existing simulation to very soon after a fire ends.
examine how fire affects forest The resilience of an ecosystem is determined in part by its level of biodiversity. A
species. How might prescribed complex ecosystem with many populations of species that perform the same function,
burns be used to manage the such as producers, is more resilient than one that has a limited number of species that
biodiversity in a forest, including perform each function. Consider two forests—one a single-species stand of mature
threatened or endangered species? pine trees and the other a multispecies stand of old and young conifers. If both stands
are impacted by identical severe wind events, the stand of mature pines will be more
severely affected by breakage and uprooting than the mixed stand. The mixed stand,
with its variety of wood characteristics and ages, will have more trees left after the
wind event. It will recover and continue to function as a forest much more quickly than
the singles-species stand of pines.
Image Credits: ©Zhukova Valentyna/Shutterstock

Biodiversity improves the resilience of an ecosystem, but only to a point. Genetic

diversity in each species in an ecosystem is also important. Human activities that alter
biodiversity or increase the rate of change, such as using pesticides and antibiotics,
fishing, and destroying rain forests, reduce genetic diversity. A reduction in genetic
diversity decreases the chance that populations can adapt to abiotic disturbances in
an ecosystem.

Predict What similarities would you expect to find in a highly resilient ecosystem?

204 Unit 4 Ecosystems: Stability and Change

 Ecosystem Resistance
Resistance is the ability of an ecosystem to resist change from a disturbance. Some
ecosystems are highly resistant to change while others have little resistance. Highly
resistant ecosystems remain essentially unchanged when a disturbance occurs.
Even the most resistant ecosystem can be stressed beyond its ability to recover. In the
past, the forests along the ridges of the Appalachian Mountains have recovered from
repeated wind, snow, and wildfire damage but are now slowly dying from the effects
of acid rain.

Resistance and Resilience in Ecosystems

FIGURE 16: Resistant ecosystems remain unchanged after a disturbance occurs, while
a resilient ecosystem quickly rebounds. This graph shows a simplified version of
how ecosystem function might respond to disturbances in resistant versus resilient
ecosystems.
Explain The concepts of
resistance and resilience
disturbance disturbance shown in the graph can be applied
resistant to other situations too. Thanks to
resilient
scientific advances and technology,
we now have many medicines to
Ecosystem function

treat diseases caused by pathogens.

Does this make humans more or
less resilient as a species? Does it
make humans more or less
resistant? Explain your reasoning.

Time

Resistant ecosystems initially show little impact caused by disturbances. However, if

disturbances become too intense, ecosystem structure and function may be severely
impacted. As shown in Figure 16, after a second disturbance, the example resistant
ecosystem is not able to recover as easily. A resilient ecosystem is often immediately
impacted by even low-intensity disturbances but can quickly recover structurally and
functionally to levels approaching the conditions before the disturbance occurred.
Image Credits: ©Dr. Jeremy P. Stovall

Gather Evidence Think back to the volcanic eruption on the island. Once the lava
cooled, plants began to grow. Is this an example of a stable ecosystem? Use evidence from
the discussion of resilient and resistant ecosystems to support your answer.

Lesson 2 Ecosystem Dynamics 205

 EXPLORATION 4

Ecological Succession

The area surrounding the Kilauea volcano on the island of Hawaii is a prime example
of what happens when an ecosystem undergoes a devastating disturbance. What
was once a lush tropical ecosystem is now covered in bare volcanic rock. Over time,
this new volcanic rock will undergo a series of changes. Ecological succession is the
sequence of biotic changes that restore a damaged community or create a community
in a previously uninhabited area. Two types of ecological succession occur: primary
and secondary.

Primary Succession

Analyze Where do Primary succession, shown in Figure 17, is the establishment and development of an
pioneer species come from? ecosystem in an area that was previously uninhabited, usually a bare rock surface.
Melting glaciers, volcanic eruptions, and landslides all begin the process of primary
succession. The first organisms that move into this area are called pioneer species.
These organisms, such as mosses and lichens, break down solid rock into smaller pieces.
Once pioneer species have made soil, plants such as grasses can begin to grow. Over
time, shrubs and trees replace the grasses to form a forest. This process continues until
a climax community is established.
Explore Online

Hands-On Lab FIGURE 17: It can take hundreds of years to establish a climax community. This diagram
shows the process of primary succession in a boreal forest.
Using GPS in Ecological
Surveys Perform a survey by
collecting and recording samples of
plant life from a given area of land.
Find and map their exact location
using GPS, and analyze the data.

a 0–15 years b 15–80 years c 80–115 years d 115–200

Moss, lichens, Shrubs, Transition to years
grasses cottonwoods, forest, alder, Hemlock-
alder thicket spruce spruce forest

Explain Do you think tall trees are the final stage of primary succession in every
biome? Explain your answer.

206 Unit 4 Ecosystems: Stability and Change

 Secondary Succession
Secondary succession is the reestablishment of an ecosystem in an area where the Model Make a model
soil was left intact, such as after a fire or flood. Because soil is already present in the that shows the difference
ecosystem, secondary succession reaches the climax community stage more rapidly between primary and secondary
than primary succession. The process of regrowth is begun by the plants, seeds, and
succession. Make sure your model
other organisms that remain after the disturbance occurs.
explains how long each step takes
As with primary succession, biodiversity of the ecosystem typically increases as and why.
secondary succession progresses. One reason for increased biodiversity is the return of
animals as the plant population grows. In addition, animals bring in seeds from plants
in other ecosystems on their fur and in their waste, which will establish new plant
populations if conditions are favorable for growth.

Data Analysis

FIGURE 18: The amount of species richness in an ecosystem is related to its stage of succession.

60
Number of woody plant species

50

40

30

20

10

0
0 50 100 150 200
Years

Analyze Think about how an ecosystem changes during secondary succession.

Refer to the graph in Figure 18 to answer the following questions.
1. When does species richness increase at the fastest rate? Why is this possible?
2. Why does the species richness not continue to rise over time? Explain.
Image Credits: ©Stephen Collins/Photo Researchers, Inc.

Succession is an ongoing process. Even after the climax community is reestablished,

changes continue to occur. Small disturbances, such as a tree falling, restart the
process. For this reason, an ecosystem is generally never really permanently
established—the processes of succession are always causing changes in an ecosystem.

Gather Evidence How does ecological succession affect biodiversity? Use evidence
gathered from this lesson to support your claims.

Lesson 2 Ecosystem Dynamics 207

 CONTINUE YOUR EXPLORATION

Language Arts Connection

FIGURE 19: A specially designed air tanker drops a load of fire retardant to slow the progress of a California wildfire.

Should Forest Fires Be Suppressed? Just five years later, a series of fires burned 3 million acres
over a three-state region. The “Big Blowup,” as it was called,
changed national thinking about fire management. State
Forest fires can cause considerable damage to forest
and Federal forest officials decided the best way to protect
ecosystems; therefore, wildland firefighters work hard to
the national forests was to completely suppress any and
contain and put out forest fires. They use heavy equipment,
all wildfires. To that end, policies were enacted that were
such as bulldozers, to stop the spread of wildfires. Sometimes
designed to stop fires completely when possible and put out
airplanes and helicopters carrying water or fire retardant are
any fire that did occur as rapidly as possible.
also used to put out the fires, as shown in Figure 19.
At the time, conservationists and foresters did not understand
Fire is a natural part of many ecosystems. It cycles nutrients
fire’s ecological importance to a forest ecosystem. They
back into the soil from plants. In some forests, shrubs growing
believed all fire was bad, because it damaged timber, an
underneath the trees are naturally removed by cyclically
economically important resource. As a result, they banned
occurring fires. In most cases, these fires leave the trees and
the use of fire to clear underbrush and improve soil. They also
other organisms living in the ecosystem unharmed. With
constructed roads, watchtowers, and ranger stations to make
increased efforts to prevent and stop forest fires, shrubs and
it easier to detect and reach any forest fire quickly.
other understory species grow thick. When a fire does occur,
it burns extremely hot and catches the trees on fire. This can In the 1930s, a firefighter corps was established that could be
have a catastrophic impact on the forest as a whole. sent anywhere a forest fire occurred. As technology advanced,
Image Credits: ©Shari L. Morris/Getty Images

airplanes and helicopters were added to the ground

After major forest fires in the late 1800s, early conservationists
equipment to drop firefighters and fire-suppression chemicals
became concerned about the effect of wildfire on future
wherever they were needed. Today, the National Interagency
timber supplies. In 1905, they convinced the United States
Fire Center (NIFC) coordinates and supports the deployment,
government to establish the U.S. Forest Service. This agency
training, and certification of firefighters, equipment, and
developed fire protection practices in an effort to conserve
support staff nationwide.
what came to be known as national forests.

208 Unit 4 Ecosystems: Stability and Change

 Through continued research, scientists found that fire can
FIGURE 20: Forest fires can cause significant economic damage to
actually be helpful to some ecosystems, and Forest Service cities and towns in their path.
officials began to realize that fire suppression led to a buildup
of fuel that made fires much more hazardous when they
did break out. This led to a change in policy that allowed for
prescribed burns to manage fuel loads in certain forests and
other wildlands, based on the ecological needs of the area.
How do officials decide where and when a wildfire should
be fought instead of being allowed to burn? Ecosystem
characteristics play a major role in these decisions. For
example, stands of Rocky Mountain lodgepole pines need
regular exposure to fires severe and intense enough to wipe
out the stand and allow a new one to grow in its place. Other
plants depend on fire as part of their reproductive strategies.
For example, the cones from sequoia trees need fire to open
and release their seeds. Fire also exposes bare soil where the
seeds can take root and opens the forest canopy, allowing
light to reach the seedlings, which helps them grow. On
the other hand, wildfires in zones near human populations
require active suppression to protect life and property. As
human development takes over what were once wild spaces,
the potential for widespread catastrophe increases.
Language Arts Connection
Climate affects fire management policy as well. Naturally Some policymakers think that natural wildfires should be
occurring events such as the yearly Santa Ana winds that blow
allowed to burn or that controlled burns should be used as a forestry
along coastal Southern California and northern Baja California
management tool. Others argue that the risk of letting fires burn or
contribute to the outbreak and spread of wildfires. Lightning
starting controlled burns pose a hazard to the forests and people.
strikes, heat waves, and droughts also increase the occurrence
of wildfires. Climate change is beginning to increase the
Select a position on whether or not to allow controlled burns.
severity of weather phenomena that contribute to wildfires. Research to learn about the pros and cons of controlled burns.
These fire events increase the amount of stored carbon Gather information and write a one-page position paper. Your paper
released into the atmosphere. All of these factors require should discuss your viewpoint and cite evidence from your research
officials to be flexible in their policy decisions.
to support your claims.
Lastly, cost figures into the development of fire management
After completing your research and writing your position paper, you
policies. Fighting wildfires is expensive in terms of hours
will take part in a classroom debate. In the debate, you will have an
worked, transportation, and equipment costs. Wildfires also
cause economic damage to communities and endanger lives.
opportunity to state and defend your position using the information
Officials must weigh these factors when determining whether you gathered in your research. Be sure to listen to the students who
Image Credits: ©EyeEm/Rob Lind/Getty Images

to practice fire-suppression policies. agree with your position and those who disagree as you make your
own arguments.

CASE STUDY: SECONDARY SUCCESSION IN THE KEY(STONE) TO Go online to choose one of

SUCCESSION AT MT. ST. HELENS DIFFERENT ECOSYSTEMS ECOSYSTEM STABILITY these other paths.

Lesson 2 Ecosystem Dynamics 209

 EVALUATE

Lesson Self-Check

CAN YOU EXPLAIN IT?

FIGURE 21: A lone seedling begins the process of colonizing a field of lava.

Volcanic eruptions play an important role in the formation of new ecosystems, but
the resulting lava flows leave behind a hard rock surface that cannot support life.
Nevertheless, living things will gradually begin to grow and thrive on this rock surface
as it undergoes chemical and physical weathering. Over time, the bare rock will no
longer be visible as it becomes covered in soil and plant life.
The Hawaiian Islands began to form more than 70 million years ago following volcanic
eruptions in the middle of the Pacific Ocean. As time passed, the process of succession
created unique tropical ecosystems. Succession from bare rock to highly diverse
vegetation takes a great deal of time. When new eruptions occur, the process of
succession begins again, and eventually a stable ecosystem returns.

Explain Refer to the notes in your Evidence Notebook and use what you learned in
Image Credits: ©Design Pics/Getty Images

the lesson about succession to explain how a plant is able to grow in the middle of a
lava field.

210 Unit 4 Ecosystems: Stability and Change

 CHECKPOINTS

Check Your Understanding 7. Ecosystem A and Ecosystem B have the same eight
species, but Ecosystem A has a more even distribution
1. Which of the following is a characteristic associated with of species than Ecosystem B. Which ecosystem is more
an organism’s niche but not with its habitat? diverse? Explain your reasoning.
a. climate
b. soil quality 8. Top predators are often keystone species in their habitat.
c. place in the food web Explain what happens to the biodiversity of an ecosystem
d. location within the ecosystem when a top predator is deliberately removed from the
ecosystem in which it lives.
2. Which of the following are factors in determining the
stability of an ecosystem? Choose all that apply.
a. the process by which it recovers after a disturbance
b. the ability to function during a disturbance
c. whether a disturbance is natural or human-made MAKE YOUR OWN STUDY GUIDE
d. the rate of recovery after a disturbance
e. the level of biodiversity in the ecosystem In your Evidence Notebook, design a study guide that
supports the main idea from this lesson:
3. An epiphyte is a plant that grows on the surface of
another plant, such as a tree. It gets water and nutrients Within an ecosystem, organisms interact with each other
from the air and its surroundings instead of from the tree. and with their environment. The stability of the ecosystem
The tree is unaffected by the epiphyte’s presence. What is determined by its biodiversity, resilience, and resistance
type of relationship does the epiphyte have with the to change.
tree? Explain your reasoning.
Remember to include the following information in your
study guide:
4. Whenever Paramecium aurelia and Paramecium caudatum
• Use examples that model main ideas.
are placed into the same culture and given a constant
supply of food under constant conditions, P. aurelia will
• Record explanations for the phenomena you investigated.
always outcompete P. caudatum, which eventually dies • Use evidence to support your explanations. Your support can
off. What factors prevents P. caudatum from surviving in include drawings, data, graphs, laboratory conclusions, and
this situation? other evidence recorded throughout the lesson.
Consider how ecosystem interactions can be used to analyze
5. Ecological succession after a disturbance usually takes
ecosystem dynamics and predict how conservation efforts will
hundreds of years in the Pacific Northwest. However,
affect the stability of these ecosystems.
succession after the Mount St. Helens eruption in 1980
has progressed much more rapidly, because some plants
and animals were in protected areas when the hot ash
and pumice fell. What conclusion can you draw about the
pace of succession from this example?

6. Use the following terms to complete the statement:

resilient, resistant
If an ecosystem is , it is generally stable
unless drastically changed by a disturbance. When a
disturbance causes a change, the ecosystem quickly
recovers when it is .

Lesson 2 Ecosystem Dynamics 211

 A BOOK EXPLAINING
COMPLEX IDEAS USING
ONLY THE 1,000 MOST
COMMON WORDS

HOW FORESTS COME BACK

How trees and flowers and animals fill
in the land again after a big change
After a disturbance in an ecosystem, biotic changes
regenerate the damaged community or create a new
community in a previously uninhabited area. Take a look RANDALL MUNR
OE
at this process of change and rebirth. XKCD.COM

212 Unit 4 Ecosystems: Stability and Change

THINGS THAT HAPPEN TO FORESTS
Sometimes, big things happen to forests that clear out lots of the old trees and
animals that live there. When these things happen, lots of new trees and fl owers
come in to fi ll in the space. After a while, big trees can grow back.
Here are a few of the things that can happen to forests:

FIRE PEOPLE WIND GETTING EATEN

In some forests, there are People cut down forests When there’s a really Most animals that eat
fires every so often that to make room for stuff or big storm, wind can trees get eaten by other
burn lots of the plants because they want to use push over lots of trees, things. If something that
and trees, along with the wood. Most of Earth’s especially if the storm eats trees moves to a new
dead sticks and leaves old forests have been cut happens when the trees part of the world where
lying on the ground. down over the years. have leaves on them. nothing eats it, it can eat
whole forests.
TREE-
EATING
ANIMAL

SMALL HOLES IN THE FOREST

Some forests have little animals that like to build their The water covers part When the water in the pool
houses in pools. If they can’t fi nd a pool to use, they of the forest, and the goes away, it leaves an
make a new one by cutting down trees and building trees that are covered open area among the trees.
a wall across the river. with water die. When the Over the years, this area
animals move away, the fills with green things and
wall falls apart. becomes a forest again.
RIVER
HOUSE WALL

WATER

BIGGER PROBLEMS
Life doesn’t just come from nowhere. When forests But if there’s a really big change, there might not be
are cleared and grow back, some of the new trees any life left in the area to grow back. Sometimes, on
grow from parts in the ground that are still alive. Other land out in the sea, hot fi re comes out of the ground
green things move in from the edges or are carried by and the rocks get hot and run like water. If this
the wind or birds. happens, nothing grows back until new life is carried
there from across the sea.

Unit 4 Thing Explainer 213

 A FOREST AFTER A FIRE
WHAT STARTS FIRES? SPACE ROCKS
Many forest fires are caused by people. Sometimes, people Big rocks falling from space can
drop burning things on the ground and forget about them, start fires, but that doesn’t happen
or they start fires to sit around and then don’t put them out. very often. As far as we know, there
Other fires start without help from people. Most of those are haven’t been any of these fires
caused by flashes of power from big storms, but some are caused since people started writing down
by hot rocks that come out of holes in the tops of mountains. things that happened.

FIRE STARTER
Fires aren’t usually started
by people who are bad at
fl ying space boats, but it
could happen!

If there are a lot of dry sticks or dead

branches on the ground, fi res can get big
and hot. These fires can spread to the tops of
These small fi res can trees and burn down forests.
be good for a forest,
Some fires burn the dead because they burn away If it goes a long time
leaves and sticks on the the leaves and sticks without raining, all the
ground but don’t really before too many of them dry stuff makes fires
bother the big trees. pile up. bigger and hotter.

BABY TREES
(INSIDE)
TREE EGGS Fires clear away the tall trees that block light from the
These things fall off ground. Some trees make tree eggs that stay closed
of trees. Then they open until there’s a fire, so they can get lots of light and
up and new trees grow grow quickly.
out of them. These tree eggs are covered in a layer of clear stuff
that keeps them from opening up. When a tree egg
gets hot in a fi re, the clear stuff gets soft and falls off,
the egg opens up, and the tree starts to grow.
SOFT
STUFF
TREE EGGS
ON THE
GROUND

214 Unit 4 Ecosystems: Stability and Change

 Go online for more
about Thing Explainer.

TREES THAT DON’T MIND FIRE GREEN THINGS RETURN

Some trees are good at living The first things to grow TREE FIGHTS
through fires. Their strong, thick up after a fire are grasses Some trees try to
skin keeps them safe from heat, and fl owers, like the be the first to grow
and some keep most of their things you pull up from up in the clear
branches near the top to keep your garden. They’re space after a fire.
their leaves away from the good at spreading and If a tree grows
burning stuff on the ground. grow very fast. taller than the ones
around it, it can
block the sun’s
BIRDS DROPPING HIDING TREES
light from the other
TREE EGGS AS THEY Some trees and green things live trees and keep
FLY AROUND through fire even if their top parts more light for itself.
burn down. They can grow back up
out of the parts left in the ground.

FAST TREES BIRDS THAT EAT SMALL ANIMALS

In the first twenty or thirty Some birds that eat small animals like to fl y over open fields like this or sit in trees
years after the forest is near the edge. When they see something running in the grass, they try to catch it.
cleared away, fast trees Since these birds like areas where forests meet open areas, they’re often spotted
grow up. They block the in trees by the side of big roads.
sun’s light from reaching
the ground, which makes BIRDS THAT EAT OTHER BIRDS (HIDING)
the grasses and small Different animals like different kinds of forest. In some areas, as the trees
plants die off. These are get bigger, different kinds of birds move in. Some birds are good at fl ying
young forests. through trees to catch other birds. Since these birds usually stay away
from the edges of forests, people don’t see them as often.

SLOW TREES OLD FORESTS

After the first trees grow up, new kinds Slow trees grow up and take the place of the fast trees.
of trees start to grow. These newer This takes a very long time—longer than a person’s life.
trees grow slowly, and they don’t need Forests that are many times older than the oldest humans
as much sun, so they can grow up in are special. They have different kinds of trees and
the shadows of the faster trees. animals than young forests have. Many of those older
forests have been cut down, and some people are trying
to save the ones that are left.

Unit 4 Thing Explainer 215

 UNIT CONNECTIONS

Environmental Science Connection FIGURE 1: Feral hogs can change

ecosystems.
Feral Hogs Feral hogs are descendants of domesticated hogs that escaped
captivity. Feral hogs are able to survive in many ecosystems due to the lack of
natural predators and their diverse foraging habits. Feral hogs change abiotic
factors in ecosystems, such as soil structure and levels of erosion. They also change
biotic factors in ecosystems, such as disrupting native plant communities and
competing for resources with organisms in similar niches.

Using library and Internet resources, research the feral hog epidemic in the United
States. Create a public service announcement that will inform landowners about the
scale of the feral hog epidemic and the potential impacts to the environment, including
changes to populations and ecosystems.

Art Connection

Image Credits: (t) Photo provided by NASA/U.S. Dept of Agriculture (USDA); (c) ©National Geographic Magazines/RALPH LEE HOPKINS/Getty Images; (b) ©Diego Grandi/Shutterstock
FIGURE 2: This photograph could be
Conservation Photography Have you ever heard the saying, “A picture is worth a
used to highlight the impact of global
thousand words”? Conservation photographers embrace this statement when they
warming on polar bear populations.
use pictures to highlight environmental problems. Candid and staged images are
used to invoke a response in the public and to advocate for conservation outcomes.
When devastating changes in ecosystems are documented in visual ways, it can
strengthen public support and involvement in critical environmental issues.

What is the difference between nature photography and conservation photography?

Using library and Internet resources, research the field of conservation photography
and the work of a particular conservation photographer. Prepare a multimedia presentation
that explains the purpose of conservation photography and introduces a photographer you
researched, selections of his or her work, and explanations of the conservation issues that
the photographer highlights.

Social Studies Connection

Environmental History Human impacts on the environment during the last several FIGURE 3: Ruins of the Mayan
centuries have been extensively studied and documented. It is clear that humans Civilization.
have changed and destabilized many modern ecosystems. There is also evidence
that ancient peoples, such as the Maya, the Nazca, and the Rapa Nui of Easter
Island, changed the landscape in dramatic ways. These changes may not have
been on the scale of modern human impacts, but the changes may have resulted in
destabilization of ecosystems that led to the downfall of these civilizations.

Using library and Internet resources, research an example of an ancient society

whose collapse may be linked to human impacts on the environment. Write a report
that provides a brief background on the civilization and evaluates the claims and evidence
that the environmental changes led to the disappearance of the society. Include an
illustration of the potential impacts the society had on the environment with a model,
graph, map, or other method.

216 Unit 4 Ecosystems: Stability and Change

UNIT PRACTICE AND REVIEW

SYNTHESIZE THE UNIT DRIVING QUESTIONS

Look back to the Driving Questions from the opening section

In your Evidence Notebook, make a concept map, of this unit. In your Evidence Notebook, review and revise
graphic organizer, or outline using the Study Guides you your previous answers to those questions. Use the evidence
made for each lesson in this unit. Be sure to use evidence to you gathered and other observations you made throughout
support your claims. the unit to support your claims.

When synthesizing individual information, remember to follow

these general steps:
• Find the central idea of each piece of information.
• Think about the relationships between the central ideas.
• Combine the ideas to come up with a new understanding.

PRACTICE AND REVIEW

1. Over the course of a year, a population of 25 penguins 4. What is the relationship between population number and
gained 5 penguins through births and lost 3 penguins carrying capacity in a stable population?
to death. In addition, 10 adult penguins moved into the a. The population number matches the carrying capacity
population through immigration and 6 adult penguins exactly through births, deaths, immigrations, and
moved out of the population through emigration. What is emigrations.
the yearly growth rate of this population? b. The population number oscillates around the carrying
a. 19 capacity as resources and population growth rates
b. -8 change slightly over time.
c. -2 c. The carrying capacity and population number increase
d. 6 when resources are scarce and decrease when
resources are abundant.
2. Which of the following would result from the expansion d. Carrying capacity and population numbers are
of a species into a new habitat with no predators and inversely proportional. An increase in carrying capacity
evenly spaced resources? Select all correct answers. will be accompanied by a decrease in population
a. logistic growth numbers.
b. uniform dispersion
5. An invasive species moves into three niches that were
c. exponential growth
once occupied by three different native species and
d. type III survivorship
outcompetes the native species, producing larger
e. partial relief from density-dependent limiting factors population numbers than the three native species
combined. What happens to the biodiversity of the
3. Which type of interaction is most responsible for energy ecosystem?
transfer in a food chain? a. Biodiversity increases because the number
a. competition of individuals increases.
b. mutualism b. Biodiversity decreases because only native species
c. parasitism count toward biodiversity in an area.
d. predation c. Biodiversity remains the same because the same
ecological niches are still being filled.
d. Biodiversity decreases because the number of species
decreases.

Unit 4 Unit Closer 217

 UNIT PRACTICE AND REVIEW

Ecosystem A is resistant to periodic small-scale floods. 9. What type of ecosystem would be most affected by a
Ecosystem B displays resilience to small and large floods. A sequence of widespread, heavy rainfall that leads to
small flood occurred as a disturbance in both ecosystems, significant regional flooding over the course of several
followed by a larger flood. The graph indicates the general months?
reaction of the two ecosystems to the disturbances. a. resilient ecosystem
Use Figure 4 to answer Questions 6–8. b. resistant ecosystem
c. both resilient and resistant ecosystems
Ecosystem Resilience and Resistance d. neither resilient nor resistant ecosystems
FIGURE 4: Resistance and Resilience in Ecosystems
10. A limiting factor keeps population size down and can be
density-dependent or density-independent. Which of
disturbance disturbance
these is the best explanation for why a disease outbreak
resistant
resilient is considered a density-dependent limiting factor?
a. Disease will only affect population size if the
population has a very low density.
Ecosystem function

b. Disease will spread at the same rate throughout the

population, regardless of population density.
c. Disease spreads more readily in a population with
closely packed individuals.
d. Disease will not spread if individuals are evenly
dispersed in their environment.

Time

Source: ©Dr. Jeremy P. Stovall UNIT PROJECT

6. How does succession relate to the decrease in ecosystem Return to your unit project. Prepare your research and
function shown in the graph? materials into a presentation to share with the class. In
a. Decreases in ecosystem function represent the your final presentation, evaluate the strength of your
ecosystem being reset to an earlier successional state. hypothesis, data, analysis, and conclusions.
b. Decreases in ecosystem function represent the Remember these tips while evaluating:
ecosystem progressing through succession back
toward a climax community. • Was your hypothesis supported by your data?
c. Climax communities cause decreases in ecosystem • Look at the empirical evidence gathered from your
function as types of species and population numbers wetland model—evidence based on observations
stabilize. and data. Does the evidence support your claim
and reasoning regarding the impact of wetlands
d. Succession ends when ecosystem function decreases.
on populations and ecosystems?
• Consider if the evidence and explanation are
7. Which ecosystem would experience more periods of
logical. Does your research contradict any evidence
exponential growth after disturbances? Explain your
you have seen?
answer.

8. Which characteristics would make Ecosystem B more

stable? Select all correct answers.
a. resilient tertiary consumers
b. resilient primary producers
c. high level of biodiversity
d. early successional state

218 Unit 4 Ecosystems: Stability and Change

UNIT PERFORMANCE TASK

Analyzing Red Squirrel Population Dynamics

The Mount Graham red squirrel (Tamiasciurus hudsonicus
FIGURE 5: Average population estimates for Mount Graham
grahamensis) is an endangered red squirrel subspecies that is
red squirrels, 1987-2010.
endemic to the Pinaleño Mountains in southeastern Arizona.
Population data for this squirrel is shown in Figure 5. Use Average Average
this information and independent research to determine Year population estimates Year population estimates
population trends for the red squirrel. Investigate whether the
1987 242 1999 530
red squirrel habitat is declining and what natural or human-
caused disturbances may be responsible for fluctuations 1988 202 2000 484
in the red squirrel population. Based on your investigation,
1989 174 2001 270
decide whether you think the Mount Graham red squirrel
population is resistant or resilient to disturbance. 1990 275 2002 292
1991 391 2003 293
1. ASK A QUESTION
1992 332 2004 276
Develop a set of questions to help guide your research and
data analysis. Focus your inquiry on population trends for the 1993 375 2005 289
Mount Graham red squirrel, how those trends relate to habitat 1994 419 2006 285
loss, and how the causes of the habitat decline are affecting
the red squirrel population. 1995 407 2007 305
1996 381 2008 273
2. CONDUCT RESEARCH 1997 392 2009 259
Investigate the Mount Graham red squirrel population. Use
1998 566 2010 216
library and Internet resources to explore how this species has
fared over the last half-century. Source: U.S. Fish and Wildlife Service. 2011. Draft Recovery Plan for the Mount Graham Red Squirrel
(Tamiasciurus hudsonicus grahamensis), First Revision. U.S. Fish and Wildlife Service, Southwest
Region, Albuquerque, NM. 85 pp. + Appendices A-D. [September 30, 2016] https://www.fws.
gov/southwest/es/arizona/Documents/SpeciesDocs/MGRS/MGRS_dRecov_Plan_Revision_Final_
3. ANALYZE DATA
May2011.pdf
Analyze your research and the population data provided.
Graph the population data in order to visualize the red
squirrel population trends. Is there evidence of disturbances, CHECK YOUR WORK
ecosystem decline, or the resilience or resistance of the
squirrel population?
A complete presentation should include
the following information:
4. CONSTRUCT AN EXPLANATION
Use your analysis to answer your questions and construct an
• guiding questions answered in the final presentation
explanation for the changes in the population of the Mount • a graph that shows the population of Mount Graham
Graham red squirrel and its habitat. red squirrels over time
• an explanation of the current status of the squirrel
and its habitat, a discussion of disturbances that
5. COMMUNICATE
affected the squirrel population, and if the squirrel
Present your findings about the Mount Graham red squirrel population has shown resilience or resistance to
and its habitat. Be sure to include whether you think the disturbance
squirrel population is resilient or resistant to disturbance. Your
• images and data that further support your
presentation should include images and data to support your
explanation
claims.

Unit 4 Unit Closer 219