POPULATION ECOLOGY

Population Ecology
▪ Population ecology is the study of 2. Immigration from external populations
populations in relation to the e.g. Bird migration.
environment. It includes environmental Factors that reduce population size
influences on population density and 1. Mortality which is the death rate from
distribution, age structure, and any source e.g. predation.
variations in population size. 2. Emigration, where individuals leave the
Why do we need to study Population ecology? population for another habitat.
Scientific goal
1. understanding the factors that influence the Other Factors that Affect Population:
size of populations Abiotic factors
2. general principles and specific ▪ sunlight & temperature
cases Practical goal ▪ precipitation / water
1. management of populations ▪ soil / nutrients
increase population size Biotic factors
endangered species • predators prey (food)
decrease population size • competitors
pests • , parasites,
2. maintain population size disease
fisheries management Intrinsic factors
maintain & maximize sustained yield • adaptations

Population 3. Population Density

group of individuals of same species in The number of individuals per unit area
same area at same time
Density = number of individuals
Characteristics of Population area (units2)
1.Geographic distribution
area that is inhabited by the Variation in density depends on:
population; the range of the population • the species
Geographical limitations: • the ecosystem
abiotic & biotic factors Practice Problem
temperature, rainfall, food, predators
habitat If scientists sampling a population of deer
counted 200 individuals in an area of 10 square
2. Size
kilometers, what is the density of this deer
▪ Changes to population size can occur
population per square kilometer?
by:
▪ Birth
A: 20 deer per square kilometer
▪ Death
▪ Immigration Measuring Density
▪ Emigration
1. Total Count Method
Natality
The birthrate, which is the ratio of total live ▪ Direct counting of populations
births to total population in a particular area ▪ Possible for few animals
over a specified period of time. ▪ Breeding colonies can be photographed
Mortality then later counted
▪ The death rate, which is also the ratio Examples
of the total number of deaths to the
▪ Human population census
total population.
Immigration ▪ Trees in a given area
2. Sampling Method
▪ The number of organisms moving into
area occupied by the population is ▪ Depends on the type of organism and its
called immigration. natural abundance and distribution. Two
Emigration broad categories:
▪ The number of organisms moving out of 1. Plot-based (quadrat) methods
the area occupied by the population is
called emigration.
Total population = Average number per
Factors that increase population size 1.
Natality is recruitment to a population quadrat × Total area/area of quadrat
through reproduction.
2. Capture-based methods 5. Growth rate
Ex. Mark –Recapture Method The amount by which a population’s
size changes in a given time.
▪ Used for very mobile or elusive species ▪
A measure of the speed of reproduction
First used in the 1890s by C. G. Peterson to
estimate fish abundance. Factors affecting population growth rate
N = MC 1. sex ratio
R how many females vs. males?
N= no. of population 2. generation time
at what age do females reproduce?
M= Marked
3. age structure
C= Captured #females at reproductive age in cohort?
R= Recaptured
Life Table
4. Population Dispersion ▪ A life table (mortality table ) is a table
• Spacing patterns within a population which shows, for each age, what
probability does a person of certain age
Types of Dispersion: will die before his or her next birthday.
1. Clumped dispersion From this starting point, a number of
▪ Individuals are clustered together or inferences can be derived:
aggregated in patches ❑ The probability of surviving any
▪ Most common pattern of dispersion particular year of age
Occurs: ❑ Remaining life expectancy for
▪ When resources (food, water, living people at different ages
spaces) are clumped together Separate for men and for women
▪ Species have a certain social behavior like because of their substantially
mating; living in group increases the different mortality rates.
effectiveness of hunting ; spreads the
work of protecting and caring for their Survivorship Curve
youngs and help excludes other ▪ Graph showing the number or
individuals from their territory (i.e. in proportion of individuals surviving at
wolves) each age for a given species or group
For example: herding animals, flocks of (e.g. males/females).
birds, schools of fish, hives of bees ▪ Graphic representation of life table ▪
Constructed for a given cohort (a group of
2. Uniform Dispersion individuals of roughly the same age) based
▪ Evenly spaced distributions, in which on a life table.
members of the population maintain a ▪ Three types
minimum distance from one another. Reasons: 1. Type I 2. Type II 3. Type III
▪ Limited resources, competition,
nesting, social interaction
▪ In plants due to competition for water,
sunlight, or available nutrients
(i.e. Creosote bushes in the Mojave
desert)
▪ In animals due to strong territoriality
(i.e. The desert lizard Uta sp)
3. Random Dispersion
▪ It is a spacing pattern based on
total unpredictability.
▪ Location of one individual is
independent of the location of the The relatively straight lines of the plots
other individuals indicate relatively constant rates of death;
▪ Least common pattern of distribution however, males have a lower survival rate
Reasons overall than females.
▪ Members of a species do not frequently
interact with one another Survivorship Curve
▪ Not heavily influenced by the
microenvironments within their
habitat Ex: Seed dispersal by the wind or
animals
 other large mammals that produce few
offspring but provide them with good care
often exhibitthis kind of curve. In contrast, a

Type III curve drops sharply at the start,

reflecting very high death rates for the
young, but then flattens out as death rates
decline for those few individuals that have
survived to a certain critical age. This type of
curve is usually associated with organisms
that produce very large numbers of offspring
Type I: High death rates in post but provide little or no care, such as
reproductive years (i.e. humans) long–lived plants, many fishes, and marine
Type II: Constant mortatility rate invertebrates. An oyster, for example, may
throughout lifespan (i.e. hydra) release millions of eggs, but most offspring
Type III: Very high early mortality rates but die as larvae from predation or other causes.
the few survivors live long and stay active in Those few that survive long enough to
reproduction (i.e. octopus) attach to a suitable substrate and begin
growing a hard shell will probably survive for
Reproductive Strategies a relatively long time.
• K-selected
Type II curves are intermediate, with a
o late reproduction
constant death rate over the organism’s life
o few offspring
span. This kind of survivorship occurs in
o invest a lot in raising offspring
Belding’s ground squirrels and some other
▪ primates
rodents, various invertebrates, some lizards,
▪ coconut
and some annual plants.
• r-selected
Population Growth
o early reproduction
▪ Refers to how the number of individuals
o many offspring
in a population increases (or
o little parental care
decreases) with time.
▪ insects
▪ many plants ▪ Controlled by the rate at which new
individuals are added to the
population -- the birth rate, and the
rate at which individuals leave the
population -- the death rate.
Population Growth Math
▪ Change in population = Births – Deaths
▪ Per capita birth rate = b
▪ Per capita death rate = d
▪ # of individuals = N
▪ Rate of population growth (r) = b – d
▪ Survivorship = % surviving
Ex: If there are 50 deer in a population, 13
die and 27 are born the next month. What is
the population size the following month?
(Answer: 27-13 = 14, so new population is
Survivorship Curve with Reproductive Strategy 64) Ex: What is the birth rate for the deer?
#Births/N = b
Answer: 27/50 = .54
Death rate (d) = 13/50 = .26
Ex: What is the rate of growth for the
deer? r = .54 -.26 = .28

Population Growth Rate Models

• Exponential growth
o Rapid growth, growth is at
maximum rate
o No constraints/ no
A Type I curve is flat at the start, reflecting environmental barrier
low death rates during early and middle life, • Logistic growth
then drops steeply as death rates increase o Environmental constraints
among older age groups. Humans and many o Limited growth
 African elephant protected from hunting

Exponential Growth
▪ If a population has a constant birth rate
through time and is never limited by
food or disease, it has what is known
as exponential growth.
▪ With exponential growth the birth rate
alone controls how fast (or slow) the
population grows.
▪ Continuous population growth in an Logistic Growth
unlimited environment can be ▪ As resources are depleted, population
modeled exponentially. growth rate slows and eventually
dN / dt = rmax N stops: logistic population growth.
▪ As population size (N) increases, rate of ▪ Sigmoid (S-shaped) curve
population increase (dN/dt) gets ▪ Carrying capacity (K) is the number of
larger. individuals of a population the
environment can support.
▪ For an exponentially growing ▪ Finite amount of resources can only
population, size at any time can be support a finite number of
calculated as: individuals.
Nt = Noert
Nt = number individuals at time t. Logistic Population Growth:
N0 = initial number of individuals.
e = base of natural logarithms. dN/dt = rmaxN(1-N/K)
r (rmax ) = per capita rate of increase.
▪ rmax = maximum per capita rate of
t = number of time intervals.
increase under ideal conditions.
▪ K= carrying capacity
▪ When N nears K, the right side of the
equation nears zero.
▪ As population size increases, logistic
growth rate becomes a small fraction of
growth rate.
• Highest when N=K/2

Logistic Growth Equation

Whooping crane coming back from near dN/dt = rmaxN(K-N)/K

K = carrying capacity of population

Ex: If a population has a carrying capacity of 900

and the rmax is 1, what is the population growth
when the population is 435?
1 x 435 (900-435)/900 = 224
What if the population is at 850? 1010?
extinction
Carrying Capacity
▪ Maximum population size that
environment can support with no
degradation of habitat
▪ varies with changes in resources
 ▪ Demographic processes that affect the
increase or decrease in population
growth are fertility/natality, mortality,
and migration.

▪ Fertility refers to the actual production

of offspring. It depends on various
factors like physical health and
nutrition, sexual behavior, culture,
emotions, economics, way of life, etc.
Thus, fertility rates vary among
countries and cultures. The birth rate is
an issue of concern for many
governments and policymakers. Some
seek to increase the national birth rate
while other countries have policies to
reduce the birth rate.

▪ Mortality rate is a measure of the

number of deaths in a particular
population depending on various
factors like age and gender distribution.
A life table is a statistical tool that
summarizes the number of deaths of a
population. It yields information about
longevity and gives a reasonable
estimate of life expectancy. Life
expectancy refers to the average
Exponential Growth vs Logistic Growth
survival time for an individual.
Limits to Population Growth
▪ Environment limits population growth ▪ Migration/Immigration is the physical
by altering birth and death rates. movement of people from one place to
1. Density-dependent factors another. It is divided into two groups of
Disease, Parasites, Resource Competition • factors: push and pull. The push factors
Populations do not show continuous are things that an immigrant is
geometric increase unfavorable about the area he is
• When density increases other coming from, while pull factors interest
organisms reduces the fertility and the immigrant in the new place.
longevity of the individuals in the
population. This reduces the rate of Population Pyramid
increase of the pop until eventually the ▪ Age structure diagram
population ceases to grow ▪ Graphical illustration that shows the
• The growth curve is defined as the distribution of various age groups & sex
sigmoid curve, S – shaped ratio in a population.
• K = carrying capacity (upper asymptote Three age categories:
or maximum value) – the maximum 1. Prereproductive (ages 0-14)
number of individuals that environment 2. Reproductive (ages 15-44)
can support 3. Postreproductive (ages 45 and up)
Age Structure
2. Density-independent factors
• Natural disasters
• Climate

Demography
▪ It is the statistical analysis of the human
population, which encompasses the
size, distribution, structure, and
changes in the population that occur
over time.
▪ Population structure is the proportion of ▪ Relative number of individuals of each
people by age, sex, ethnicity, age
education, and other parameters.
Human Population Growth
For most of human existence, the population
grew very slowly. There were many limiting
factors that kept the human population low:
food was not always readily available.
diseases were rampant.

About 500 years ago, the human population

began to grow at a staggering rate. Reasons for
this include:
• The beginning of agriculture and
industry made life much easier and
much safer.
• Food is available on a regular basis.
• Goods can be shipped around the
world.
• Improved sanitation and living
conditions eliminated the high levels of
diseases.
• Death rate dropped while birth rate
increased.
World Population

Share of the World Population

World Population distribution