CHAPTER 8 POPULATION ECOLOGY
THE WOLF
WATCHERS
Endangered gray wolves return to the
American West
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THE WOLF WATCHERS
Endangered gray wolves return to the American West
At the end of this unit you will
know:
• The ecological significance of a
population
• How and why populations change
• Different approaches to reaching
carrying capacity
• The role of predators in regulating
populations
• Ways of applying population
dynamic to conservation and
ecosystem management decisions
Learning Outcomes
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THE WOLF WATCHERS
Endangered gray wolves return to the American West
Main
concept
Population size and makeup can
fluctuate or remain stable.
Population stability is often
dependent on predators. When
human impact results in reduced
predation, we may need to
manage the system ourselves.
Case Study
Yellowstone Gray Wolf
Restoration Project
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THE WOLF WATCHERS
Endangered gray wolves return to the American West
Case Study – Yellowstone Gray Wolf Restoration
Project
History of decline – Humans hunted wolves,
destroyed habitat by conversion for agriculture, and
hunted wolves’ food sources—elk, deer, and bison.
Wolf populations in Yellowstone had also declined as
a result of predator control programs.
Protected under the Endangered Species Act of 1973
In 1987, 41 wolves were
reintroduced and outfitted with radio
collars allowing researchers to track
the size of wolf packs, their food
sources, and movement patterns.
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THE WOLF WATCHERS
Endangered gray wolves return to the American West
Populations, all the individuals of a species living
together in the same area, fluctuate naturally over
time.
Populations increase with birth and immigration and
decrease with death and emigration.
Response is often based on access to food, water,
nesting sites, and predation.
Population dynamics can be predictable or more
random.
Information from tracking helps aid recovery and plan
conservation strategies for wolves and for many other
threatened plants and animals.
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Populations fluctuate in size and have varied distributions
TERMS TO KNOW:
Population
Population dynamics
Minimum viable
population
Below a minimum number
of individuals, a species may
not be viable long term.
Courtship rituals,
flocking, schooling, foraging,
and genetic variability are
often dependent on
population size.
Doug Smith,
Population
Biologist
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Populations fluctuate in size and have varied distributions
Successful population
density varies by species.
Too low and individuals
may not be able to find
mates or only mates that
are closely related.
TERMS TO KNOW:
Environmental impact
statement
Population density
Population distribution
Clumped distribution
Random distribution
Uniform distribution
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Populations fluctuate in size and have varied distributions
Successful population
density varies by species.
Too low and individuals
may not be able to find
mates or only mates that
are closely related.
In a population that is too
dense, competition,
fighting, and disease can
become problems.
Location and spacing of
individuals within a
population may be
influenced by a variety of
factors.
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Populations fluctuate in size and have varied distributions
Social species such as
wolves, elk, and prairie
dogs provide examples of
clumped distribution.
Individuals are found in
groups within the habitat.
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Populations fluctuate in size and have varied distributions
Individuals are spread out
irregularly.
Random distribution –
Species that disperse
randomly in an environment
like wind-blown seeds that
germinate where they land
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Populations fluctuate in size and have varied distributions
Individuals spaced evenly
possibly due to territorial
behavior or something that
suppresses growth.
Creosote bushes in the
desert are an example of
uniform distribution.
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Populations display various patterns of growth
Mathematical models
describe population growth
over time.
Variables include birth rate—
number of births per 1000
individuals per year—minus
death rate—deaths per 1000
individual per year.
Growth rate can also be
determined by comparing two
distinct points over time.
320 to 343 represents an
increase of 23 animals, or a
7% growth rate (23/320).
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Populations display various patterns of growth
Mathematical models
describe population growth
over time
Variables include birth rate—
number of births per 1000
individuals per year—minus
death rate—deaths per 1000
individual per year.
Growth rate can also be
determined by comparing two
distinct points over time.
320 to 343 represents an
increase of 23 animals, or a
7% growth rate (23/320).
8
Populations display various patterns of growth
Mathematical models
describe population growth
over time
Variables include birth rate—
number of births per 1000
individuals per year—minus
death rate—deaths per 1000
individual per year.
Growth rate can also be
determined by comparing two
distinct points over time.
320 to 343 represents an
increase of 23 animals, or a
7% growth rate (23/320).
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Populations display various patterns of growth
Without environmental limits,
a population will reach its
maximum per capita rate of
increase (r), or biotic potential.
Increase in population is
exponential with species
exhibiting high biotic potential.
New populations in a
environment will often have
high biotic potential.
Loss of predators can also
result in exponential growth.
Examples include spotted
knotweed and deer mice.
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Populations display various patterns of growth
Without environmental limits, a
population will reach its
maximum per capita rate of
increase (r), or biotic potential.
Increase in population is
exponential with species
exhibiting high biotic potential.
New populations in a
environment will often have
high biotic potential.
Loss of predators can also result
in exponential growth.
Examples include spotted
knotweed and deer mice.
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Populations display various patterns of growth
Exponential growth can’t last
forever:
Resources become scarce.
Individuals starve or are
unable to find habitat for
reproduction.
Aggression and competition
increase.
There is pressure from
predation.
Population size increasing
while the growth rate
decreases is logistical
growth—the S-shaped curve.
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Populations display various patterns of growth
Exponential growth can’t last
forever:
Resources become scarce.
Individuals starve or are
unable to find habitat for
reproduction.
Aggression and competition
increase.
There is pressure from
predation.
Population size increasing
while the growth rate
decreases is logistical
growth—the S-shaped curve.
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Populations display various patterns of growth
Carrying capacity (K) –
Population size that be
sustained indefinitely
without long-term damage
to the environment
K Depends on
growth factors—resources
needed to survive and
reproduce.
Carrying capacity can
increase or drop as resource
availability changes.
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A variety of factors affect population growth
TERMS TO KNOW:
Population growth rate
Birth rate
Death rate
Biotic potential (r)
Exponential growth
Logistic growth
Carrying capacity
Limiting factors are resources needed for survival but that may be in short supply.
This scarcity will determine carrying capacity.
Resistance factors, such as predation, competition, and disease, will also contribute
to controlling population size and growth.
These factors are density-dependent, but other factors such as natural disaster are
density-independent since they would occur regardless of the population size.
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A variety of factors affect population growth
Effects of density-dependent factors
increase as populations grow.
 Disease
 Competition
 Predation
TERMS TO KNOW:
Density dependent
Density independent
Reproductive strategies
r-selected species
K-selected species
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A variety of factors affect population growth
Effects of density independent factors
affect a population, regardless of its
size, but can serve to decrease the
population.
 Storm
 Fire/Flood
 Avalanche
TERMS TO KNOW:
Density dependent
Density independent
Reproductive strategies
r-selected species
K-selected species
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A variety of factors affect population growth
High rate of population increase and
well adapted to exploit unpredictable
environments
Will increase quickly as resources
become available
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A variety of factors affect population growth
Low reproductive rates and very
responsive to environmental conditions
Decrease or increase slowly as resource
availability changes
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A variety of factors affect population growth
K-selected species tend to be stable in
undisturbed areas.
Slow increases and decreases in response to
the environment.
r-selected species with rapid reproductive potential sometimes have sudden
population growth with high peaks which may overshoot carrying capacity
followed by sudden crashes. Some populations will level off near carrying
capacity while others will continue to overshoot and crash.
TERMS TO KNOW:
Boom-and-bust cycles
Extirpation
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A variety of factors affect population growth
Some populations
overshoot carrying capacity,
drop below it, and increase
and overshoot it again until
they settle down close to
carrying capacity.
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A variety of factors affect population growth
Example – Isle
Royale, Michigan
Wolves preyed upon
moose and the
moose population
dropped, followed
by a drop in the wolf
population. As
moose populations
recovered, so did the
wolves’ until a Parvo
epidemic reduced
wolves to their
lowest point.
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The loss of the wolf emphasized the importance of an
ecosystem’s top predator
Black line shows winter browse line from herbivores
Populations do not exist in
isolation. Like the
Yellowstone example
demonstrates, the
addition of a keystone
species like the wolf can
result in observable
changes in the behavior of
its prey. Cascading effects
on other community level
processes may follow.
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The loss of the wolf emphasized the importance of an
ecosystem’s top predator
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The loss of the wolf emphasized the importance of an
ecosystem’s top predator
Without wolves, beavers
thrive and build dams that
create lakes and ponds. Elk
stay in the willow thickets
and overgraze willow needed
by the beavers.
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The loss of the wolf emphasized the importance of an
ecosystem’s top predator
With wolves, willows regrow because
elk feed in the meadows, where they
can watch for wolves rather than in
the willow thickets. With more
willows, beavers return and the
wildlife populations recover.
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PERSONAL CHOICES THAT HELP
International Wolf Center www.wolf.org
US Fish and Wildlife www.fws.gov
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UNDERSTANDING THE ISSUE
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UNDERSTANDING THE ISSUE
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ANALYZING THE SCIENCE
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www.actionbioscience.org/biodivers
ity/johnson.html
EVALUATING NEW INFORMATION
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MAKING CONNECTIONS