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Do Now
• How would you define a “Population”?
• What would an ecologist consider as
“population dynamics”?
• What does the phrase “minimum
viable population” mean?
CHAPTER 7 POPULATION ECOLOGY
THE WOLF
WATCHERS
Endangered gray wolves return
to the American West
A population comprises all the
individuals of a species that live in the
same geographic area and interbreed.
Ecologists who study changes in
population size and makeup
(population dynamics) find that
some population sizes change
predictably while others tend to
fluctuate randomly.
Every population has a minimum
viable population, or the smallest
number of individuals that would
still allow a population to persist or
grow.
Before humans started killing
Yellowstone wolves in the 1900s, an
estimated 300–400 wolves lived in the
park.
At least 136 wolves were
killed from 1914–1926 as
part of the U.S. Government
Predator Control Program.
Humans threatened the wolf
population by destroying
wolves’ natural habitat,
cutting down trees to build
farms.
Humans also starved wolves by
hunting wolves’ usual sources of
food – deer, elk, and bison.
Do Now
• What is population density?
• Why is it important for the survival
of a species?
• How can growth rates differ
amongst populations?
The Yellowstone Gray Wolf Restoration
Project has reintroduced 41 wolves to
Yellowstone since 1994.
There are now more than 5,000 gray
wolves in the lower 48 states, about
100 of which are in Yellowstone.
The number of wolves distributed
over the acreage of Yellowstone is
their population density – the
number of individuals per unit
area.
Population density is important:
Too low = hard to find mates
Too high = increased competition,
spread of disease, fighting
Scientists predict annual
population growth rate by
subtracting birth rate
(births/1,000 individuals)
from death rate
(deaths/1,000 individuals).
Scientists also look at different
points in time:
From 2009–2010, wolf population
increased from 320 to 343, a
growth rate of 7 percent.
The exponential
growth of a
population
occurs when
there are no
limits to growth.
Exponential growth is typically
seen when a species enters a
new environment or there is
an influx of new resources.
A population must
have a high birth rate
and a low death rate
for exponential growth
to occur.
A loss of predators can lead to
exponential growth in a prey species:
Yellowstone elk doubled between 1914
and 1932 after predator program
introduced.
Exponential
growth can’t
last forever: as
a population
fills its
environment,
its growth rate
decreases.
Clicker Question
Population Size (Number of Individuals)
Which population is experiencing exponential growth?
30
J-Curve
20
S-Curve
10
Population A
Population B
0
5
10
15
20
25
30
Breeding Cycles
A. Population A
B. Population B
C. Both populations
D. Neither population
Population size is affected by densitydependent and density-independent factors.
A species’ reproductive strategies affect
its growth potential.
r -selected species, such as
deer mice, adapt well to
unpredictable environments
and have a high rate of
increase.
K-selected species, such as wolves,
reproduce slowly and have a lower
rate of increase.
Boom-and-bust cycles occur when
predator and prey populations
increase and decrease.
Before the wolves were
reintroduced, the U.S. Fish and
Wildlife Service prepared an
environmental impact
statement on possible effects.
Scientists learned that the presence or absence
of wolves affects the entire ecosystem.
Many animal and plant
populations are declining
worldwide from habitat loss,
introduction of non-native
species, and predator
removals.
The main reason species are
endangered is habitat destruction:
People remove habitats or resources
and break needed connections within
ecosystems.
More than 18,000 threatened plant and
animal species are on the International
Union for Conservation of Nature’s Red
List.
Understanding population
dynamics helps managers
monitor, protect, and
restore populations.
Predators are an important
component to every ecosystem.
Do you think it is right for predators
to be reintroduced to their home
habitats even if they pose a threat to
human property? Why or why not?