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Chapter 5: Population
Ecology
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
The Spread of Shakespeare's Starlings
• In the 1800s and early 1900s, introducing
foreign species of animals and plants to North
America was a popular, unregulated activity
• In 1890, a group of Shakespeare enthusiasts
released about 120 starlings in New York's
Central Park
– It was part of a project to
bring to America every
bird species mentioned
in Shakespeare’s works
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• Today, the starling range extends from Mexico
to Alaska
• Their
population
is estimated
at well over
100 million
Current
1955
Current
1955
1945
1935
1925
1945
1905
1915
1935
1925
1925
1935
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Over 5 million starlings have been counted in a
single roost
• Starlings are omnivorous, aggressive, and
tenacious
• They cause
destruction and
often replace
native bird species
• Attempts to
eradicate starlings
have been
unsuccessful
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• The starling population in North America has
some features in common with the global
human population
– Both are expanding and are virtually
uncontrolled
– Both are harming other species
• Population ecology is concerned with
changes in population size and the factors
that regulate populations over time
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
35.1 Populations are defined in several ways
• Ecologists define a population as a singlespecies group of individuals that use common
resources and are regulated by the same
environmental factors
– Individuals in a population have a high
likelihood of interacting and breeding with one
another
• Researchers must define a population by
geographic boundaries appropriate to the
questions being asked
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
POPULATION STRUCTURE AND DYNAMICS
35.2 Density and dispersion patterns are important
population variables
• Population density is the number of
individuals in a given area or volume
• It is sometimes possible to count all the
individuals in a population
– More often, density is estimated by sampling
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• One useful sampling technique for estimating
population density is the mark-recapture
method
Figure 35.2A
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• The dispersion pattern of a population refers
to the way individuals are spaced within
their area
– Clumped
– Uniform
– Random
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• Clumped dispersion is a pattern in which
individuals are aggregated in patches
– This is the most
common
dispersion
pattern in nature
– It often results
from an unequal
distribution of
resources in the
environment
Figure 35.2B
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• A uniform pattern of dispersion often results
from interactions among individuals of a
population
– Territorial behavior and competition for water
are examples of such interactions
Figure 35.2C
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Random dispersion is characterized by
individuals in a population spaced in a
patternless, unpredictable way
– Example: clams living in a mudflat
– Environmental conditions and social
interactions make random dispersion rare
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
35.3 Idealized models help us understand
population growth
• Idealized models describe two kinds of
population growth
– exponential growth
– logistic growth
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Example of Exponential Growth
Describes the rate of expansion of a population under
ideal, unregulated conditions.
Figure 35.3A
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• Logistic growth is slowed by populationlimiting factors
– It tends to level off at
carrying capacity
– Carrying capacity
is the maximum
population size
that an environment
can support at a
particular time
with no degradation
to the habitat
Figure 35.3B
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How carrying capacity plays a role in exponential
growth and logistic growth:
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• The logistic growth model predicts that
– a population's growth rate will be low when
the population size is either small or large
– a population’s growth rate will be highest
when the population is at an intermediate
level relative to the carrying capacity
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
35.4 Multiple factors may limit population growth
• Density-dependent limiting factor- a
factor that causes population growth to
decreases a result of the population size
– Competition
– Predation
– Parasitism
– Disease
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• Density-independent limiting factorsaffect all populations in similar ways,
regardless of the population size
– Weather
– Natural Disasters
– Seasonal Cycles
– Human Activities
• Damming rivers
• Clear-cutting forests
• DEVELOPMENT
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• Abiotic factors may limit many natural
populations
– Aphids show
exponential
growth in the
spring and
then rapidly
die off when
the climate
becomes hot
and dry in the
summer
Figure 35.4B
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Most populations are probably regulated by a
mixture of factors
– Density-dependent birth and death rates
– Abiotic factors such as climate
and disturbances
• Populations often
fluctuate in number
– A natural population
of song sparrows
often grows rapidly
and is then drastically
reduced by severe
winter weather
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Figure 35.4C
35.5 Some populations have "boom-and-bust"
cycles
• Some populations go through boom-and-bust
cycles of growth and decline
• Example: the population cycles of the lynx
and the snowshoe hare
– The lynx is one of the main predators of the
snowshoe hare in the far northern forests of
Canada and Alaska
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– About every 10 years, both hare and lynx
populations have a rapid increase (a "boom")
followed by a sharp decline (a "bust")
Figure 35.5
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Recent studies suggest that the 10-year cycles of
the snowshoe hare are largely driven by
excessive predation
– But they are also influenced by fluctuations in
the hare's food supply
• Population cycles may also result from a time
lag in the response of predators to rising prey
numbers
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THE HUMAN POPULATION
35.8 Connection: The human population has been
growing exponentially for centuries
• The human population as a whole has doubled
three times in the last three centuries
• The human population now stands at about 6.7
billion and may reach 9.3 billion by the year
2050
Current Clock
• Most of the increase is due to improved health
and technology
– These have affected death rates
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How to Calculate Growth Rates:
 Four influences:
 Birth rate 
 Death rate 
 Immigration 
 Emigration 
 Birth + Immigration – Death – Emigration =
Population Count
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• The history of human population growth
Figure 35.8A
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• The ecological footprint represents the amount
of productive land needed to support a nation’s
resource needs
• The ecological capacity of the world may
already be smaller than its ecological
footprint
http://www.myfootprint.org/
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• Ecological footprint in relation to ecological
capacity
Figure 35.8B
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• The exponential growth of the human
population is probably the greatest crisis ever
faced by life on Earth
Figure 35.8C
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35.9 Birth and death rates and age structure affect
population growth
• Population stability is achieved when there is
zero population growth
– Zero population growth is when birth rates
equal death rates
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• The demographic transition is the shift from
high birth and death rates to low birth and
death rates
– During this
transition,
populations
may grow
rapidly until
birth rates
decline
Figure 35.9A
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• The age structure of a population is the
proportion of individuals in different agegroups
– Age structure affects population growth
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
RAPID GROWTH
SLOW GROWTH
ZERO GROWTH/DECREASE
Kenya
United States
Italy
Male
Female
Male
Female
Ages 45+
Ages 45+
Ages 15–44
Ages 15–44
Under
15
Percent of population
Male
Female
Under
15
Percent of population
Percent of population
Figure 35.9B
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• Age-structure diagrams not only reveal a
population's growth trends
– They also indicate social conditions
• Increasing the status and education of
women may help to reduce family size
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings