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Population Ecology
Chapter 45
Population Ecology
Certain ecological principles govern the
growth and sustainability of all
populations--including human
populations
Limits to Growth
A
population’s growth depends on
the resources of its environment
 Moose/Wolf
study on Isle Royal
Human Population Problems
 Over
6 billion people alive
 About
 Most
2 billion live in poverty
resources are consumed by the
relatively few people in developed
countries
Population
A
group of individuals of the same
species occupying a given area
 Can
be described by demographics
– Vital statistics such as size, density,
distribution, and age structure
Population Age Structure
 Divide
population into age
categories
 Population’s
reproductive base
includes members of the
reproductive and pre-reproductive
age categories
Density & Distribution


Number of
individuals in some
specified area of
habitat
Crude density
information is
more useful if
combined with
distribution data
clumped
nearly uniform
random
Figure 45.2
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Determining Population Size
 Direct
counts are most accurate but
seldom feasible
 Can
sample an area, then extrapolate
 Capture-recapture
mobile species
method is used for
Capture-Recapture Method
 Capture,
mark, and release
individuals
 Return
sample
 Count
later and capture second
the number of marked
individuals and use this to
estimate total population
Assumptions in
Capture-Recapture
 Marking
has no effect on mortality
 Marking
has no effect on likelihood
to being captured
 There
is no immigration or
emigration between sampling
times
Changes in Population Size
 Immigration
 Emigration
 Births
adds individuals
subtracts individuals
add individuals
 Deaths
subtract individuals
Zero Population Growth
 Interval
in which number of births
is balanced by number of deaths
 Assume
no change as a result of
migration
 Population
size remains stable
Per Capita Rates
 Rates
 Total
per individual
number of events in a time
interval divided by the number of
individuals
 Per
capita birth rate per month =
Number of births per month
Population size
r
 Net
reproduction per individual per
unit time
 Variable
combines per capita birth
and death rates (assuming both
constant)
 Can
be used to calculate rate of
growth of a population
Exponential Growth Equation
G = rN
G
is population growth per unit time
 r is net reproduction per individual
per unit time
 N is population size
Exponential Growth


Population size expands
by ever increasing
increments during
successive intervals
The larger the
population gets, the
more individuals there
are to reproduce
Figure 45.4
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Effect of Deaths

Population grows exponentially as long as per capita
death rates are lower than per capita birth rates
25% mortality
between divisions
Figure 45.5
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Biotic Potential
 Maximum
rate of increase per
individual under ideal conditions
 Varies
 In
between species
nature, biotic potential is rarely
reached
Limiting Factors
 Any
essential resource that is in
short supply
 All
limiting factors acting on a
population dictate sustainable
population size
Carrying Capacity (K)
 Maximum
number of individuals that
can be sustained in a particular
habitat
 Logistic
growth occurs when
population size is limited by carrying
capacity
Logistic Growth Equation
G = rmax N (K-N/K)

G = population growth per unit time

rmax = maximum population growth rate
per unit time

N = number of individuals

K = carrying capacity
Logistic Growth


As size of the population increases,
rate of reproduction decreases
When the population reaches carrying
capacity, population growth ceases
Logistic Growth Graph
initial carrying
capacity
new carrying
capacity
Figure 45.6
Page 812
Overshooting Capacity


Population may
temporarily
increase above
carrying capacity
Overshoot is
usually followed by
a crash; dramatic
increase in deaths
Reindeer on St. Matthew’s Island
Figure 45.6
Page 812
Density-Dependent Controls
 Logistic
growth equation deals
with density-dependent controls
 Limiting
factors become more
intense as population size
increases
 Disease,
competition, parasites,
toxic effects of waste products
Density-Independent Controls
 Factors
unaffected by population
density
 Natural
disasters or climate changes
affect large and small populations
alike
Life History Patterns
 Patterns
of timing of reproduction
and survivorship
 Vary
among species
 Summarized
in survivorship
curves and life tables
Life Table
 Tracks
age-specific patterns
 Population
is divided into age
categories
 Birth
rates and mortality risks are
calculated for each age category
Survivorship Curves
Graph of age-specific survivorship
Figure 45.8
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Predation and Life History
 Guppy
populations vary in life history
characteristics and morphology
 Differences
 Variation
have genetic basis
seems to be result of directional
selection by predators
Human Population Growth
 Population
 Rates
now exceeds 6 billion
of increase vary among countries
 Average
annual increase is 1.26 percent
 Population
continues to increase
exponentially
Side-Stepping Controls
 Expanded
into new habitats
 Agriculture
increased carrying
capacity; use of fossil fuels aided
increase
 Hygiene
and medicine lessened
effects of density-dependent controls
Future Growth
 Exponential
growth cannot continue
forever
 Breakthroughs
in technology may
further increase carrying capacity
 Eventually,
density-dependent
factors will slow growth
Fertility Rates
 Worldwide,
average annual rate of
increase is 1.26%
 Total
fertility rate (TFR) is average
number of children born to a woman
 Highest
in developing countries, lowest
in developed countries
Age Structure Diagrams
Show age distribution of a population
Figure 45.14
Page 821
Rapid
Growth
Slow
Growth
Zero
Growth
Negative
Growth
Population Momentum
 Lowering
fertility rates cannot
immediately slow population growth
rate
 Why?
There are already many future
parents alive
 If
every couple had just two children,
population would still keep growing
for another 60 years
Slowing Growth in China
 World’s
most extensive family
planning program
 Government
rewards small family
size, penalizes larger families,
provides free birth control, abortion,
sterilization
 Since
5.7
1972, TFR down to 1.8 from
Effects of Economic
Development
 Total
fertility rates (TFRs) are highest in
developing countries, lowest in
developed countries
 When
individuals are economically
secure, they are under less pressure to
have large families
Population Sizes in 2001
Asia
3.7 billion
Europe
727 million
Africa
816 million
Latin America
525 million
North America
316 million
Oceania
31 million
Resource Consumption
 United
States has 4.7 percent of the
world’s population
 Americans
have a disproportionately
large effect on the world’s resources
 Per
capita, Americans consume more
resources and create more pollution
than citizens of less developed
nations