Population Ecology

1
Population Ecology

• Chapter 45

2
Population Ecology

• Certain ecological principles govern the growth
  and sustainability of all populations--including
  human populations

3
Limits to Growth

• A populations growth depends on the resources of
  its environment
• Moose/Wolf study on Isle Royal

4
Human Population Problems

• Over 6 billion people alive
• About 2 billion live in poverty
• Most resources are consumed by the relatively few
  people in developed countries

5
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

6
Population Age Structure

• Divide population into age categories
• Populations reproductive base includes members
  of the reproductive and pre-reproductive age
  categories

7
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
Figure 45.2 Page 808  
random
8
Determining Population Size

• Direct counts are most accurate but seldom
  feasible
• Can sample an area, then extrapolate
• Capture-recapture method is used for mobile
  species

9
Capture-Recapture Method

• Capture, mark, and release individuals
• Return later and capture second sample
• Count the number of marked individuals and use
  this to estimate total population

10
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

11
Changes in Population Size

• Immigration adds individuals
• Emigration subtracts individuals
• Births add individuals
• Deaths subtract individuals

12
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

13
Per Capita Rates

• Rates per individual
• Total 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

14
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

15
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

16
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 Page 810
17
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 Page 811
18
Biotic Potential

• Maximum rate of increase per individual under
  ideal conditions
• Varies between species
• In nature, biotic potential is rarely reached

19
Limiting Factors

• Any essential resource that is in short supply
• All limiting factors acting on a population
  dictate sustainable population size

20
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

21
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

22
Logistic Growth

• As size of the population increases, rate of
  reproduction decreases
• When the population reaches carrying capacity,
  population growth ceases

23
Logistic Growth Graph
initial carrying capacity
new carrying capacity
Figure 45.6 Page 812
24
Overshooting Capacity

• Population may temporarily increase above
  carrying capacity
• Overshoot is usually followed by a crash
  dramatic increase in deaths

Reindeer on St. Matthews Island
Figure 45.6 Page 812
25
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

26
Density-Independent Controls

• Factors unaffected by population density
• Natural disasters or climate changes affect large
  and small populations alike

27
Life History Patterns

• Patterns of timing of reproduction and
  survivorship
• Vary among species
• Summarized in survivorship curves and life tables

28
Life Table

• Tracks age-specific patterns
• Population is divided into age categories
• Birth rates and mortality risks are calculated
  for each age category

29
Survivorship Curves

• Graph of age-specific survivorship

Figure 45.8 Page 815
30
Predation and Life History

• Guppy populations vary in life history
  characteristics and morphology
• Differences have genetic basis
• Variation seems to be result of directional
  selection by predators

31
Human Population Growth

• Population now exceeds 6 billion
• Rates of increase vary among countries
• Average annual increase is 1.26 percent
• Population continues to increase exponentially

32
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

33
Future Growth

• Exponential growth cannot continue forever
• Breakthroughs in technology may further increase
  carrying capacity
• Eventually, density-dependent factors will slow
  growth

34
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

35
Age Structure Diagrams

• Show age distribution of a population

Figure 45.14 Page 821
Negative Growth
Slow Growth
Zero Growth
Rapid Growth
36
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

37
Slowing Growth in China

• Worlds most extensive family planning program
• Government rewards small family size, penalizes
  larger families, provides free birth control,
  abortion, sterilization
• Since 1972, TFR down to 1.8 from 5.7

38
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

39
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
40
Resource Consumption

• United States has 4.7 percent of the worlds
  population
• Americans have a disproportionately large effect
  on the worlds resources
• Per capita, Americans consume more resources and
  create more pollution than citizens of less
  developed nations