Population Ecology

1
Population Ecology

• Chapter 27

2
Population Ecology

• Certain ecological principles govern the growth
  and sustainability of all populations
• Human populations are no exception

3
Limits to Growth

• A populations growth depends on the resources of
  its environment
• Deer introduced to Angel Island
• Population outstripped resources

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
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
in-text, p. 688
13
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

14
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

15
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

16
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

17
Exponential Growth

• Population size expands by ever increasing
  increments during successive intervals
• The larger the population gets, the more
  individuals there are to reproduce

18
Effect of Deaths

• Population will grow exponentially as long as per
  capita death rates are lower than per capita
  birth rates

25 mortality between divisions
19
Biotic Potential

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

20
Limiting Factors

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

21
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

22
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

23
Logistic Growth

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

carrying capacity
Time
24
Click to view animation.
animation
25
Overshooting Capacity

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

26
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

27
Density-Independent Controls

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

28
initial carrying capacity
new carrying capacity
Fig. 40.6, p. 690