Population Ecology

Chapter 27

Population Ecology

• Certain ecological principles govern the

growth and sustainability of all

populations

• Human populations are no exception

Limits to Growth

• A population’s growth depends on the

resources of its environment

• Deer introduced to Angel Island

– Population outstripped resources

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

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

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

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

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 adds individuals

• Emigration subtracts individuals

• Births add individuals

• Deaths subtract individuals

population growth per unit time

net population growth rate

per individual per unit time

number of individuals

= X

in-text, p. 688

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 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

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

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

Biotic Potential

• Maximum rate of increase per individual

under ideal conditions

• Varies between species

• In 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

carrying capacity

Time

animation

Click to view animation.

Overshooting Capacity

• Population may temporarily increase above carrying capacity

• Overshoot is usually followed by a crash; dramatic increase in deaths

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

Fig. 40.6, p. 690

initial carrying capacity

new carrying capacity