Population EcologyPopulation Ecology

Chapter 45Chapter 45

Population EcologyPopulation Ecology

Certain ecological principles govern the Certain ecological principles govern the

growth and sustainability of all growth and sustainability of all

populations--including human populations--including human

populations populations

Limits to GrowthLimits to Growth

A population’s growth depends on A population’s growth depends on

the resources of its environmentthe resources of its environment

Moose/Wolf study on Isle Royal Moose/Wolf study on Isle Royal

Human Population ProblemsHuman Population Problems

Over 6 billion people aliveOver 6 billion people alive

About 2 billion live in povertyAbout 2 billion live in poverty

Most resources are consumed by the Most resources are consumed by the

relatively few people in developed relatively few people in developed

countries countries

PopulationPopulation

A group of individuals of the same A group of individuals of the same

species occupying a given areaspecies occupying a given area

Can be described by demographicsCan be described by demographics

– Vital statistics such as size, density, Vital statistics such as size, density,

distribution, and age structuredistribution, and age structure

Population Age StructurePopulation Age Structure

Divide population into age categoriesDivide population into age categories

Population’s reproductive base Population’s reproductive base

includes members of the reproductive includes members of the reproductive

and pre-reproductive age categoriesand pre-reproductive age categories

Density & DistributionDensity & Distribution

Number of Number of individuals in some individuals in some specified area of specified area of habitathabitat

Crude density Crude density information is more information is more useful if combined useful if combined with distribution with distribution

datadata

clumped

nearly uniform

random Figure 45.2

Page 808  

Determining Population SizeDetermining Population Size

Direct counts are most accurate but Direct counts are most accurate but seldom feasibleseldom feasible

Can sample an area, then extrapolateCan sample an area, then extrapolate

Capture-recapture method is used for Capture-recapture method is used for mobile speciesmobile species

Capture-Recapture MethodCapture-Recapture Method

Capture, mark, and release Capture, mark, and release individualsindividuals

Return later and capture second Return later and capture second samplesample

Count the number of marked Count the number of marked individuals and use this to estimate individuals and use this to estimate total populationtotal population

Assumptions in Assumptions in Capture-RecaptureCapture-Recapture

Marking has no effect on mortalityMarking has no effect on mortality

Marking has no effect on likelihood Marking has no effect on likelihood to being capturedto being captured

There is no immigration or There is no immigration or emigration between sampling emigration between sampling timestimes

Changes in Population SizeChanges in Population Size

Immigration adds individualsImmigration adds individuals

Emigration subtracts individualsEmigration subtracts individuals

Births add individualsBirths add individuals

Deaths subtract individualsDeaths subtract individuals

Zero Population GrowthZero Population Growth

Interval in which number of births Interval in which number of births

is balanced by number of deathsis balanced by number of deaths

Assume no change as a result of Assume no change as a result of

migrationmigration

Population size remains stablePopulation size remains stable

Per Capita RatesPer Capita Rates Rates per individualRates per individual

Total number of events in a time Total number of events in a time

interval divided by the number of interval divided by the number of

individualsindividuals

Per capita birth rate per month =Per capita birth rate per month =

Number of births per month Number of births per month

Population sizePopulation size

rr

Net reproduction per individual per Net reproduction per individual per

unit timeunit time

Variable combines per capita birth Variable combines per capita birth

and death rates (assuming both and death rates (assuming both

constant)constant)

Can be used to calculate rate of Can be used to calculate rate of

growth of a populationgrowth of a population

Exponential Growth EquationExponential Growth Equation

GG = = rNrN

GG is population growth per unit time is population growth per unit time rr is net reproduction per individual is net reproduction per individual

per unit timeper unit time NN is population size is population size

Exponential GrowthExponential Growth

Population size expands Population size expands by ever increasing by ever increasing increments during increments during successive intervalssuccessive intervals

The larger the The larger the population gets, the population gets, the more individuals there more individuals there are to reproduceare to reproduce

Figure 45.4Page 810

Effect of DeathsEffect of Deaths

Population grows exponentially as long as per capita Population grows exponentially as long as per capita

death rates are lower than per capita birth ratesdeath rates are lower than per capita birth rates

25% mortality

between divisions

Figure 45.5Page 811

Biotic PotentialBiotic Potential

Maximum rate of increase per Maximum rate of increase per

individual under ideal conditionsindividual under ideal conditions

Varies between speciesVaries between species

In nature, biotic potential is rarely In nature, biotic potential is rarely

reachedreached

Limiting FactorsLimiting Factors

Any essential resource that is in Any essential resource that is in

short supplyshort supply

All limiting factors acting on a All limiting factors acting on a

population dictate sustainable population dictate sustainable

population sizepopulation size

Carrying Capacity (Carrying Capacity (KK))

Maximum number of individuals that Maximum number of individuals that

can be sustained in a particular habitatcan be sustained in a particular habitat

Logistic growth occurs when Logistic growth occurs when

population size is limited by carrying population size is limited by carrying

capacitycapacity

Logistic Growth EquationLogistic Growth Equation

G = rG = rmaxmax N (K-N/K)N (K-N/K)

G = G = population growth per unit timepopulation growth per unit time

rrmaxmax = = maximum population growth rate maximum population growth rate

per unit timeper unit time

N = N = number of individualsnumber of individuals

K = K = carrying capacitycarrying capacity

Logistic GrowthLogistic Growth

As size of the population increases, As size of the population increases, rate of reproduction decreasesrate of reproduction decreases

When the population reaches carrying When the population reaches carrying capacity, population growth ceasescapacity, population growth ceases

Figure 45.6Page 812

initial carrying capacity

new carrying capacity

Logistic Growth GraphLogistic Growth Graph

Overshooting CapacityOvershooting Capacity

Population may Population may temporarilytemporarily increase above increase above carrying capacitycarrying capacity

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

Figure 45.6Page 812

Reindeer on St. Matthew’s Island

Density-Dependent ControlsDensity-Dependent Controls

Logistic growth equation deals with Logistic growth equation deals with

density-dependent controlsdensity-dependent controls

Limiting factors become more Limiting factors become more

intense as population size intense as population size

increasesincreases

Disease, competition, parasites, Disease, competition, parasites,

toxic effects of waste productstoxic effects of waste products

Density-Independent ControlsDensity-Independent Controls

Factors unaffected by population Factors unaffected by population

densitydensity

Natural disasters or climate changes Natural disasters or climate changes

affect large and small populations affect large and small populations

alikealike

Life History PatternsLife History Patterns

Patterns of timing of reproduction Patterns of timing of reproduction

and survivorshipand survivorship

Vary among speciesVary among species

Summarized in survivorship Summarized in survivorship

curves and life tablescurves and life tables

Life TableLife Table

Tracks age-specific patterns Tracks age-specific patterns

Population is divided into age Population is divided into age

categoriescategories

Birth rates and mortality risks are Birth rates and mortality risks are

calculated for each age categorycalculated for each age category

Survivorship CurvesSurvivorship Curves

Graph of age-specific survivorship Graph of age-specific survivorship

Figure 45.8Page 815

Predation and Life History Predation and Life History

Guppy populations vary in life history Guppy populations vary in life history

characteristics and morphologycharacteristics and morphology

Differences have genetic basisDifferences have genetic basis

Variation seems to be result of directional Variation seems to be result of directional

selection by predatorsselection by predators

Human Population Growth Human Population Growth

Population now exceeds 6 billionPopulation now exceeds 6 billion

Rates of increase vary among countriesRates of increase vary among countries

Average annual increase is 1.26 percentAverage annual increase is 1.26 percent

Population continues to increase Population continues to increase

exponentiallyexponentially

Side-Stepping ControlsSide-Stepping Controls

Expanded into new habitats Expanded into new habitats

Agriculture increased carrying capacity; Agriculture increased carrying capacity;

use of fossil fuels aided increaseuse of fossil fuels aided increase

Hygiene and medicine lessened effects Hygiene and medicine lessened effects

of density-dependent controlsof density-dependent controls

Future GrowthFuture Growth

Exponential growth cannot continue Exponential growth cannot continue

foreverforever

Breakthroughs in technology may Breakthroughs in technology may

further increase carrying capacity further increase carrying capacity

Eventually, density-dependent Eventually, density-dependent

factors will slow growthfactors will slow growth

Fertility Rates Fertility Rates

Worldwide, average annual rate of Worldwide, average annual rate of

increase is 1.26%increase is 1.26%

Total fertility rate (TFR) is average Total fertility rate (TFR) is average

number of children born to a womannumber of children born to a woman

Highest in developing countries, lowest Highest in developing countries, lowest

in developed countriesin developed countries

Age Structure DiagramsAge Structure Diagrams

Show age distribution of a populationShow age distribution of a population

Rapid Growth

Slow Growth

Zero Growth

Negative Growth

Figure 45.14Page 821

Population MomentumPopulation Momentum

Lowering fertility rates cannot Lowering fertility rates cannot immediately slow population growth immediately slow population growth raterate

Why? There are already many future Why? There are already many future parents aliveparents alive

If every couple had just two children, If every couple had just two children, population would still keep growing population would still keep growing for another 60 yearsfor another 60 years

Slowing Growth in ChinaSlowing Growth in China

World’s most extensive family World’s most extensive family

planning program planning program

Government rewards small family Government rewards small family

size, penalizes larger families, size, penalizes larger families,

provides free birth control, abortion, provides free birth control, abortion,

sterilizationsterilization

Since 1972, TFR down to 1.8 from 5.7Since 1972, TFR down to 1.8 from 5.7

Effects of Economic Effects of Economic Development Development

Total fertility rates (TFRs) are highest in Total fertility rates (TFRs) are highest in

developing countries, lowest in developing countries, lowest in

developed countriesdeveloped countries

When individuals are economically When individuals are economically

secure, they are under less pressure to secure, they are under less pressure to

have large familieshave large families

Population Sizes in 2001Population 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 ConsumptionResource Consumption

United States has 4.7 percent of the United States has 4.7 percent of the world’s populationworld’s population

Americans have a disproportionately Americans have a disproportionately large effect on the world’s resourceslarge effect on the world’s resources

Per capita, Americans consume more Per capita, Americans consume more resources and create more pollution resources and create more pollution than citizens of less developed than citizens of less developed nationsnations