Chapter 52Population Ecology

Population density

Density = # of ind per unit area or volume

3 ways to measure› Aerial count of herds, tall trees in savanna› Random (sample) plots to count individuals

or nests, burrows, tracks› Mark-recapture

Capture, mark, release, recapture Pop size = total recaptured X (marked & released)

marked & recaptured

Patterns of population dispersion – p. 1138

Clumped – individuals in patches› Most common

Uniform – territoriality Random

› Ex – wind blown seeds

Factors affecting population size

Age at 1st reproduction # of offspring/year – natality (birth

rate) Life expectancy – death rate Movement of organisms

› Immigration› emigration

Survivor ship CurveType I – human – greatest survival to old age

Type II – squirrel – medium survival

Type III – clam – greatest deaths early

Life History Strategies

Opportunistic species (r-selection) Density-independent

› Population densities tend to fluctuate below carrying capacity or have little competition so overshoot & crash

› Many offspring quickly› No parental care› Short lifespan› Small offspring› Weeds, salmon, insects, starfish, mice

Equilibrium species (k-selection) Density dependent

› Few young› Parental care› Reproduce at more mature age› Long lifespan› Large offspring› Birds, whales, elephants, humans,

hardwood trees

Population Growth Models

Exponential model – typical of populations introduced to a new area or whose # have been reduced› Ex – single pair of elephants produce 19

million in 750 years› Single bacterium divides every 20 mins,

can cover the earth 1 ft deep in 36 hours› Begin with a few individuals and unlimited

resources, but can lead to crash as resources become limited

Logistic Model› The per capita rate of growth is reduced as

the population nears the carrying capacity› Carrying capacity – maximum population

size that can be maintained in a particular area

Carrying capacity

# of population that can be maintained indefinitely in the ecosystem› dN = rN(K-N)

dt KIf population (N) is small, equation works as

if it were just rNAs N increases, growth slows b/c it is being

multiplied by a fractionIf N=K, growth is zero

Limiting Factors

Density dependent LFs – only come into play or intensify when the population becomes denser› Food› Disease› Sunlight for plants› Hormonal changes› Space, nesting sites› predation

Density-independent LFs – affect both large & small populations the same› Climate, weather› Disasters – flood, drought, fire

Human Population GrowthPresent population 7.1 billion