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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