density = # of ind per unit area or volume 3 ways to measure › aerial count of herds, tall trees...
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Chapter 52Population Ecology
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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
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Patterns of population dispersion – p. 1138
Clumped – individuals in patches› Most common
Uniform – territoriality Random
› Ex – wind blown seeds
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Factors affecting population size
Age at 1st reproduction # of offspring/year – natality (birth
rate) Life expectancy – death rate Movement of organisms
› Immigration› emigration
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Survivor ship CurveType I – human – greatest survival to old age
Type II – squirrel – medium survival
Type III – clam – greatest deaths early
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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
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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
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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
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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
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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
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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
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Density-independent LFs – affect both large & small populations the same› Climate, weather› Disasters – flood, drought, fire
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Human Population GrowthPresent population 7.1 billion