CHAPTER 51
Population Ecology
ECOLOGY BASICS
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Terms to know…
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Ecology
Branch of biology
 Relatively new science
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Biotic factors
 Abiotic factors
 Environmental science
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Population
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ecology + human interactions
same species, same area, same time
Population ecology

numbers + changes
FEATURES OF POPULATIONS
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Properties that individuals lack:
Population density
 Population dispersion
 Birth/death rates
 Growth rates
 Survivorship rates
 Age structure
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Properties that communities lack:
Common gene pool
 Reproductive success
 Evolution
 Economic importance (crops, forests, game
animals…)
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POPULATION DYNAMICS
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Density – the number of individuals per unit of area
or volume at a given period of time
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Affected by:
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Habitat
Season
Other populations
Weather
Limiting factors: the number of individuals in a population is
controlled by the ability of the environment to support it
 Density-dependent factors – the effect increases as
population density increases
 Examples:
 Density-independent factors – affects the size of a
population but is not influenced by changes in population
density; typically abiotic
 Examples:
POPULATION DYNAMICS…
Dispersion – spacing in relation to other
members of the population
 Three basic varieties:
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Clumped (aggregated distribution,
patchiness)
1.
Individuals are concentrated in specific areas
Reasons: distribution of resources, social behavior of
animals (herds, family groups), reproduction
Advantages: reduced chance of predation
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Uniform
2.
Individuals are fairly evenly spaced
Reasons: social behavior of animals (territories), high
levels of competition between individuals
Advantages: reduced competition
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3.
Random
Individual spacing is unrelated to others in the population
 Does not occur often in nature
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CHANGES IN POPULATION SIZE
Per capita – per individual
 Natality – average per capita birth rate (b)
 Mortality – average per capita death rate (d)
 Immigration – individuals entering a local
population (i)
 Emigration – individuals leaving a local
population (e)
 Population growth rate (r):
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r = (b + i) – (d + e)
If r = positive number  population is increasing
If r = negative number  population is decreasing
If r = zero  population is staying the same
Examples:
INTRINSIC RATE OF INCREASE
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Maximum rate of increase when:
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Conditions are ideal
Resources are abundant
Population density is low
rmax
 Factors which influence this:
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Age at which reproduction begins
 The fraction of the life span devoted to reproduction
 The number of reproductive cycles
 The number of offspring produced each cycle
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Different species have different intrinsic rates…
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Small organisms have high rates (bacteria); large
species have low rates (elephants)
EXPONENTIAL POPULATION GROWTH
Optimal conditions allow a constant per capita
population growth (rmax)
 The larger the population gets, the faster it grows
 J shape curve:
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Organisms cannot reproduce this way
indefinitely because of increased:
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competition, predation, disease, wastes
LOGISTIC POPULATION GROWTH
Population growth rate nears zero
 Occurs near the environment’s limits to support
the population
 Carrying capacity (K) – the largest population
that an area can maintain indefinitely, assuming
no changes in the environment
 S shape curve:
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