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Populations &
Limits to Growth
Characteristics of a Population
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Geographic Distribution: area inhabited by
a population
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bacteria: 1 cm3
whale: millions km2
Density: no. of individuals per area
 Growth Rate
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Population Growth
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number of births
number of deaths
number of individuals that enter/leave a
population
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Populations Grow: births > deaths
Populations Shrink: births < deaths
Immigration: movement of individuals into the
area
Emigration: movement of individuals out of an
area

Young leave to find mates, food shortage
Exponential Growth
Ideal conditions & unlimited resources
 individuals in a population reproduce at a
constant rate
 individual number increases slowly but
becomes larger and larger until it
approaches an infinitely large size

Logistic Growth
with limited resources, population growth
slows or stops after a period of exponential
growth
 produces an S shaped curve
 Carrying Capacity: the size of the
population when the growth rate is 0

Limits to Growth

Limiting Factor: a factor that causes
population growth to decrease
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Competition
Predation
Parasitism and disease
Drought and climate extremes
Human Disturbances
Limits to Growth

Density Limiting Factors: depends on the
population size
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Become limiting only when the population
density, or size, becomes a certain level
Doesn’t affect small, scattered populations
Density Limiting Factors
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Competition: food, water, space, sunlight
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Predation: population control
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More individuals living in area, the sooner available
resources are used up
Between same or different species
Sea otters and killer whales
Wolves and Moose
As the wolves feed upon the moose, the moose
population falls which causes the wolf population to fall;
the moose population can then rise which rises the wolf
population
Parasitism and Disease: similar to predators

wasp cocoon on a caterpillar
Density-Independent Factors

affect population regardless of size
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unusual weather or natural disasters
human activities: damming rivers or clearcutting forests
crash in population size
 population may build up or stay small
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