Chapter 4 Notes,
Population Ecology
Population Characteristics
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Three main characteristics of a population are.
Population density is the number of
organisms per unit of area.
Dispersion is the spatial distribution or the
pattern of spacing or how the population is
arranged within an area.
Population growth rate explains how fast a
population grows over time.
Population Density
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Population density is a number that can be
determined by dividing the number of organisms
in an area, by the size of the area.
For example, let's say the Alaska Department of
Fish and Game conducted a moose population
survey and they counted 500 moose in a 100
square mile area. The population density would
be 500 / 100 = 5 moose per square mile.
100 square miles would be 10 miles wide by 10
miles long. That's roughly about the size of the
Wasilla and Palmer area.
Dispersion (Spatial Distribution)
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Uniform dispersion – organisms that are
solitary and/or territorial often have a uniform
dispersion pattern. (bear or wolverine)
Clumped dispersion – organisms that live in
colonies, herds, flocks, or packs have a
clumped dispersion pattern. (like a pack of
wolves or a herd of caribou)
Random dispersion – organisms that are not
very territorial or do not live in big groups tend
to have a more random dispersion due to the
availability of good habitat. (snowshoe hare)
Dispersion (Spatial Distribution)
Population-Limiting Factors
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Limiting factors that affect population size are
called population-limiting factors.
Density-independent factors are limiting
factors that are not dependent upon the density
of the population. These factors are usually
abiotic. Examples of density-independent factors
could be droughts, floods, extreme heat or cold,
tornadoes, hurricanes, snowfall etc.
Many moose and deer die in years of heavy
snowfall regardless of their density.
Deep Snowfall's Effect on Moose
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Moose are extremely vulnerable to predation in
the deep snow. Especially when the snow is
hard enough to support the weight of a wolf.
Population-Limiting Factors
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Density-dependant factors are limiting factors
that are affected by the population density.
Density-dependant factors are often biotic factors
like predation, disease, parasites, and
competition for limited resources.
Predation is high when the density of prey
species is high. Prey are much easier to catch.
Diseases and parasites spread more easily when
densities are high because there is more contact
between individuals within a population.
Population Characteristics
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Another important characteristic of populations is
the population growth rate.
The population growth rate explains how fast a
population grows.
Two primary factors you need to know in order to
determine a population's growth rate are the
natality (birth rate) and mortality (death rate).
Two other factors that are also important are
emigration and immigration. Emigration is
moving away from the population. Immigration is
moving into a population.
Population Growth Rate Models
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One way to understand population growth rates
is to graph them to produce a model.
An exponential growth model shows that as a
population grows, its growth rate increases.
When this happens the population experiences
a period of slow growth called the lag phase.
Then, after several generations, the curve
begins to change into a steep j-shaped curve.
The j-shape represents the rapid increase in
growth, which is called exponential growth.
Exponential Growth Model
Population Growth Rate Models
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Another type of model that shows population
growth rates is called the logistic growth model.
The logistic growth model is similar to the
exponential growth model, except the growth rate
slows down or stops once the population reaches
the carrying capacity.
The carrying capacity is the maximum number of
individuals in a species that a habitat can support.
Logistic growth occurs when deaths exceeds
births, or emigration exceeds immigration.
Population Growth Models
Reproductive Patterns or Strategies
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The rate strategy is
used by r-strategists
High natality and
mortality
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Low parental care
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Short life spans
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Begin breeding early
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Fish, mice, rabbits
Reproductive Patterns or Strategies
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The carrying-capacity
strategy is used by kstrategists
Low natality and
mortality
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High parental care
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Long life spans
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Begin breeding late
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Elephants, bears,
humans