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Given adequate conditions populations
will grow. The growth of a population
depends on several factors: Birth rate,
death rate, immigration, and
emigration.
There
are three basic interactions between species.
When species populations grow interactions start to occur.
1.) Competition: The outcome is negative for both species
involved
2.) Symbiosis: The outcome is beneficial for both species
involved
3.) Predation-parasitism: The outcome is beneficial for one
species and negative for the other.
Competition
The competitive exclusion principle states that two species that
have the same exact requirements cannot coexist in the exact
same habitat.
Exponential Growth:
If a population has a constant growth rate (which is determined
by birth rate and death rate) that is not limited by food or
resources the growth is said to be exponential
.
Constant exponential growth is not
usually seen in nature. The
availability of resources and space
usually control the population of an
organism. What is one species that has exhibited exponential
growth throughout its existence?
Natural populations usually exhibit
something called logistic growth.
There is an upper limit to the
number of individuals the
environment can support. Ecologists
refer to this as the "carrying
capacity" of the environment. Populations
in this kind of environment show what is known as logistic growth.
Carrying capacity refers to the
maximum, equilibrium number of
organisms of a particular species that
can be supported indefinitely in a
given environment.
Organisms that live in stable
environments tend to make few,
"expensive" offspring. Organisms that
live in unstable environments tend to
make many, "cheap" offspring.
r-selection, for
those species that produce many
The two evolutionary "strategies" are termed
"cheap" offspring and live in unstable
environments and K-selection for
those species that produce few
"expensive" offspring and live in
stable environments.
The following table compares some characteristics of organisms which are
extreme r or K strategists:
r
K
Unstable environment, density
independent
Stable environment, density dependent
interactions
small size of organism
large size of organism
energy used to make each individual
is low
energy used to make each individual is high
many offspring are produced
few offspring are produced
early maturity
late maturity, often after a prolonged period of
parental care
short life expectancy
long life expectancy
each individual reproduces only once
individuals can reproduce more than once in
their lifetime