Populations & Communities

5.1 - Populations
Population
- a group of organisms of the same species that live in
    an area and interbreed.

Population Growth
 - depends on...
    - birth rate(the number of individuals born each year)
    - death rate(the number of individuals who die each year)
    - immigration(the number of individuals who move into an area)
   - emigration(the number of individuals who leave an area)
 - can be...
    - exponential growth if the numbers increase by a certain
       factor every time period.
    - logistic growth if the population starts at a minimal number
       then grows to a maximum number based on the area's
       carrying capacity.

The carrying capacity of an environment is the largest population that an environment can support at any given time.  There are two types of factors that influence an area's carrying capacity:
  - density-dependent factors are affected by the number of
     organisms present in the environment.  An example could be
     the number of available nesting sites for birds.
  - density-independent factors are not affected by the number
     of organisms in the environment.  Examples could include
     weather, floods, fires, etc.

5.2 - Interactions in Communities
Predation
- one organism kills another for food.
   - The prey is killed and eaten by the predator.
Parasitism - a relationship where one organism benefits while the
      other is harmed.
   - The parasite feeds on its host without killing it.
Herbivory - herbivores feed on plants, usually without killing them.

Symbiosis - a relationship in which two different organisms live in
     close association with each other.
  - Mutualism (+/+) - both sides benefit
  - Commensalism (+/o) - one side benefits, the other is unaffected.

Coevolution - the back-and-forth evolutionary adjustment
    between two species that interact.

5.3 - Shaping Communities
A niche is the unique position occupied by a species within its community, both in terms of its physical use of its habitat and its function within the community.  (In other words, an organism's niche describes how it interacts with other organisms and the abiotic factors that make up its environment.)

There are two types of niches that organisms occupy:
 - fundamental niche: the largest ecological niche where an
      organism or species can survive.
 - realized niche: the actual niche that a species occupies.
A species' realized niche is almost always smaller than its fundamental niche because competition between species limits the resources that are available for each species to use.

Competitive exclusion occurs when one species eliminates a competing species from an area by out-competing the other species for resources.

Some species avoid competing with each other by dividing up resources.  They may feed in slightly different ways, places, or times.

Predators can limit competition by keeping the populations of competing species under control.

A keystone species is a species that affects the survival and population size of many other species, and is therefore critical to the functioning of the ecosystem.