Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Types of Competition

Competition happens when organisms compete for limiting sources, including nutrients, water, light, space.  Competition does not include direct killing of a competitor, as this would be considered predation.

When competition occurs between members of the same species, it is considered intraspecific

When competition occurs between members of  different species, it is considered interspecific.

As members of the same species will have the same resource demands, intraspecific competition will usually be more intense than interspecific.

Interspecific competition is usually less intense, as different species will have somewhat different requirements, allowing for some resources to not be contested.  

II.  Competitive interactions

When intraspecific competition is very strong, and when one species is much better able to gather resources than the other, the weaker competator may be eliminated.  This precess is termed competitive exclusion.

In the past, this was thought to have been an important process in the natural world.  However, the best examples of this have only been documented from artificial experiments conducted under laboratory conditions, when two species are forced to live together.

In the 'real' world, evolution will tend to favor those individuals which minimize interspecific competition, as these individuals will be more fit.  Thus, we would expect competing species to evolve differences from each other over time.

The shift of bodies and/or behavior to lessen interspecific competition over time is termed character displacement.

Also, in the 'real' world the environment changes so rapidly, that time is ususally not sufficient to allow competitive exclusion to take place, as at one time one species may be favored, but at another time the other may be.  Because of this environmental change, not interspecific competition, is usually far more important determinant of species population size.

III.  Competition in determinate vs. plastic growth species.

Classic competition, in which weak individuals or species are removed from an area by stronger individuals or species, only can occur when the adult size of organisms are genetically determined.  Because adults will be roughly the same size (no matter the amount of resources) competition will control how many of those adults will exist in a region.  This type of control is important for mammals, birds, and other species showing determinate growth.

However, many other species do not have a set adult size.  For these species, adult size will be determined by the density of other individuals in the environment, combined with the amount of resources available.  Species which show such plastic growth include many fish, reptiles,  plants.

For these species, the number of organisms in the environment will not effected.  Rather their size will be. The more individuals in an environment, and the less resources in the environment, the smaller adults will be.  The fewer individuals in the environment, and the more resources in that environment, the larger adults will be (up to a point).

If you are working for the Wisconsin DNR and want to encourage growth of large perch or bass, what would you want to do, given that these fish show plastic growth?


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Created 2 September 2011, Last Update 09 September 2011

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