Intraspecific Competition is defined as the struggle between members of a population for scarce resources. As in intraspecific cooperation, there are two basic types of competition:

1. Adapted or programmed intraspecific competition results from aggressive social behavior such as dominance hierarchies and territoriality. Here only certain individuals high in the peck-order, or holding territories, succeed in breeding. This is sometimes called contest competition because it involves aggressive contests between competing individuals.
2. Unadapted or incidental intraspecific competition results from the accidental interaction between individual organisms utilizing the same resources, for resources used by one are unavailable to others. This is sometimes called scramble competition because everybody is involved in a mad scramble for the scarce resources.

Notice the parallels between intraspecific competition and cooperation. Both can be induced by adapted (evolved) or incidental (accidental) processes. Both are associated with the problem of obtaining resources or avoiding being used as a resource by others. It should not surprise us that these two powerful principles have led to the parallel evolution of aggressive and cooperative social behaviors in many species.

A major consequence of intraspecific competition is that the survival and/or reproduction of individual organisms normally declines as the density of the population rises (see figure). This is commonly called "density dependence" in the literature. However, this term has probably generated more controversy and confusion than any other single issue in ecology. The argument started between the two Australian ecologists A. J. Nicholson and H. G. Andrewartha. Nicholson studied populations of blowflies living in corpses and concluded that density-dependent struggle for food, what we call intraspecific competition, was the main factor regulating population size. Andrewartha studied populations of rose thrips and found that numbers were largely determined by weather which, of course, is not dependent on the density of thrips. The controversy is largely semantic -- a confusion about the meaning of the terms regulation, stability, feedback and so on -- but even so it continues to this day.

e-mail Instructor?

Use BACK key to return to Sessions