Competition occurs when members of the same or different species utilize shared resources that are in limited supply, thereby reducing one another's individual fitness and population abundance through the depletion of those resources. This definition encompasses two mechanisms of competitive interaction. Exploitation competition involves the depletion of resources such that another individual is disadvantaged. Interference competition is a direct interaction, usually of an aggressive nature, for instance when one individual excludes another from a preferred habitat. Competition has long been viewed as a challenge to species coexistence, requiring sufficient differences between species to prevent competitive exclusion. Thus niche specialization becomes a key consideration in community assembly, a topic we return to in Chapter 10. Competition has been demonstrated in many different settings, however, and when it occurs it often is asymmetrical, with one species able to exclude a second species, which persists by occupying habitats or using resources largely unutilized by the superior species (Begon et al. 2005). Competition may be less evident a force than predation and herbivory, perhaps because it acts more gradually (Gurevich 2002). In addition, competition may often be diffuse, emanating from many species rather than just a pair-wise interaction.

A rigorous demonstration of competition generally requires evidence of an adverse effect of numbers of one population upon the abundance, growth, or survival of individuals of another population under reasonably natural conditions, and also some insight into the mechanism. However, many studies simply document some overlap in resource use, from which competition is inferred, and some differences, from which niche partitioning is inferred. Although such studies must be viewed as weak evidence, they make up a large portion of the literature on competition. Thus we shall first consider the evidence in support of resource partitioning, and then look to other lines of evidence including experimental and natural comparisons. Finally, since unrestrained competition ultimately should result in the elimination of all but the best competitors, it is necessary to ask how commonly this occurs. Physical disturbance, floods in particular, appear to be important in counteracting strong competition in a number of instances.

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