A separate problem is the competition between different species exploiting the same or similar resources. The kinds of resources different species exploit and generally their ecological niches are usually not identical, but sometimes they may be very close to each other leading to behavioral reaction toward another species similar to that of its conspecific. However, the differences in ecological niches imply that one species is more efficient than another or that environment is more favorable for one species than for another, so that one species is dominant and other subordinate in a given habitat. Competition of different species in different habitats can be analyzed applying the isoleg theory developed by Michael L. Rosenzweig. This theory is based on the theory of free habitat selection and it allows analyzing both the quality of the habitats and the dominance of one species over another. Consequently, with the help of this theory we may learn the behavioral mechanism of interspecific competition: elimination of one species by another or the coexistence of both of them.
The interspecies competition is not a war of one species against another. There is no good empirical evidence of animals being hostile more to individuals of another species than to its conspecific. If two species compete for the same resources, it is a competition of individuals not of species. What makes the interspecific competition different from intraspecific competition are differences in competing abilities between species, which can be larger than between individuals of a single-species population.
Other related phenomena are competition among sibs within a single family and the competition for mates. For the parents, it is often of selective advantage to produce more progeny than they can support. This leads to a strong competition among sibs mediated by their kinship and the manipulation of their parents. For example, an asynchrony in egg hatching among birds may produce large differences among sibs and an earlier elimination of the weakest ones ifresources are scarce. It leads to contest competition within a nest. Competition for mates in order to reproduce is a subject of a separate phenomenon shaped by sexual selection.
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