Direct and Indirect Effects of N-species Competition

A measure of net interspecific competition is proposed which incorporates both direct and indirect effects between each species pair in a community. Indirect effects of one species on another may be mediated through other species in the community such that changes in the first species induce changes in other species which in turn directly affect the second species. Even in a competitive community these indirect effects are not necessarily always of a competitive nature and may sometimes actually act to oppose the effects of direct competition. Species pairs which are strong competitors in isolation may exhibit little net competition or even appear mutualistic depending on the other species present in the community.An expression for the net competitive effects between species pairs is derived in terms of the elements of the α-matrix representation of the direct competitive effects. Examples calculated from the literature suggest that indirect competitive effects may often act to counterbalance direct competition between otherwise strong competitors in natural communities.