Summary
The effects of interspecific competitive encounters have traditionally been viewed in terms of species replacement, but recent workers have recognised the importance of weak or diffuse competition, which leads only to reduced niche volume. Evidence of such niche restriction has been obtained from the study of bird communities, where it has been shown that species often expand niche dimensions in areas with few species. For example, one-third as many bird species are present on the island of Puercos as on the nearby Panamanian mainland and the insular birds forage in a wider range of habitat types than conspecific mainland birds. Unfortunately this rather direct method of relating changes in niche width to changes in competitive background. is often difficult to apply but the importance of competition can be evaluated in other ways. It is generally conceded that if competition occurs at all, it will be strongest among phenotypically similar species. More explicitly, the intensity of competition which a species faces should be inversely related to its phenotypic isolation, where such isolation is a complex function of the phenotypic distance between a species and coexisting species which share limiting factors.