Niche restriction is usually attributed to interspecific competition: a species does not spread from its optimal niche into less suitable ones because competing species, better adapted to these niches, prevent it from doing so. However, the chapters dealing with interspecific competition have shown that evidence for this assumption is in many cases far less convincing than generally assumed. But why, then, are niches restricted? This chapter deals with this problem by firstly demonstrating that niches may be restricted even if potentially interacting species are absent, and secondly by suggesting other, non-competitive mechanisms responsible for niche restriction. It further proposes alternative mechanisms for explaining niche segregation.

Evidence for niche restriction even in the absence of potentially interacting species, and mechanisms responsible

Gill parasites of fishes are particularly good models for examining this question because the distribution of parasites can be easily mapped, because the number of species is limited (even in the richest communities they do not exceed about 30 species and are usually much fewer), and because an almost unlimited number of replicas are available. Rohde, in a series of papers (e.g., Rohde 1976a, b; 1977a, b, c; 1978a, b; 1980b; and reviews 1989; 1991; 1994a; 2002), has shown that monogenean gill parasites of marine fish from all latitudes use microhabitats that are sometimes very restricted, even when competing species do not exist or, when they do exist, are not present on individual fish. Figure 5.1 gives an example.