A Critical Evaluation of Intrinsic and Extrinsic Factors Responsible for Niche Restriction in Parasites

Abstract

Intrinsic and extrinsic factors responsible for niche restriction in parasites and particularly in Monogenea of fish are discussed. Interspecific competition may result in competitive exclusion of one or several species, or it may lead to a change in the microhabitat of one or all co-occurring species (interactive site segregation), but there is no evidence that such effects lead to evolutionary changes and avoidance of competition, i.e., to selective site segregation. That extrinsic factors, particularly competition, are not of such great evolutionary significance among parasites as is usually assumed, is indicated by the following observations. Most effects of competitive exclusion may also be caused by intraspecific crowding. Interspecific effects may also be positive, enhancing the chances of species co-occurring. Host-mediated effects are usually more harmful to the species evoking them than to competing species. Parasite species with coinciding or overlapping microhabitats often show no interactions. Related species commonly have widely overlapping niches. Only a small proportion of the niches available to ectoparasites of fish is filled. Species with mouthparts of different sizes may use the same food in the same habitat, i.e., such differences do not necessarily indicate segregation. Evidence for the effect of species intrinsic factors is that in many cases microhabitats are extremely restricted, even though competing species do not exist and cannot have existed in the past, for instance in fish species at high latitudes with only one species of ectoparasite. Circumstantial evidence shows that an important factor responsible for niche restriction in parasites is selection to increase intraspecific contact and thus mating. Niche diversification is self-augmenting, and in a continuously expanding niche space populations would be diluted to such a degree that mating would become impossible without the counteracting selection for niche restriction. The probability that two species show complete niche coincidence is infinitesimally small even without competition, and it is therefore not permissible to use niche differences as proof for competition.