Supplementary Mechanisms for Niche Restriction Specialization and Avoidance of Interspecific Competition

It has never been suggested that selection for more effective mating is the only mechanism leading to niche restriction. An important ultimate cause of niche restriction may be specialization not just for more effective mating, but for more effective use of resources (Price, 1980), such as food and sites for attachment. A widely accepted view is that the primary function of niche restriction is the avoidance of intra- and interspecific competition. There is indeed evidence that microhabitat width and host ranges may be affected by the presence of other species, but many studies have shown that, in parasites, competition is of minor importance (reviews in Rohde, 1991, 1994, 1999). Sukhdeo et al. discuss interspecific competition in Chapter 11 (this volume) and I therefore mention only some major points. Most species of marine fish harbour few parasite species, and abundance of infections is low (Fig. 9.7). Consequently, many vacant niches are available, or, in other words, resources are in oversupply and there is no need for competition to occur. Packing rules derived from spatial scaling laws, which predict unimodal distributions skewed to the left in plots of species numbers against the size of species and a decline of body-size ratios of species of adjacent sizes with increasing size of organisms, apply to few, if any, parasites (Rohde, 2001b; Fig. 9.8), because species are not densely packed and do not compete for limiting resources. Some recent studies demonstrating the availability of empty niches and the lack of competition are by Sasal et al. (1999): digeneans of Mediterranean fish; Buchmann (1989), Dzika and Szymanski (1989), Koskivaara etal. (1992), Bagge and Valtonen (1999): monogeneans of freshwater fish; and Ramasamy et al. (1985): monogeneans of marine fish. Sousa (1994) has reviewed the evidence for interspecific interactions in parasite communities and concluded that such effects are important in some parasite communities, but not in others. However, although interspecific effects occur, evidence for their evolutionary significance does not exist. It may well be that, generally, such effects, where they occur, are intermittent and have no lasting effect on community structure (Price, 1980).

In summary, parasite data clearly indicate that niche space is largely empty (Figs 9.7 and 9.8). This strongly suggests that avoidance of competition is not an important factor in niche restriction and segregation, much less important than the necessity to find suitable habitats for survival (by specialization) and mating partners (mating hypothesis). As stated by Rohde (1979): '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.'

Fig. 9.7. (a) Number of species of metazoan ectoparasites on the heads and the gills per species of marine teleosts (5666 fish of 112 species examined). Note: maximum number 27 parasite species, but most fish species with fewer than five. If 27 is considered to be the maximum possible, the percentage of empty niches on all fish species would be 84.1%. (b) Abundance (= mean number of metazoan ectoparasites of all species per host species). Note: maximum abundance more than 3000, but most species with fewer than five. (From Rohde, 1998.)

Fig. 9.7. (a) Number of species of metazoan ectoparasites on the heads and the gills per species of marine teleosts (5666 fish of 112 species examined). Note: maximum number 27 parasite species, but most fish species with fewer than five. If 27 is considered to be the maximum possible, the percentage of empty niches on all fish species would be 84.1%. (b) Abundance (= mean number of metazoan ectoparasites of all species per host species). Note: maximum abundance more than 3000, but most species with fewer than five. (From Rohde, 1998.)

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