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). Sukhdeoet 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 Sasalet al. (1999):
digeneans of Mediterranean fish; Buchmann (1989), Dzika and Szymanski
(1989), Koskivaaraet al. (1992), Bagge and Valtonen (1999): monogeneans
of freshwater fish; and Ramasamyet 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 com-
petition 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 counter-
acting selection for niche restriction.’Niche Restriction and Mate Finding in Vertebrate Hosts 189