INTRASPECIFIC AND INTERSPECIFIC INTERACTIONS IN LOW-DENSITY POPULATIONS IN RESOURCE-RICH HABITATS

Parasites on the gills of marine fish are used to study intra- and interspecific interactions. The model has several advantages: there are many replicas, habitats are small and can be easily examined, ectoparasite numbers vary from none to hundreds of more than 15 species, data can be quantitatively evaluated, fish are easily available in large numbers, all but one or few environmental parameters can be kept constant in "natural" experiments, in marine habitats uncontrolled variability can be largely excluded, only space for attachment is in limited supply. Evidence is provided that many niches are not utilized. The observations that vacant niches are available, that some fish species have many congeneric parasite species whereas others have few or none, and that there are insignificant effects of potentially competing species on microhabitats and on infection intensities do not support the view that interspecific competition has great ecological/evolutionary importance. Morphological evidence for interspecific competition is ambiguous. Expansion of microhabitats at great infection intensities indicates intraspecific competition. The observations that micro-habitat restriction leads to increased intraspecific contact, that adult stages often have more restricted sites than larval or asexually reproducing stages, that sessile and rare species have more restricted sites than mobile and common ones, and that sites may become narrower at the time of mating, provide evidence that an important function of niche restriction is enhancement of the chances to mate. Differences in microhabitat overlap between congeners compared to that between non-congeners, and differences in the structure of copulatory organs between spatially segregated and non-segregated congeners suggest that an important function of niche segregation is reinforcement of reproductive barriers. Evidence is given that most animals species are likely to occur at low densities in resource-rich habitats, and that conclusions based on the study of gill parasites can be applied to them. In particular, a discussion of a diversification equation modelled on the logistic equation for population growth shows that there is a vast number of vacant niches. The general conclusion is that the major problem for most animals is not to avoid competition but to find suitable sites for feeding and mating.