Parasites and the Coexistence of Animal Host Species

Data from the literature suggest that the phylogenetic backgrounds, body sizes and morphologies, and feeding behaviors of animal species serve to promote parasite-host specificity. Differences in these factors among coexisting animal species appear to result in hosts with parasite faunas characteristic of their species. There is laboratory and field evidence that suggests parasites may constitute barriers to sympatry of host species with similar parasite susceptibilities. The factors that promote parasite host specificity are the same as those often cited by field ecologists as being responsible for reducing competition among coexisting animal species. If future experimentation demonstrates that maintenance of low levels of sharing of parasites is necessary in allowing for animal coexistence, then competition may be of secondary importance in determining the patterns of coexistence of similar animal species (e.g., birds). If competition for resources is of importance in determining "community structure" or species richness, it may have its greatest impact in determining relations between disparate taxonomic groups (e.g., Brown et al. 1979). This results from large phylogenetic differences allowing potentially competing animals to consume the same foods, etc., without being susceptible to one another's parasites. Communities as we know them may be capable of supporting more species of mammal, insect, etc. The factor limiting species richness may be the availability of a wide variety of host phylogenetic background, or other ways of avoiding the overlap of parasites among host species. Evaluation of these suggestions requires, at a minimum, a data base providing information on changes in parasite communities with changes in climatic type, latitude, host species richness, etc., as well as some understanding of the differences between the parasite faunas of differing taxonomic classes of host, and the effects of body size-morphology and food type upon parasite faunas. Such a systematic data base is unavailable.