PHYLOGENY, ECOLOGY, AND THE RICHNESS OF PARASITE COMMUNITIES IN VERTEBRATES

The makeup of parasite communities is the result, among other factors, of interactions between the evolutionary history and ecological chara cteristics of hosts. This study evaluates the relative importance of s ome ecological factors (host body size, diet, habitat, latitude, and t he mean number of parasite individuals per host) as determinants or co rrelates of parasite community richness in vertebrates, before and aft er controlling for potential effects of host phylogenetic relationship s. Data were obtained from the literature on 596 parasite communities belonging to one of four distinct types: gastrointestinal parasite com munities of fish, birds, or mammals, and ectoparasite communities of f ish. There were positive correlations between the number of hosts samp led and mean species richness of the parasite community of each genus. In analyses treating host genera as independent statistical observati ons and using estimates of parasite species richness corrected for hos t sample size, positive correlations were observed between richness an d host body size in gastrointestinal communities of all three groups o f vertebrates. The mean number of parasite individuals per host also w as correlated positively with species richness. In fish, richness incr eased with increases in the proportion of animal food in the host diet . Aquatic birds had richer parasite communities than their terrestrial counterparts, whereas marine fish had richer gastrointestinal parasit e communities than freshwater fish. The richness of ectoparasite commu nities on fish showed no association with any of the ecological variab les investigated. Using host genera as independent points in the analy ses may lead to biased results since some host lineages are descended from recent common ancestors, and are therefore not truly independent. The comparative analysis was repeated using phylogenetically independ ent contrasts derived from the phylogeny of hosts. Once the effects of host phylogeny were removed, somewhat different results were obtained : host body size showed no relationship with parasite species richness in birds, and there was no evidence that habitat transitions resulted in significant changes in parasite species richness in any of the typ es of communities studied. Of the ecological factors studied, the comp arative analyses suggest that only host body size can be an important determinant of parasite community richness in certain host groups. Thi s study illustrates clearly the need to control for phylogeny in inves tigations of host-parasite interactions.