Parasite faunas of freshwater fish: the relationship between richness and the specificity of parasites.

The relationship between the host specificity of parasites and the richness of the assemblages in which they occur was examined among the parasite faunas of Canadian freshwater fishes. The prediction tested was that rich faunas would consist of both generalist and specialist parasites, whereas poor faunas would include only generalists, a pattern known as nestedness. An index of nestedness was computed for parasite faunas of fish species in 5 large families (Salmonidae, Cyprinidae, Catostomidae, Centrarchidae and Percidae) and compared with the value expected if parasite faunas are random assemblages of parasites. There was no evidence of nested patterns among any of the 5 families of fish hosts. However, since both measures of host range of parasites (i.e. number of known host species) and richness of parasite faunas are affected by how intensely the different parasites and hosts have been studied, tests of nestedness may be flawed. After correcting both variables for study effort, negative correlations were found between the mean host range of parasites and the richness of the faunas to which they belong. In other words, parasites in rich faunas occurred on average in fewer host species, because of the many specialists, than parasites in poor faunas, which are mainly generalists. This relationship was apparent in all fish but the Salmonidae; fish species in this family have been introduced to new areas much more frequently than other fish, and their parasite faunas have thus had a distinct recent history. The general trend observed in non-salmonid fish suggests that parasite colonization and speciation may have been facilitated in some fish species, but not in others.