Nested assemblages resulting from host size variation: the case of endoparasite communities in fish hosts

Nested species subsets are a common pattern in many types of communities fo und in insular or fragmented habitats. Nestedness occurs in some communitie s of ectoparasites of fish, as does the exact opposite departure from rando m assembly, anti-nestedness. Here, we looked for nested and anti-nested pat terns in the species composition of communities of internal parasites of 23 fish populations from two localities in Finland. We also compared various community parameters of nested and anti-nested assemblages of parasites, an d determined whether nestedness may result simply from a size-related accum ulation of parasite species by feeding fish hosts. Nested parasite communit ies were characterised by higher prevalence (proportion of infected fish) a nd intensities of infection (number of parasites per fish) than anti-nested communities: the two types of non-random communities did not differ with r espect to parasite species richness, however. In addition, the correlation between fish size and the number of parasite species harboured by individua l fish was much stronger in nested assemblages than in anti-nested ones, wh ere it was often nil. These results were shown not to be artefacts of sampl ing effort or host phylogeny. They apply to both assemblages of adult and l arval parasites, which were treated separately. Since species of larval par asites are extremely unlikely to interact with one another in fish hosts, t he establishment of nestedness appears independent of the potential action of interspecific interactions. The species composition of these parasite co mmunities is not determined from within the community, but rather by the ex trinsic influence of host feeding rates and how they amplify differences am ong parasite species in probabilities of colonisation or extinction. Nested patterns occur in parasite communities whose fish hosts accumulate parasit es in a predictable fashion proportional to their size, whereas anti-nested communities occur in parasite communities whose fish hosts do not, possibl y because of dietary specialisation preventing them from sampling the entir e pool of parasite species available locally. Thus, nestedness in parasite communities may result from processes somewhat different from those generat ing nested patterns in free-living communities. (C) 2001 Australian Society for Parasitology Inc. Published by Elsevier Science Ltd. All rights reserv ed.