COMMUNITY STRUCTURE - LARVAL TREMATODES IN SNAIL HOSTS

In species assemblages of larval trematodes in individual snail hosts, fewer multispecies infections are observed than might be expected by chance. Both interspecific competition and the isolating effect of het erogeneity in recruitment may explain this pattern of community struct ure. Here, we analyzed the expected and observed frequency of double i nfections, using data culled from 62 studies. Our analysis included 29 6,180 host snails. Of these, 62,942 were infected with one or more spe cies of trematode (23% pooled over all studies, 24% average across stu dies). By incorporating information from subsamples, we were able to e stimate the proposed isolating effect of heterogeneity in recruitment. Surprisingly, spatial and temporal heterogeneity as well as different ial prevalence among host size classes typically led to intensificatio n of interactions (average increases in interactions by +19%, +19%, an d +23%, respectively), while partitioning among host species usually l ed to isolation of potential competitors (a -1% average decrease in in teractions). We calculated the expected number of interspecific double infections by applying rules of independent assortment to the frequen cy of trematode species. The majority of the 14,333 expected interacti ons did not persist; only 4,346 double infections were actually observ ed (a 69% decrease, 62% average). Competition, via a variety of inters pecific competitive mechanisms by dominant species, is the structuring process most consistent with this paucity of observed multispecies in teractions. How important is competition? Overall, we estimated that 1 3% (10% average) of the trematode infections were lost to interspecifi c interactions. Subordinate species in particular suffered very high l osses.