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 heterogeneity inrecruitment may explain this pattern of community structure. Here, we analyzed the expected and observed frequency of double infections, using data culled from 62 studies. Our analysis included 296,180 host snails. Of these, 62,942 were infected with one or more species of trematode (23% pooled over all studies, 24% average across studies). By incorporating information from subsamples, we were able to estimate the proposed isolating effect of heterogeneity in recruitment. Surprisingly, spatial and temporal heterogeneity as well as differential prevalence among host size classes typically led to intensification f interactions (average increases in interactions by +19%, +19%, and +23%, respectively), while partitioning among host species usually led to isolation of potential competitors (a -1% average decrease in interactions). We calculated the expected number of interspecific double infections by applying rules of independent assortment to the frequency of trematode species. The majority of the 14,333 expected interactions did not persist; only 4,346 double infections were actually observed (a 69% decrease, 62% average). Competition, via a variety of interspecific competitive mechanisms by dominant species, is the structuring process most consistent with this paucity of observed multispecies interactions. How important is competition? Overall, we estimated that 13% (10% average) of the trematode infections were lost to interspecific nteractions. Subordinate species in particular suffered very high losses.