ANALYSIS OF LARVAL TREMATODE COMMUNITIES

We can compare natural communities with null models of communities to indicate how they differ from random assemblages of species (i.e., how much structure is present). However, because null models draw on obse rved values of species' prevalences, whatever structure already exists in natural communities affects the composition of a null model and we akens its comparative power. To address this, we developed formulae to estimate ''pre-interactive'' species prevalences permitting a more se nsitive quantification of community structure. Nonetheless, if a null model deviates from the community that we base it on, it is difficult to separate the effects of heterogeneity in recruitment from competiti on. We have developed a method to test for each independently. Applyin g our analytical techniques to a well-studied guild of larval trematod es in the salt marsh snail Cerithidea californica revealed that compet itive interactions among species were the most significant structuring force. Interestingly, spatial heterogeneity acted to significantly in tensify species co-occurrences. This differs from previous studies, wh ich argued that the isolating effects of spatial heterogeneity, not co mpetition, structure these communities by reducing co-occurrences.