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.