In this chapter we will examine the relationships among populations within freshwater mollusc communities. We define the term ‘community’ broadly, noting as we do that its component mollusc populations may be of such diverse ecological character that the likelihood of interactions among them may be less than the likelihood of interactions between them and other elements of the benthos. The artificiality of the concept of the freshwater mollusc community does not, however, erase its utility.

Developing a theme first opened in Chapters 2 and 3, here we review a large body of literature approaching diet and habitat from a comparative standpoint. Differences in the gut content of co-occurring gastropod species can be substantial, but this seems primarily due to differing habitat choice, rather than selective grazing. Variation in substrate preference seems central in gastropod communities, with other aspects of the local environment (especially depth) important as they impact substrate. The overlap in diet and habitat observed within bivalve communities seems great.

We next review the several studies that have applied ordination techniques to variation in species distribution within freshwater mollusc communities, and offer several original analyses. The elements of most communities seem to associate into subsets according to features of the habitat. We also find one situation where species seem to aggregate apart, and cases where little structure of any sort is apparent. Sampling scale seems to be a key to detecting interspecific pattern in distribution.