REGIONAL PATTERNS OF MUSSEL SPECIES DISTRIBUTIONS IN NORTH-AMERICAN RIVERS

North American freshwater mussels are a highly threatened group with h alf of the fauna already federally listed as threatened or endangered, candidates for listing, or believed extinct. Using data From 16 river systems, I examined distributional attributes of mussel species to pa in insight into the importance of regional-scale processes vs local-sc ale processes to species distribution patterns. There was no evidence of density compensation or saturation, which would have indirectly ind icated that competition was important in structuring mussel communitie s. Rather, there was a positive correlation between summed species den sities and regional richness. indicating that regional forces may be s trongly contributing to community structure. Incidence, abundance and nestedness patterns all indicated a hierarchical niche structure for t hese mussel assemblages. I hypothesize that these hierarchical pattern s may be the result of differences in colonization potentials among mu ssel species as a result of different fish-host requirements among mus sels, and of the abundance and distribution of those host fishes.