Many freshwater ecosystems are threatened and losing biodiversity at rates equivalent to tropical rainforests.  Losses include not only species extinctions, but declines in the abundance of common species and shifts in species dominance patterns.   Effects of losses on ecosystem processes are complex and understudied.  Both species identity and species richness can be important, and can vary in importance with species composition, trophic level and foodweb structure, environmental context, and spatial and temporal scale.  Studies with organisms as diverse as small benthic insects and large bivalves have shown that species within ‘trait-based’ functional groups may be contributing differently to ecosystem function, and thus cannot be assumed to be redundant.  Ecosystem services provided by unionid mussels are linearly related to community biomass and hydrologic residence times and some species have a greater impact due to metabolic and behavioral differences under different environmental conditions.  At the scale of a mussel bed, field and laboratory experiments demonstrate potentially strong interactions between mussel species with dominant species regulating factors such as periphyton biomass, but also increasing the body condition of rarer mussel species.  At the scale of whole rivers, species richness becomes important, with different species thriving and driving ecosystem services under different environmental conditions.