Abstract
To increase the generality of biodiversity–ecosystem function theory, studies must be expanded to include real communities in a variety of systems. We modified J. W. Fox's approach to partition the influence of species richness on standing crop biomass (net biodiversity effect) of 21 freshwater mussel communities into trait‐independent complementarity, trait‐dependent complementarity (species with particular traits dominate without impacting other species), and dominance effects (species with particular traits dominate at the expense of others). Overall, species‐rich mussel communities have greater biomass than predicted based on average biomass across the region. This effect is largely due to trait‐independent complementarity with less abundant species having higher body condition and reduced metabolic rates in species‐rich communities. These measures are positively correlated with spatial and temporal thermal variation, suggesting that use of thermal niches as habitat may be important to species coexistence and performance, and emphasizing that knowledge of species traits and environmental context are important to understanding biodiversity–ecosystem function dynamics.