Freshwater benthic science, through studies of resource heterogeneity and organic matter processing, has developed theoretical understanding of the relationships between biodiversity and ecosystem function. Initial theories explaining species diversity effects on ecosystem function were largely based on terrestrial studies, however, recent studies have tested the functional importance of species diversity in both terrestrial and aquatic ecosystems. These studies have shown additive effects of leaf litter species identity and nonadditive effects of species interactions on breakdown dynamics based on full-factorial experiments and analyses of single- and mixed-species litter. In stream ecosystems, nonadditive effects of litter species diversity have resulted in synergistic and antagonistic effects on breakdown dynamics. Differences in the directional effects (positive or negative) of litter species diversity have been explained by site-specific abiotic and biotic variables. Further exploration into resource diversity beyond the species-level has shown that litter species evenness mediates effects of litter species diversity on breakdown. Plant genes explain intraspecific variation in litter breakdown and macroinvertebrate assemblages, and recent advances in molecular techniques have shown that litter mixing alters microbial community diversity and litter processing. Freshwater benthic science has great potential to further expand theoretical understanding of global environmental change impacts on biodiversity and ecosystem function.