Initial research informing biodiversity–ecosystem functioning (B–EF) theory focused largely on terrestrial plant species richness effects on productivity. Recent studies in stream ecosystems have further advanced understanding of B–EF beyond species richness by testing effects of species diversity (richness and composition) at multiple trophic levels. Recent meta-analyses of stream B–EF studies across multiple trophic levels found stronger evidence for composition- than richness-functioning relationships. Here, we further examine effects of resource and consumer diversity on stream organic matter processing (OMP) to identify general patterns and potential mechanisms of non-additivity across spatial and temporal heterogeneity. We also review multi-trophic consumer response patterns to resource diversity to assess how consumer diversity responses compare to independent resource and consumer effects on OMP in streams. Consistent emergent patterns include: (1) Top-down (i.e. consumer) diversity effects are common among vertebrate, invertebrate and microbial trophic levels and are generally explained by species evenness; (2) bottom-up (i.e. resource) diversity effects are mediated by species evenness and vary both spatially and temporally and (3) consumer responses to resource diversity that best explain resource diversity effects are predominantly seen at the microbial level. Resource and consumer diversity effects are driven by dominance of functionally distinct taxa. However, response of consumers to resource diversity only partially explain resource diversity effects, suggesting functional differences between how naturally colonizing and manipulated consumer assemblages use OM resources. The challenges facing general ecology and the advancement of B–EF theory include an improved understanding of how environmental heterogeneity and temporal and spatial variation influence B–EF patterns. Copyright © 2009 John Wiley & Sons, Ltd.