Forest soils and streams receive substantial inputs of detritus from deciduous vegetation. Decay of this material is a critical ecosystem process, recycling nutrients and supporting detrital-based food webs, and has been attributed, in part, to leaf litter species composition. However, research on why speciose leaf litter should degrade differently has relied on a bottom-up approach, embracing interspecific variation in litter chemistry. We hypothesized that preferential feeding by an aquatic detritivore interacts with species-specific leaf palatability and slows decay of speciose leaf litter. We addressed this by offering four single- and mixed-species leaf resources to field densities of a leaf-shredding consumer. Mixing leaf species resulted in slower total leaf decomposition. Decreases in mixed-species decomposition was partly explained by preferential feeding by the consumers in one case, but the lack of preferential feeding in other mixtures suggested an interactive effect of feeding and microbial degradation. Loss of riparian tree biodiversity may have implications for in-stream consumer-resource interactions.