Effects of species number and identity on the breakdown rate of leaf litter were estimated in a laboratory experiment using leaf-eating insects, three species of Plecoptera, as detritivores. We found significant differences between the different species on this process in single-species experiment, but not when animal biomass was accounted for. When species were combined the effect of species identity was strongly reduced and rendered insignificant, whereas the number of species had a significant effect. This shows that rates of ecosystem processes may benefit from species richness even when all species belong to the same guild, which is in contrast to hypotheses predicting redundancy within guilds. Facilitation between species and negative interactions, where intraspecific interactions are greater than interspecific interactions, are two potential mechanisms which could explain increasing decomposition rates with species richness.