Decomposition of senesced primary production starts processing chains in aquatic systems. Shredding macroinvertebrates convert coarse particulate organic matter (CPOM) to fine particulate organic matter (FPOM) that supports 2 other feeding groups, collecting and filtering macroinvertebrates. This linkage is often invoked by aquatic ecologists, but the effect of detritivore assemblage composition on production of FPOM is relatively understudied. I manipulated detritivore assemblage composition (Limnephilus sp., Caecidotea sp., and Hyalella azteca) in aquatic mesocosms stocked with green speckled alder leaves (Alnus incana rugosa). I measured production rate, size distribution, and stoichiometry of FPOM produced through time. Detritivore species richness had a positive effect on FPOM production resulting from inclusion of the functionally dominant shredder, Limnephilus sp., in mixed-species treatments (e.g., sampling effect). Mixed-species treatments had significantly faster particle production than predicted from single-species treatments. The significant increases in particle production in mixed-species treatments could have resulted from release of Limnephilus sp. from intraspecific competition, facilitation between shredders, or both processes. FPOM size distribution and C∶N varied significantly among treatments and was affected by species interactions in mixed-species treatments. The presence of Limnephilus sp. significantly skewed the FPOM size distribution and increased the mass of particles >250 µm by ∼60%. These results suggest that the specific shredding insects in an assemblage could strongly affect production of FPOM and the size distribution and stoichiometry of FPOM produced by the benthos of a stream.