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Functional trait diversity within species assemblages ultimately determines whether or not ecosystem processes are sensitive to shifts in relative abundances or species composition. For example, trait variation suggests detritivores process detritus at different rates and make different relative contributions to whole-assemblage processing, which is therefore likely sensitive to global change-driven shifts in assemblage composition. Here, we address three main questions: 1) what is the magnitude of interspecific variation in detritus processing within the detritivore guild in high-elevation wetlands, 2) which traits drive interspecific variation in processing among larval caddisflies, 3) are larval caddisflies’ single-species effects additive in multi-species assemblages? We used a series of microcosm experiments to quantify species-specific coarse and fine particulate organic matter (CPOM and FPOM) processing for ten closely-related larval caddisfly species and three non-caddisfly species. Next, we compared trait-based models including life history, dietary, and extrinsic trait predictors to determine which intrinsic and extrinsic traits best explain the interspecific variation among the larval caddisflies. Finally, we compared processing by mixed caddisfly assemblages in microcosms and in natural ponds to additive predictions based on individual species processing rates. We found considerable interspecific variation in biomass-specific CPOM (thirteen-fold differences) and FPOM (eight- fold differences) processing. Furthermore, on a mass-specific basis, amphipods and chironomids processed a similar amount of detritus as caddisflies, suggesting the importance of non-shredder taxa for processing CPOM may be greater than previously recognized. Next, the best trait model of CPOM processing included development rate, percent CPOM in the diet, and primary case material. Similarly, the best model for FPOM processing included development rate and percent of total detritus in the diet. Finally, additive predictions were strikingly similar to realized 21 processing in mixed caddisfly assemblages in both microcosms and natural ponds, with the largest difference being a 15% overestimate of CPOM processing in one microcosm experiment. Thus, as global change drives species range shifts, invasions, and extinctions, additivity of species-specific processing suggests that single-species rates may be useful for understanding functional consequences of shifting detritivore assemblages. Furthermore, a trait-based approach to predicting species-specific processing could support preparation of additive estimates of whole-assemblage processing. 22
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