Detritus processing, ecosystem engineering and benthic diversity: a test of predator-omnivore interference

1. Interference between species from different functional groups may influence ecosystem functioning and biological diversity. This study tested whether interactions between predacious cutthroat trout and an omnivorous signal crayfish modified the crayfish's trophic and engineering effects within a detrital-based, stream benthic community. 2. We show in a trough experiment that omnivorous crayfish through their trophic and engineering roles enhance detritus decomposition, reduce particulate organic matter (POM) accumulation, and diminish diversity in leaf packs. 3. In crayfish troughs by day 30, leaf dry weight loss was 1.8-fold greater, whereas POM trapped in leaf packs was 80% less, than of those in controls, and the abundance, biomass and taxon richness of benthos in leaf packs were lower than those in controls. Predatory cutthroat trout did not affect those variables and did not interfere with the crayfish. 4. Crayfish and cutthroat trout both decreased fine material sedimentation in the troughs. 5. Thus, with no interference from cutthroat trout, the signal crayfish acted as ecosystem processors and engineers, and strongly influenced detrital processing, benthic diversity, and the accumulation of POM and fine sediments.