Journal of Animal Ecology 2004 73, 756– 766 © 2004 British Ecological Society Blackwell Publishing, Ltd. Detritus processing, ecosystem engineering and benthic diversity: a test of predator–omnivore interference YIXIN ZHANG, JOHN S. RICHARDSON and JUNJIRO N. NEGISHI* Department of Forest Sciences, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada Summary 1. Interference between species from different functional groups may influence ecosys- tem 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, bio- mass 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 ecosys- tem processors and engineers, and strongly influenced detrital processing, benthic diversity, and the accumulation of POM and fine sediments. Key-words: cutthroat trout, signal crayfish, POM accumulation, headwater streams, strong interactors. Journal of Animal Ecology (2004) 73, 756– 766 Introduction Non-trophic interguild interactions of species from dif- ferent functional groups, such as predators and detri- tivores, have important direct and indirect influences on community structure (Kerfoot & Sih 1987) and ecosystem functioning (Loreau 2000). In detritus-based stream ecosystems, detritivorous consumers do not regulate allochthonous energy inputs (Polis, Anderson & Holt 1997). However, detritivores play an important role in processing organic matter, which involves a sig- nificant portion of the material cycling and energy flow that influence the nutrient and material input–output balances that govern ecosystem processes (Cummins 1973; Wallace et al. 1997; Zhang et al. 2003). Predacious fish may have direct and indirect effects on the function of detritivores and their efficiency of litter processing (e.g. Short & Holomuzki 1992; Ruetz, Newman & Vondracek 2002) and benthic diversity (Nyström, McIntosh & Winterbourn 2003). Among a variety of species interactions, interference is ubiquitous in natural communities (Amarasekare 2002); interference is the non- consumptive, negative interaction between species that may affect the distribution of consumers (Sutherland 1983). In stream ecosystems, the knowledge of how inter- ference influences effects of detritivores on ecosystem functioning within benthic assemblages is still limited. Forest stream ecosystems are characterized by a large amount of allochthonous organic input from the surrounding watershed. Leaf litter decomposition is a critical carbon pathway of organic matter and nutrient flux in forested headwater streams (Richardson 1991). As primary and secondary consumers, by feeding on periphyton, macrophytes, detritus and benthic inverte- brates, crayfish can be important in ecosystem func- tioning in terms of nutrient cycling and energy flow (Usio 2000; Parkyn, Collier & Hicks 2001). Early food web models predicted that omnivory destabilizes com- munities and should be rare in nature (Pimm & Lawton 1978). However, recent empirical and theoretical studies reveal that omnivory and multiple trophic modes occur commonly and are often important in influencing Correspondence: Yixin Zhang (tel. +1 604 8223685; fax +1 604 8229102; e-mail yixin@interchange.ubc.ca). *Present address: Junjiro N. Negishi, Department of Geo- graphy, National University of Singapore, AS2 1 Arts Link, Singapore, 117570.