Wednesday, June 6, 2007 - 4:45 PM
331

Direct Effects of Predatory Invertebrates versus Indirect Effects of Benthic Fishes on the Trophic Structure of a Dryland Stream

Ryan R. McShane and David E. Cowley. Department of Fishery and Wildlife Sciences, New Mexico State University, P.O. Box 30003, MSC 4901, Las Cruces, NM 88003

Dryland river systems possess depauperate fish communities relative to temperate or tropical streams and generally lack highly predaceous fishes.  Lack of top fish predators may indicate limited top-down control in trophic interactions, but predatory invertebrates could fulfill this role.  We investigated direct and indirect effects of fish trophic guilds and predatory invertebrates on the invertebrate assemblage and benthic resources of a dryland stream.  Two enclosure/exclosure experiments, with open controls, tested the effects of three fishes—insectivorous Gila pandora, detritivorous Agosia chrysogaster and algivorous Catostomus plebeius—and invertebrate predators on patch utilization by invertebrates.  Although the fish trophic guilds are probably not strictly defined (i.e., detritivores and algivores may employ omnivory), we measured changes in benthic resources to elucidate direct and indirect effects.  In enclosures with insectivores, invertebrate abundance, detritus and periphyton were weakly impacted compared with controls; however, with detritivores and algivores, organic matter and invertebrates were significantly reduced.  In fish exclosures, invertebrate abundance was reduced equivalently to detritivore and algivore enclosures, but detritus and periphyton were more weakly affected.  In dryland streams, indirect effects of fish grazers on the invertebrate assemblage appear stronger than direct effects of fish predators but approximately correspondent with invertebrate predators.

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