212 Predator diet shifts modify the outcome of interspecific competition via nonlethal mechanisms

Tuesday, May 19, 2009
Ambassador Ballroom
Emily J. Cholak , Department of Biology, Clarion University, Clarion, PA
Andrew M. Turner , Department of Biology, Clarion University, Clarion, PA
Recent studies show that predators may influence community structure by causing shifts in prey traits. Prior studies have focused on indirect interactions induced by variation in predator feeding rates. However, prey traits also depend on predator diet, independent of variation in feeding rates. Here we ask how shifts in predator diet influence the outcome of interspecific competition in a guild of pulmonate snails. Using experimental mesocosms containing caged predators, we manipulated interspecific competition and predator diet in a factorial manner and measured growth of Helisoma trivolvis. Competition was manipulated by adding a suite of three other gastropod species. Predator diet treatments consisted of crayfish fed Helisoma, crayfish fed the competing snail species, or crayfish fed annelid worms, with overall mass of prey held constant. Predator diet had strong effects on Helisoma growth. Growth was highest in annelid worm diet treatments but reduced by 22% in the Helisoma diet treatment. Interspecific competition depressed growth of Helisoma trivolvis by 14% relative to no competition treatments. The growth depression attributable to competition depended on predator diet. Competition effects were weakest in the Helisoma diet treatment. These results show that shifts in predator diet can influence the outcome of competition via nonlethal mechanisms.