Ecological Interactions Among Predators in Experimental Stream Communities

Field observations on the effect of brook trout on an assemblage (sometimes described as a guild) of predatory salamanders suggest that the interactions among predators in small, headwater streams may be important in determining the structure of this assemblage of predators and thereby, the predation pressure felt by species at lower trophic levels. The interactions among predators and the potential role of those interactions in headwater stream communities was examined in 16 replicated experimental streams. I examined the ecological interactions among four species of co—occurring predators, adult Salvelinus fontinalis (98—122 mm standard length [SL]), larval Gyrinophilus porphyriticus (36—58 mm snout—vent length [SVL]), larval Eurycea bislineata (13—21 mm SVL), and Cambarus bartonii (6—17 mm carapace length [CL]), in an experiment that began in mid—June and ran for 3 mo. Brook trout affected both survival and growth of the larger salamander species, Gyrinophilus, and Salvelinus and Gyrinophilus affected the growth of the smaller salamander Eurycea and the crayfish Cambarus. The presence of Gyrinophilus had no effect on relative condition or fecundity of Salvelinus. Larger predators caused Cambarus and Eurycea to alter their activity levels and habitat, but Gyrinophilus made no adjustments to the presence of Salvelinus. Results demonstrate that Gyrinophilus is vulnerable to predation/aggression from brook trout, but that Eurycea Cambarus are able to behaviorally avoid predation by both large predators at a significant cost to growth. The effect of brook trout on the two species of salamander in the experimental streams parallels the observed differences between salamanders in trout and trout—free waters in the field. Thus, interactions among predators in headwater streams directly and indirectly influence population dynamics of the interacting species by affecting survival, growth, behavior, and habitat use. The interactions among these predators can determine, in part, the predation environment experienced by individuals at lower trophic levels.