Document: STE-3-52-37

Indirect effects of competition: Increased vulnerability to predators.

KOHLER, S.L.* ¹, L.C.HINZ, JR. ² and M.J.WILEY ²

Western Michigan University, Kalamazoo, MI 49008, USA ¹
The University of Michigan, Ann Arbor, MI, USA ²

Abstract:
The caddisfly Glossosoma nigrior has strong competitive effects on many other primary consumers in trout streams. Strong resource depression by Glossosoma should affect the foraging behavior of its competitors in ways (e.g., increased time spent foraging, increased movement rate while foraging) that should increase their vulnerability to predators. We tested this hypothesis in laboratory experiments using a perlid stonefly and the mottled sculpin as predators and mayfly larvae (Baetis tricaudatus) as prey. We manipulated periphyton abundance on stones in laboratory flumes, and allowed predators (either 1 sculpin or 5 stoneflies) access to 3 size classes of Baetis larvae for 24 h. For both predators, Baetis mortality increased exponentially with decreasing periphyton availability. Both the stonefly and sculpin consumed significantly more small Baetis than the other size classes. Observations of invertebrate and trout populations in 12 streams support the operation and importance of this mechanism in natural communities. In these streams, Glossosoma populations varied markedly over a 4 yr period within and among streams because of pathogen-induced mortality. A path analysis model of hypothesized trophic interactions in these communities indicated a significant, positive effect of Glossosoma abundance on juvenile brook trout growth rate. Because trout rarely consume Glossosoma, we conclude that this effect actually represents the trait-mediated indirect effect of Glossosoma on the vulnerability of its competitors to trout, which can only be represented in a path model as a direct effect of Glossosoma on trout. Thus, Glossosoma appears to have strong negative effects on its competitors through exploitative competition and by influencing their foraging behavior, which modifies their interaction with invertebrate and vertebrate predators.

Keywords: trait-mediated indirect effects, competition, predation, streams, Glossosoma

This abstract is being presented at: 10:30 AM in session:
Oral Session #55: Invertebrates in Streams: Foodwebs.