PARENT SESSION
Symposium 16: Incorporating trophic diversity into the biodiversity-ecosystem functioning debate: Lessons from aquatic ecosystems
Organized by: P McIntyre, K France, JE Duffy, and AS Flecker
Thursday, August 11, 8:00 AM - 11:30 AM, Meeting Room 517 A, Level 5, Palais des congrès de Montréal

Predator diversity strengthens trophic cascades in kelp forests by altering herbivore behavior.

Byrnes, Jarrett^*,1, Stachowicz, John¹, Hultgren, Kristin¹, Hughes, A. Randall¹, Olyarnik, Suzanne¹, Thornber, Carol², ¹ University of California, Davis, Davis, CA² University of Rhode Island, Kingston, RI

ABSTRACT- Although human-mediated extinctions and local extirpations disproportionately affect top-predators, our knowledge of the effects of such diversity declines on ecosystem functioning is generally limited to basal trophic levels. Experiments have shown that increasing plant diversity can increase primary production while, in contrast, increasing herbivore species richness may decrease standing plant biomass through complementary patterns of consumption between herbivores. Will similarly increasing predator richness cascading through a system and increasing algal biomass, or will the presence of a diverse community of herbivores stymie any trophic cascade? To answer this question, we assembled mesocosms containing kelp (Macrocystis sp.) and 5 herbivore species (Pugettia producta, Strongylocentrotus franciscanus, S. purpuratus, Tegula brunnea, and T. funebralis) and exposed them to equivalent densities of potential predators (Cancer productus, C. magister and Pycnopodia helianthoides) either singly or in combination. All treatments contained kelp and herbivores, but ranged from 0 to 3 predator species, with 1 and 3 species treatment experiencing the same total predator density. We found that herbivory on kelp decreased with increasing predator diversity, suggesting that predator diversity may enhance the trophic cascade. Different predators did not consume different prey however, eliminating complementary predation as a possible mechanism. To examine the role of prey behavioral responses, we exposed herbivores to physical and chemical cues from predators but protected them from direct mortality. We found that kelp consumption declined with increasing predator diversity by an amount similar to that observed in our predation experiments. This pattern was driven by differential inhibition of two of the main herbivores by two predators: C. productus reduced Pugettia feeding and Pycnopodia reduced S. purpuratus feeding. Our results suggest that decreasing the number of predator species can have a major effect on community structure and functioning by maintaining the biomass of a foundation species, kelp, through behavioral modifications of herbivores.

Key words: biodiversity and ecosystem function, trophic cascade, behavior, kelp bed