Oral Session #95: Trophic Structure.
Presiding: D. Wise
Thursday, August 8. 1:00 PM to 3:45 PM. Palo Verde Room, Radisson.

The strength of trophic cascades.

Shurin, Jonathan*,1, Seabloom, Eric1, 1 National Center for Ecological Analysis and Synthesis, Santa Barbara, CA

ABSTRACT- Top-down control of trophic structure is often highly variable both within and among ecosystems. We explored the roles of relative body sizes of predators and prey, their metabolic types, the production-to-biomass ratio of plants, and system productivity in determining the strength of the indirect effects of predators on plants. We used a well-studied food chain model with three trophic levels that is parameterized based on allometric relationships for rates of ingestion and metabolic efficiency. The model predicts that invertebrate and ectotherm predators and herbivores should propagate cascades to a greater degree than vertebrates and endotherms. Increasing the herbivore-to-plant body-size ratio strengthened the effects of cascades, while predator body-size was predicted to have no effect. Increasing system productivity or the production-to-biomass ratio of the plant magnified cascades. Since both herbivore:plant body size ratios and plant P/B are greater in pelagic than terrestrial systems, the model predicts stronger cascades in the water than on land. A meta-analysis of trophic cascade experiments in six different ecosystems found support for some of the model's predictions, but not others. We discuss features of natural systems that are not incorporated in the model and their implications for the intensity of trophic cascades.

KEY WORDS: trophic cascades, cross-ecosystem comparison, body size, productivity