The strength of trophic cascades across ecosystems: Predictions from allometry and energetics

1. 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 (P : B) of plants, and system productivity in determining the strength of the indirect effects of predators on plants. 2. 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 because of their higher metabolic efficiency. 3. 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 plant P : B magnified cascades. Because herbivore : plant body size ratios and plant P : B are both generally greater in aquatic than terrestrial systems (especially those with unicellular producers), the model predicts stronger cascades in water than on land. This prediction is supported by a recent cross-system comparison of trophic cascade experiments. 4. We discuss features of natural systems that are not incorporated into the model and their implications for the intensity of trophic cascades across ecosystems.