Herbivores, resources and risks: alternating regulation along primary environmental gradients in savannas.

Herbivores are regulated by predation under certain environmental conditions, whereas under others they are limited by forage abundance and nutritional quality. Whether top-down or bottom-up regulation prevails depends both on abiotic constraints on forage availability and body size, because size simultaneously affects the risk of predation of herbivores and their nutritional demands. Consequently, ecosystems composed of similar species can have different dynamics if they differ in resource supply. Here, we use large herbivore assemblages in African savanna ecosystems to develop a framework that connects environmental gradients and disturbance patterns with body size and trophic structure. This framework provides a model for understanding the functioning and diversity of ecosystems in general, and unifies how top-down and bottom-up mechanisms depend on common underlying environmental gradients.