Body size and the importance of bottom-up and top-down forces in Serengeti mammals

Rate of increase is related to metabolic body size in mammals and this affects the direction of regulation. There are many examples of both top-down and bottom-up regulation in terrestrial ecosystems but the factors that determine when one or the other occurs remain poorly understood.  We show here, using forty years of data from the highly diverse mammal community of the Serengeti ecosystem, East Africa, that the cause of regulation for a particular species is determined by two factors, the diversity of both the predator and prey guilds, and the body size of that prey species relative to other prey and predators.  In Serengeti, small ungulates are exposed to more predators due to opportunistic predation than are larger ungulates, suffer greater predation rates and show strong predator regulation.  A threshold occurs at prey body sizes of ca. 150 kg, above which ungulate species have few natural predators and exhibit bottom-up regulation through food limitation.  Thus, biodiversity allows both predator limitation (top-down) and resource limitation (bottom-up) to act simultaneously to regulate herbivore populations.  This result would apply generally in systems where there is a diversity of predators and prey.