Food Web Effects of Prey Size Refugia: Variable Interactions and Alternative Stable Equilibria

Predators can have highly variable effects on the abundance and composition of food webs, ranging from strong to weak effects of top predators. Typical food web models assume that individual prey are identical in their susceptibility to predators throughout their lives, but many prey species become less vulnerable to predators through ontogeny. A simple set of models is explored where prey must pass through a vulnerable stage prior to achieving a predator-invulnerable size refuge. As productivity of the environment increases, the proportional impact of predators decreases because more individuals become and remain in the invulnerable adult stage. The addition of a competitor prey species that can not achieve size refuge results in contrasting outcomes. At low productivity, the small species wins in competition, and the system is strongly consumer controlled. At high productivity, the large species wins due to the presence of predators, and the system becomes less consumer controlled. At intermediate productivity, either the small or the large species can win depending on initial conditions, and the system can be either strongly or weakly consumer controlled. Such alternative stable equilibria derived from models with prey size refugia may help to explain many natural situations.