Food-Web Dynamics on Some Small Subtropical Islands: Effects of Top and Intermediate Predators

Abstract

Certain models predict that the effect of top predators on producers depends on the number of trophic levels in the system (Fretwell, 1977; Oksanen et al., 1981); in a three-trophic-level system the effect is positive (as in Hairston et al. (1960)), whereas in a fourtrophic-level system (top predators, intermediate predators, herbivores, and producers) the effect is negative. These models assume that each co nsumer level eats only the next level down. Freshwater systems showed the predicted response when a fourth trophic level was experimentally added to a three-level system—intermediate predators decreased, herbivores increased and producers decreased (Carpenter et al., 1987; Persson et al., 1988; Power, 1990). In terrestrial systems with both top and intermediate predators, the top predators often feed on both intermediate predators and herbivores, particularly when the top predators are vertebrates and the intermediate predators and herbivores are arthropods (Schoener, 1989). This and other complex trophic interactions, which may necessitate different models, may be common in terrestrial systems (Polis, 1991; Strong, 1992) and also occur in some aquatic systems (Persson et al., 1992).