OMNIVORY AND THE INDETERMINACY OF PREDATOR FUNCTION: CAN A KNOWLEDGE OF FORAGING BEHAVIOR HELP?

In 1960, N. G. Hairston, F. E. Smith, and L. B. Slobodkin proposed that terrestrial ecosystems are composed of three trophic levels: predators, herbivores, and plants. Under this model, predators act in a predictable manner to suppress herbivore populations, freeing plant populations from the strong effects of herbivory. However, empirical work has recently demonstrated that many predators exhibit trophic-level omnivory, consuming both herbivores and other predators. This creates a problem for terrestrial ecologists: predator function is indeterminate, because predators may operate from either the third trophic level (“intermediate predators”) or the fourth trophic level (“omnivorous top predators”) and have opposite effects on herbivore and plant populations. Here we attempt to use a basic understanding of the foraging behavior of predators and their prey to make predictions about predator function. A simulation model produces four predictions: (1) actively foraging predators may be effective regulators of sedentary herbivore populations; (2) sit-and-wait predators are unlikely to suppress populations of sedentary herbivores, but may act as omnivorous top predators, suppressing populations of widely foraging intermediate predators and thereby increasing herbivore densities; (3) among widely foraging predators attacking a common herbivore prey, predators that are large relative to the body size of their prey will be more mobile, and therefore more vulnerable to predation by sit-and-wait omnivores, compared to predators that are similar in size to their prey; and (4) widely foraging omnivores, unlike sit-and-wait omnivores, are unlikely to disrupt herbivore population suppression generated by intermediate predators, and may instead enhance herbivore suppression. These predictions appear to explain the results of several experimental studies of the function of predatory arthropods in terrestrial ecosystems.