Toward a trophic theory of species diversity

Abstract

Efforts to understand the ecological regulation of species diversity via bottom-up approaches have failed to yield a consensus theory. Theories based on the alternative of top-down regulation have fared better. Paine's discovery of keystone predation demonstrated that the regulation of diversity via top-down forcing could be simple, strong, and direct, yet ecologists have persistently failed to perceive generality in Paine's result. Removing top predators destabilizes many systems and drives transitions to radically distinct alternative states. These transitions typically involve community reorganization and loss of diversity, implying that top-down forcing is crucial to diversity maintenance. Contrary to the expectations of bottom-up theories, many terrestrial herbivores and mesopredators are capable of sustained order-of-magnitude population increases following release from predation, negating the assumption that populations of primary consumers are resource limited and at or near carrying capacity. Predation sensu lato (to include Janzen-Connell mortality agents) has been shown to promote diversity in a wide range of ecosystems, including rocky intertidal shelves, coral reefs, the nearshore ocean, streams, lakes, temperate and tropical forests, and arctic tundra. The compelling variety of these ecosystems suggests that top-down forcing plays a universal role in regulating diversity. This conclusion is further supported by studies showing that the reduction or absence of predation leads to diversity loss and, in the more dramatic cases, to catastrophic regime change. Here, I expand on the thesis that diversity is maintained by the interaction between predation and competition, such that strong top-down forcing reduces competition, allowing coexistence.

Keywords: carrying capacity; interspecific competition; predation; species diversity; trophic cascades.

Fig. 1.

Trade-off between competition and predation as regulators of species diversity. Under zero predation, competition is postulated to be strong, favoring β- over α-diversity via competitive exclusion. Intense (“normal”) predation damps competitive interactions, increasing the fraction of the regional species pool that coexists as α-diversity up to an optimum level of predation. Introduction of superpredators can result in supraoptimal predation, decimation of prey populations, and diminished diversity (dashed line). Interspecific and intraspecific density dependence is strong where competition is high and predation low (at left) and decreases as the strength of predation increases (toward the right). Density-dependent predation increases from left to right. Reciprocal effects of predators on competing prey species (“apparent competition”) is common in nature but not fully captured by the model illustrated.