Cascades

Abstract Carnivorans are proposed to initiate several kinds of linked community responses to predation or other forces. Ecological cascades result from predation, competition, pollination, nutrient transport, or facilitation of scavenging, and may descend or ascend trophic levels. Trophic cascades are more narrowly described as effects of predation on herbivore densities or behaviors that in turn affect plant attributes. Evidence exists for some strong ecological cascades initiated by carnivorans. There are two recognized types of trophic cascades. In a density-mediated cascade, predation reduces herbivore densities over an area large enough to be considered a population response. This affects such plant attributes as production, biomass, or reproduction. In a behaviorally mediated trophic cascade, predation risk affects herbivore behavior heterogeneously across a landscape, which affects vegetation attributes. Cascades are now regarded as a general consequence of carnivoran predation, but field studies show highly variable effect sizes, or weak evidence for a cascade. Behaviorally mediated trophic cascades seem to be strongest where a single carnivoran species preys on a single herbivore species, where the carnivoran is an ambush predator, and where the landscape produces heterogeneous ambushing success. The Yellowstone system in the US has produced many reports of trophic cascades resulting from the reintroduction of wolves, but some of them have been discredited because of confounding ecological variation, small effect sizes over the short term, or sampling biases.