Ungulate herbivory: Indirect effects cascade into the treetops

Abstract

Changes in population numbers of top (apex) predators are increasingly acknowledged to promote major shifts in ecosystem organization. The early evidence was both experimental and observationally based: for instance, predatory starfish can influence the ability of species to coexist on marine rocky shores (1); bass, by consuming grazing minnows, alter primary production in freshwater streams (2); and sea otters, by eating sea urchins, themselves major consumers of marine benthic algae, indirectly exert a major influence on the biological performance of these primary producers (3). The initial impression was that such potent top-down effects were “all wet” (4) and that terrestrial ecosystems might be fundamentally different from aquatic ones; in this issue of PNAS, Pringle et al. (5) add to a growing body of evidence suggesting the contrary. Some reasons for this initial impression seem obvious: many terrestrial apex predators have been hunted to near or local extinction; many of the more charismatic species now enjoy stringent legal protection, which hampers or denies any manipulation; and terrestrial ecosystems themselves are less experimentally tractable than their aquatic counterparts, in part because of extreme longevity of the plant community and because of the great spatial scale required to retain a semblance of natural reality. Nonetheless, an important role for apex predators is increasingly recognized as these predators persist in fragmented habitats (e.g., coyotes; ref. 6), are introduced to islands (e.g., fox–seabird–vegetation linkages in the Aleutian Islands; ref. 7), are reintroduced to historic habitats (e.g., wolves into Yellowstone; ref. 8), when comparable habitats with and without a massive human presence are examined (e.g., the mountain lion–amphibian connection in Zion National Park; ref. 9), and in the rare cases in which sufficient time series data exist to develop tri-trophic models (10). One conclusion is that sites with a …