Are wolves helping Yellowstone’s aspen recover by changing where elk browse?

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Behaviorally mediated trophic cascades occur when herbivores’ fear of predators enhances plant productivity. Based mainly on systems with small-bodied predators, these cascades have been proposed to be strong and widespread in natural ecosystems. However, more recent synthesis has suggested that their occurrence may depend on hunting mode, with passive, sit-and-wait predators having much stronger effects than active, coursing predators.

A proposed cascade involving a wide-ranging active predator holds that the reintroduction of wolves (Canis lupus) to Yellowstone National Park, USA, is helping trembling aspen (Populus tremuloides) recover by causing elk (Cervus elaphus) to avoid foraging in risky areas. Although widely accepted and popularized, this proposed cascade had not been adequately tested. To assess whether wolves influence aspen, researchers used tree rings to obtain detailed demographic data on aspen stands and monitored browsing levels for three years in experimental elk exclosures arranged across a gradient of predation risk.

Historical failure of aspen regeneration varied widely among stands: the last recruitment year ranged from 1892 to 1956, and the data did not indicate an abrupt halt in recruitment. This pattern appeared more consistent with a gradual increase in elk numbers than with a rapid shift in elk foraging behavior after wolves were eliminated. Estimates of relative survivorship of young aspen vulnerable to browsing indicated that aspen were not currently recovering in Yellowstone, even with a large wolf population. In the experimental test of the cascade hypothesis, the impacts of elk browsing on aspen demography were not diminished at sites where elk faced higher predation risk from wolves. The findings suggest that further evaluation is needed of how predator and prey life histories and ecological context mediate trophic cascades.