Restoring Yellowstone’s aspen with wolves

Wolves (Canis lupus) were reintroduced to Yellowstone National Park in 1995–1996. We present data on a recent trophic cascade involving wolves, elk (Cervus elaphus), and aspen (Populus tremuloides) in Yellowstone’s northern winter range that documents the first significant growth of aspen in over half a century. Results indicate reduced browsing and increased heights of young aspen during the last 4–5 years, particularly at high predation risk sites (riparian areas with downed logs). In contrast, young aspen in upland settings generally showed continued suppression with only a slight decrease in browsing levels and only a slight increase in height. Our findings are consistent with the combined effects of a behaviorally-mediated and density-mediated trophic cascade. Results provide an improved perspective for understanding trophic dynamics and spatially variable plant community growth patterns in this recovering ecosystem. 2007 Elsevier Ltd. All rights reserved. The strength of top-down forces (trophic cascades) in terrestrial food webs is highly debated, as few examples exist illustrating the cascading effects of large mammalian carnivores in structuring terrestrial ecosystems (Ripple and Beschta, 2004a; Borer et al., 2005; Hebblewhite et al., 2005; Ray et al., 2005). Predators can directly influence prey populations through density-mediated effects (lethal), which in turn affect herbivory pressure on plants. A second, less understood mechanism involves behaviorally-mediated effects (nonlethal) whereby herbivores, under the risk of predation, alter foraging patterns and habitat use (Lima and Dill, 1990; Schmitz et al., 1997). For prey species, foraging decisions made in a landscape of fear may differ from a foraging strategy based on maximizing nutrient intake in the absence of predators (Laundré et al., 2001). Although little studied, the magnitude of behaviorally-mediated effects on plants may be equal to or even greater than cascading effects resulting from dener Ltd. All rights reserved ; fax: +1 541 737 3049. u (W.J. Ripple). sity-mediated effects of predation (Schmitz et al., 1997). While the effects of predation most likely occur as a combination of behaviorally-mediated and density-mediated effects on plants, disentangling the relative influence of these effects remains difficult (Ripple and Beschta, 2004a). We recently examined a potential trophic cascade involving wolves (Canis lupus), elk (Cervus elaphus), and aspen (Populus tremuloides) in Yellowstone National Park. Aspen clones, with longevities of centuries and perhaps millennia, and other woody browse species in the park’s northern winter range have declined substantially since wolf extirpation in the 1920s, likely attributable to ungulate browsing (Ripple and Larsen, 2000; National Research Council (NRC), 2002; Barmore, 2003; Beschta, 2005). Although highly controversial, wolves were reintroduced into Yellowstone National Park during 1995 and 1996 following approximately seven decades of absence. After ten years