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Linking Wolves, Ungulates, and Aspen Recruitment
Bill Ripple, Professor, Department of Forest Resources, Oregon State University, Director of Oregon State's “The Aspen Project”
Using historical aspen (Populus tremuloides) diameter data and aspen increment cores collected in 1997 and 1998, we analyzed aspen overstory recruitment in Yellowstone National Park (YNP) over the last 200 years. We found that successful aspen overstory recruitment occurred on the northern range of YNP from the middle to late 1700s to the 1920s, after which it essentially ceased. We found that aspen recruitment ceased during the same years (1920s) that gray wolves (Canis lupis) were extirpated from the park. We hypothesize that wolves may positively influence aspen recruitment through a trophic cascades (top down) affect on elk (Cervus elaphus). For prey species such as elk, foraging decisions made under the risk of predation may differ from an optimal foraging strategy based only on maximizing nutrient intake. We suggest that predation risk effects can have a specific influence on elk herbivory at multiple spatial scales and that elk may have historically avoided foraging in certain aspen and riparian habitats due to the risk of predation from wolves.
In 1999, we initiated a long-term study of the influence of reintroduced YNP wolves on elk herbivory and recruitment of aspen as well as other woody browse species. Since then, we have found no recruitment of aspen on upland sites except for areas protected from browsing (e.g. jackstrawed woody debris). In valley bottom riparian areas, we found aspen releasing on a few very high risk sites (i.e. canyon, gully). These releasing aspen (2-4 m tall with 2003 seasonal growth) are taller than previously documented maximum heights for young aspen in the northern range following the reintroduction of wolves. Results appear to (1) indicate a trophic cascades effect, based on predation risk, at a local scale and (2) illustrate that terrain features may play a supportive role in aspen recruitment by providing increases in predation risk to elk. In additional areas, willow (Salix spp.) has been growing taller since wolf reintroduction.
Overall, these results offer rare empirical evidence on the indirect effects of a top carnivore in a terrestrial food chain and supports theory on predation risk effects and trophic cascades. If the aspen and willow of Yellowstone continue to grow taller and expand in canopy cover, the numerous benefits to ecosystem processes will include stream channel stabilization, flood plain restoration, and higher water tables. Through a trophic cascades effect of improved habitat, wolves may be beneficial to numerous species of vertebrates and invertebrates such as fish, birds, beaver, and butterflies, as well as many other species of wildlife. For more information, see “The Aspen Project” website www.cof.orst.edu/aspen.