How predators influence communities: Fire, wolves, elk and aspen trophic cascades, case studies from the Rocky Mountains

Background/Question/Methods

Food webs consist of a combination of top-down (predator-driven) and bottom-up (resources-driven) effects. The strong bottom-up effects that prevail in all systems can attenuate top-down effects. Thus, in most ecological settings, the top-down and bottom-up trophic interactions result in top-down effects being a trickle rather than a trophic cascade . Trophic cascades and trickles are relationships in which an apex predator produces direct effects on its prey and indirect changes in faunal and floral communities.  In the case of a trophic trickle, an apex predator is present, but has limited indirect effects on vegetation. This paper reports on relationships between wolf (Canis lupus) predation, elk (Cervus elaphus) herbivory, and aspen (Populus tremuloides Michaux) recruitment, a possible trophic cascade, in the Northern Rocky Mountains. Observations were stratified across three valleys that had three different observed wolf population levels (high, moderate, and low), which represent three levels of long-term predation risk. Ecological characteristics were comparable in all valleys. We used standard wildlife and forestry survey methods to measure tri-trophic ecological relationships.  Response variables included elk density, elk vigilance, and aspen recruitment. The study objective was to examine the relative influence of top-down and bottom-up factors and the context-dependence of these relationships via data gathered during a three-year time span.

Results/Conclusions

Results show complex elk responses to bottom-up (fire) and top-down (predation risk) factors that could influence habitat use by elk. In sum, bottom-up and top-down forces worked together in valleys that contained well-established wolf populations, and to a lesser degree in a valley with a low wolf population. We found some evidence that support trophic cascades, but evidence also of trends that suggest a trophic trickle. A bottom-up force such as fire may be as important as apex predation in stimulating aspen recruitment above browse height.Thus, top-down and bottom-up processes may be important joint engineers of aspen communities. In the Southern Rockies, these dynamics are being investigated on The High Lonesome Ranch, a north-central Colorado landscape that lacks both wolves and fire, and contains a high elk population. Long-term, landscape-scale research focuses on how reinstating top-down (apex predation) and bottom-up (fire) effects may yield information useful in creating aspen communities more resilient to climate change. The research involves evaluating the role of cougar (Puma concolor) predation in an elk-dominated ecosystem, through experiments using replicated combinations of ungulate exclosures, mechanical disturbances, and prescribed fire.