Risk-induced changes in prey behavior have many important effects in predator-prey communities; however, fitness consequences of predation risk to prey remain unclear. As a modifier of both top-down (i.e., predation) and bottom-up (i.e., nutrients or food) processes, predator-induced behavioral plasticity may impact prey population dynamics more than the immediate reduction of prey density. One challenge for ecologists is to link individual foraging behavior to population dynamics, while integrating the effects of predation risk. Terrestrial carnivores are important drivers of ecosystem structure, function and the conservation of biodiversity. Therefore, understanding the impacts of risk effects is critical for predicting the ecological impacts of predators on prey communities in the midst of changing anthropogenic (e.g., introduction or re-colonization) or climate-related factors.
Natural recovery of cougars (Puma concolor) on the National Bison Range (Montana, USA) allowed me to evaluate anti-predator behavioral responses of elk (Cervus elaphus), mule (Odocoileus hemionus) and white-tailed (O. virginianus) deer to an increase in the risk of predation by a solitary, stalking predator. I observed the behavior and diet of ungulates to test predictions based on foraging theory that assess the energetic consequences of predation-sensitive foraging decisions. All prey species reduced: 1) activity during crepuscular periods when cougars were most active, 2) use of risky habitats, and 3) overall activity time. However, ungulates minimized the trade-off between foraging time and scanning for predators by increasing vigilance only when a cougar was within the same drainage. By comparing observed diets with predictions from a linear programming model, I demonstrated that prey species experiencing the most predation risk appeared to trade-off maximizing energy-intake with minimizing time spent meeting foraging requirements. Whereas the least vulnerable prey selected diets that maximized energy intake, they still incurred a net cost due to reduced feeding time. Feeding time appeared to drive these relations, supporting the importance of food limitation in terrestrial herbivores, and demonstrating the effect of predation risk on food limitation. Even with a modest reduction in feeding time, ungulate prey incurred a 5-38% reduction in daily surplus energy intake Ì¢âÂ" a substantial cost of risk on foraging that could impact survival and reproduction.