Trophic interactions mediate the response of predator populations to habitat change

Identifying the mechanisms by which globally pervasive changes in habitat affect predators is a central, yet challenging, endeavor in applied ecology. Cryptic shifts in trophic interactions are potentially important but widely underappreciated mechanisms shaping predator population response to habitat change. Here, we assessed the extent to which variation in trophic interactions explained differences in predator populations at both local and landscape scales. We integrated stable isotope analyses, GPS tagging, and long-term territory occupancy information to characterize the trophic ecology of spotted owls in the Sierra Nevada, California, USA where population trends and densities vary among forest landscapes with contrasting land uses and disturbance regimes. Trophic interactions appeared to influence spotted owl space use and territory occupancy rates with emergent consequences for landscape-scale patterns in population abundance and trends. Specifically, consumption of woodrats and pocket gophers, which varied with habitat conditions, was associated with smaller home ranges and lower territory extinction probabilities. Moreover, spotted owls consumed significantly more woodrats and pocket gophers in landscapes with stable (national parks) and high-occupancy (private lands) populations than in landscapes with declining owl populations (national forests). Collectively, our results suggest that trophic responses to local habitat conditions can affect predators at multiple spatial scales and that managing for important prey species habitat may benefit predator populations. Because trophic interactions mediate species' responses to anthropogenic pressures in many ecological systems, our approach to integrating stable isotopes with behavioral, fitness, occupancy, and demographic data offers a tractable avenue for researchers elsewhere to quantify such relationships.