OOS 20-3
Effects of recovering large carnivores on elk are context dependent in multiple-predator-prey systems
Recovering gray wolves, mountain lions and grizzly bears across western North America are adding complexity to ecological communities. Most of the ecological and policy focus of carnivore impacts have been focused on the well-publicized effects of recovering wolves in Yellowstone on elk. However, the impacts of recovering predation by wolves will be highly contextual especially in complex predator-prey systems. Here, we test effects of recovering gray wolves on dynamics of a declining elk population in the Bitterroot valley of western MT, 200km west of Yellowstone, 10-years after wolves first recolonized this system with an existing mountain lion population. To compare relative cause-specific mortality rates of carnivores, we radiocollared 125 adult female elk and 287 juvenile elk calves from 2012-14. We estimated cause-specific mortality rates using cumulative incidence functions (CIF) in a time-to-event survival-modeling framework. We estimated mountain lion densities in winter 2012/13 using spatially-explicit capture recapture models, and wolf densities using radiocollars on overlapping wolf packs. Diet of wolves and mountain lions were estimated through scat analyses of large ungulate prey.
Results/Conclusions
Densities of carnivore species were 15 (95% CI 10-25) wolves/1000 km2, versus 48.5 (95% CI 52-139) mountain lions/1000 km2. Under these conditions of 3-4 times higher mountain lion densities, our results showed correspondingly higher mountain lion mortality on both adult female elk and juvenile elk during their first-year of life. Adult female mortality was more than twice as likely to be caused by mountain lions as wolves. Juvenile survival was 5-6 times as likely to be caused by mountain lions (CIF probability of mortality by lions = 0.19) as wolves (0.03). Cause-specific mortality rates did not correspond to the importance of elk in the diet of wolves or lions; elk were an important component (>50%) of both carnivore species diet, but the much higher densities of mountain lions mediated impacts on prey. We constructed a simple two-predator, 2 prey model that evaluated relative impacts of two predators on shared prey that showed relative densities of both predator and prey, moreso than prey specialization, diet composition, and functional responses explained most of the variation in relative impacts of carnivores on prey. While these results are perhaps obvious in hindsight, they emphasize an oft-overlooked aspect of multiple predator-multiple-prey systems: that relative densities of large carnivores are perhaps one of the most important predictors of relative impacts, and the least studied.