Risk effects in elk: sex-specific responses in grazing and browsing due to predation risk from wolves

Risk effects in ungulates are poorly understood but have recently been implicated as an important driver of elk (Cervus elaphus) population dynamics since wolves were reintroduced into the Greater Yellowstone Ecosystem (GYE) of Montana and Wyoming, USA. From December to May in 2004, 2005, and 2006, we recorded the daily presence of wolves on 3 sites occupied by the Upper Gallatin elk population in the northwest corner of the GYE. We estimated the proportion of grasses, conifers, evergreen shrubs, and woody stems in 980 elk fecal samples collected from those 3 sites and tested whether wolf presence affected elk diets. The winter of 2005 was extremely mild allowing us the opportunity to investigate how elk-wolf interactions might change if winter snowpack continues to decline in western North America due to global warming. Snow accumulation consistently favored browsing, and diets during the mild winter were dominated by grass, very similar to the spring diet. In normal winters, adult males grazed less than adult females except when wolves were near because females decreased grazing in response to wolves. Adult males decreased browsing on conifers by half whereas adult females doubled conifer browsing on days when wolves were near. Overall, the sexes had different diets when wolves were absent but showed strong overlap when wolves were present. Diet shifts due to wolves may be causing trophic cascades that have gone unrecognized and probably carry nutritional consequences for wintering elk.