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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. [ABSTRACT FROM AUTHOR]

We quantified the vigilance levels of elk (Cervus elaphus) preyed on by wolves (Canis lupus) in Yellowstone National Park, between January and May in 2005 and 2006, and used Akaikes information criterion to compare a set of 38 regression models for vigilance levels. These models combined up to 9 predictor variables of 3 types: characteristics of the prey group (herd size and composition), characteristics of the predator (wolf pack size, distance away, and the presence/absence of a kill), and characteristics of the local environment (distance to woodland edges, snow depth, and snow cover). The set of models spanned a range of complexity from simple univariate models to complex combinations with up to 3 variables of each type. Complex models incorporating the characteristics of the wolf pack, the structure of the elk herd, and the environmental conditions had higher information content than simple models. Although univariate models of vigilance detect significant relationships, they have low information content relative to multivariate models. These results show that elk assesses factors of several types when assessing risk and deciding how much time to allocate to vigilance. In particular, we found that all well-supported models of vigilance included several “prey” variables and several “predator” variables. This result highlights the need to consider information about predators when trying to explain variation in vigilance levels in prey. [ABSTRACT FROM AUTHOR]