Good animals in bad places : evaluating landscape attributes associated with elk vulnerability to wolf predation

Abstract

Vulnerability of prey to predators is heavily influenced by their respective physical and behavioral characteristics; however their interactions with landscape, and climate, collectively termed "environmental vulnerability," may also assume considerable importance. Landscape or habitat-dependent prey vulnerability is well-studied in smaller taxa systems where environmental factors primarily influence encounter rates, however the impact of environmental vulnerability in large mammal systems, where the probability of a successful attack may be more important than encounter probability, is poorly understood. This study utilized 18 years of survival and mortality data for radio-collared elk (Cervus elaphus), in concert with abundance, distribution, and habitat use data prior to and following restoration of wolves (Canis lupus) to Yellowstone National Park to evaluate the relationship between environmental vulnerability and elk mortality. Logistic regression was used to model the odds of mortality for 108 elk in 1257 animal sample intervals from 1991-2009 across a range of environmental conditions and gradients of wolf predation risk to evaluate: 1) The relationship between landscape, habitat, and environmental attributes and elk vulnerability to wolf predation and 2) Changes in the attributes related to elk mortality before and after wolf colonization. In the absence of wolf predation, mortality risk for elk was primarily associated with physical attributes of elk due to age and condition, factors that are known to influence starvation mortality. Following wolf reintroduction mortality risk was related to these elk physical characteristics, but more so to characteristics of the landscape and climate within an animal's home range. These apparent environmental influences were strong enough to result in substantial changes in distribution and abundance of elk in the study system to the extent that by the end of the study elk almost exclusively utilized areas with high probability of wolf encounter, but also a high probability of escape due to the collective characteristics of the landscape promoting predation refuges. The strong influence of environmental vulnerability is likely to affect the abundance and distribution of elk and wolves across their range, and has broad applications to large mammal predator-prey dynamics in general.