Disease and migratory tactic mediate the nutritional benefits of irrigated agriculture to mule deer

Abstract

Agriculture can alter the nutritional landscape for herbivores in ways that can augment nutritional condition, reproduction, and survival. Ecological benefits associated with human modified landscapes, however, potentially alter environmental cues in ways that appear beneficial but ultimately have negative effects on fitness or population growth. We tested the hypothesis that the expected nutritional benefit of agriculture would come with a tradeoff associated with increased transmission of chronic wasting disease using a partially migratory population of mule deer (Odocoileus hemionus) in northern Wyoming, USA. Agriculture provided a substantial nutritional buffer to resident deer by augmenting nutritional condition in autumn and enhancing recruitment of offspring—a resident that spent 35% of its summer in agriculture had 1.2 percentage points more body fat in December and was 18 percentage points more likely to recruit offspring to December than a resident that spent 15% of its summer in agriculture. During winter, migrants and residents selected for home ranges closer to agriculture, but only residents selected for home ranges overlapping directly with agriculture. Proximity to agriculture in winter, however, decreased adult survival for migrants and residents (29 percentage points decreased probability of survival for every 1-km closer to agriculture) and increased the probability of having CWD at time of death. The nutritional benefits of agriculture likely increase the viability of a resident tactic, but the benefits may be offset if the nutrition gained from using agriculture does not outweigh the risks associated with disease.