Short-term responses to a human-altered landscape do not affect fat dynamics of a migratory ungulate

Abstract

According to risk-sensitive foraging theory, animals should make foraging decisions that balance nutritional costs and gains to promote fitness. Human disturbance is a form of perceived risk that can prompt avoidance of risky habitat over acquisition of food. Consequently, behavioural responses to perceived risk could induce nutritional costs. Population declines often coincide with increases in human disturbance, which likely is associated with direct and indirect habitat loss. Nevertheless, behavioural and physiological responses to perceived risks associated with human disturbance could be an added nutritional deficit with population-level repercussions. Using GPS-collar data from three populations of migratory mule deer Odocoileus hemionus exposed to a gradient of established industrial energy development on winter ranges where direct and indirect habitat loss were well documented, we evaluated whether exposure and behavioural responses to human disturbance alter changes in nutritional condition (i.e. fat reserves) over winter. Although animals exhibited behaviours indicative of perceived risk of human disturbance, such as increased movement rates and avoidance of infrastructure, exposure and behavioural responses to human disturbance had little to no measurable effect on fat loss over a winter. Instead, catabolism of fat reserves occurred primarily as a function of the amount of fat animals had entering winter, suggesting that, in the short term, animals were able to mitigate the energetic costs of perceived risk of human disturbance over winter. Animals, however, did not appear to overcome persistent food limitations, and animals with less food availability lost more fat over winter. Our findings heed caution in using short-term behavioural and physiological responses to inform long-term nutritional consequences of human disturbance. Although animals appear to mitigate the energetic costs of perceived risk, food limitations exacerbated by broader-scale avoidance of food near human disturbance may be the primary pathway causing the frequently observed population declines following human disturbance to pristine landscapes.