Responses to natural gas development differ by season for two migratory ungulates

While migrating, animals make directionally persistent movements and may only respond to human-induced rapid environmental change (HIREC), such as climate and land-use change, once a threshold of HIREC is surpassed. In contrast, animals on other seasonal ranges (e.g., winter range) make more localized and tortuous movements while foraging and may have the flexibility to adjust the location of their range and the intensity of use within it to minimize interactions with HIREC. Because of these seasonal differences in movement, animals on seasonal ranges should avoid areas that contain any level of HIREC, however, during migration, animals should use areas that contain low levels of HIREC, avoiding it only once a threshold of HIREC has been surpassed. We tested this hypothesis using a decade of GPS collar data collected from migratory mule deer (Odocoileus hemionus; n = 56 migration, 143 winter) and pronghorn (Antilocapra americana; n = 70 migration, 89 winter) that winter on and migrate through a natural gas field in western Wyoming. Using surface disturbance caused by well pads and roads as an index of HIREC, we evaluated behavioral responses across three spatial scales during winter and migration seasons. During migration, both species tolerated low levels of disturbance. Once a disturbance threshold was surpassed, however, they avoided HIREC. For mule deer, thresholds were consistently ~3%, whereas thresholds for pronghorn ranged from 1% to 9.25% surface disturbance. In contrast to migration, both species generally avoided all levels of HIREC while on winter range. Our study suggests that animal responses to HIREC are mediated by season-specific movement patterns. Our results provide further evidence of ungulates avoiding human disturbance on winter range and reveal disturbance thresholds that trigger mule deer and pronghorn responses during migration: information that managers can use to maintain the ecological function of migration routes and winter ranges.