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 1-9.25% 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.