Forest disturbance shapes habitat selection but not migratory tendency for partially migratory ungulates

Abstract

In forest management settings, disturbance resets forests to earlier successional stages, typically improving forage conditions for mule deer. Examining how forest disturbance influences mule deer behavior is important for guiding forest and wildlife management. We used GPS collar data collected between 2017 and 2019 from 136 adult female mule deer in three populations throughout western Montana, United States, to investigate how disturbance from burns (wildfire and prescribed fire) and timber harvest influenced three aspects of space-use behaviors: (1) probability of migration from winter range to summer range, (2) home range (second-order) selection by migrants on summer range, and (3) within-home range (third-order) selection. We hypothesized that deer would maximize use of disturbances during summer for nutritional benefits, predicting that deer with higher proportionate disturbance in their winter home range would be less likely to migrate away from those disturbances during summer. We predicted that migrants would select disturbances at the second and third orders. We found that proportionate disturbance in winter home ranges had no effect on the probability of migration. Among migrants, deer generally selected burns, timber harvests, and open-canopy habitat at the second order in all study areas, with particularly strong selection for 6- to 15-year-old disturbances. At population levels, selection for disturbances ceased at the third order. At individual levels, however, third-order selection for burns increased with availability, whereas selection for harvests decreased, suggesting burns may satisfy more resource needs than harvests. Our results emphasize how space-use fidelity constrains mule deer habitat selection. During summer, adherence to migratory strategies constrains the habitat available for second-order selection, preventing deer from exploiting disturbances that would otherwise be available had they remained resident in wintering areas. Second-order selection then determines disturbance availability within home ranges, affecting third-order behaviors. Although variance in selection behaviors among individuals was high, population-level patterns were remarkably similar among study areas, suggesting these responses may be generalizable to mule deer throughout the northern Rocky Mountains. Forest management practices like timber harvest, prescribed burns, and wildfire management within higher elevation areas of summer range used by migrants could yield the greatest nutritional benefits for mule deer.