Selection for Forage and Avoidance of Predation Risk by Partially Migratory Mule Deer
Migration by ungulates has traditionally been thought of as a strategy that increases access to forage quality or reduces exposure to risk of predation, but the benefits of migration may be waning globally. In partially migratory populations, the persistence of both migrant and resident strategies is an intriguing ecological phenomenon, because migrants and residents often face contrasting fitness consequences. Partial migration is common in mule deer (Odocoileus hemionus), a species that has experienced widespread declines across the western United States during recent decades. Mule deer seldom switch between migratory strategies throughout their lifetime, which may make them less resilient to environmental change than more behaviorally plastic ungulate species. To indicate the mechanisms maintaining partial migration, we investigated how predation risk, forage quality, and habitat selection in relation to these factors varied between migrant and resident mule deer. First, we developed resource selection functions (RSFs) for wolves and mountain lions to estimate predation risk. Then, we modeled forage quality throughout mule deer summer ranges. We then compared forage quality (kcal/m2 ) and predation risk in migrant and resident summer ranges of 3 partially migratory populations across Western Montana. We found no substantial differences in forage quality between migrant and resident summer ranges, and predation risk did not differ predictably between the 2 groups. We used RSFs to assess how home range (2nd order) and within-home range (3rd order) selection varied between migrants and residents. At the 2nd order, neither migrants or residents selected forage or avoided wolf predation risk, but both groups avoided mountain lion predation risk. At the 3rd order, both migrants and residents selected for forage and avoided wolf and mountain lion predation risk. Given their exposure to similar forage and risk conditions between groups, and similar habitat selection patterns, our results suggest that the benefits of a migrant strategy did not outweigh those of a resident strategy during our study. Within mule deer populations, partial migration may be maintained due to changes in the relative benefits of migration over time. Mule deer behavior was consistent across different ecosystem types and migratory strategies, suggesting a general mechanism for summer habitat selection may exist for mule deer in forested environments of the Northern Rockies.