Winter resource selection by female mule deer Odocoileus hemionus: functional response to spatio-temporal changes in habitat

Populations of mule deer Odocoileus hemionus have declined throughout most of their historic range in recent decades, and habitat alteration has been hypothesized as one potential cause of those declines. Consequently, understanding how patterns of behaviour change as landscapes are altered through time may provide important insights into mechanisms underlying observed demographic trends in populations of mule deer. We examined resource selection in relation to habitat change by mule deer on the Tex Creek winter range in southeastern Idaho, USA. We created a GIS-based map of habitats available to mule deer during two time periods, past (1985-1986) and current (2007-2009), to document changes in habitat over time. We then modeled past and current patterns of resource selection by mule deer based on locations obtained from visual observation, radio-telemetry and GPS collars. Abundance and distribution of juniper Juniperus spp., aspen Popuhts tremuloides, sagebrush steppe Artemisia spp. and riparian habitat did not change significantly between past and current time periods. In contrast, we documented an increase in grassland from 3.5 to 30.7% of our study area, and a corresponding decrease in agricultural land, which provides high-quality forage for mule deer, from 37.8 to 12.5% of our study area. Patterns of resource selection largely were similar between the two time periods. Nevertheless, mule deer significantly increased selection of agricultural fields and areas far from roads between the 1980s and 2007-2009. In addition, juniper stands were strongly selected in all years, and importance values (use x availability resealed to 100%) for grassland and sagebrush steppe increased between past and current time periods. Our results indicated that mule deer responded behaviourally to declining availability of high-quality forage (i.e. agricultural land) by increasing selection of agricultural fields. Such functional responses in habitat selection may have important consequences for dynamics of mule deer populations, but additional research is necessary to link those responses to demography and population performance. Although juniper encroachment often is thought to have affected mule deer negatively on many summer ranges, strong selection for juniper in our study highlights the potential importance of that species on winter range, likely because it provides both thermal and hiding cover.