- The western United States has experienced large-scale degradation due to land use and land cover changes, invasion of annual grasses, and expansion of woody plants into grass and shrublands and the resultant altered fire regimes. These landscape-scale changes have coincided with declining mule deer (Odocoileus hemionus) populations, making habitat loss and degradation presumptive drivers for the decline. I investigated how wildfire, invasive annual grasses, and western juniper expansion influenced habitat use and survival for a partially migratory population of mule deer in the John Day Basin, Oregon, USA. In addition, I compared forage quality and quantity between migratory and resident deer summer ranges.
I used GPS radio collar data to develop resource selection functions (RSF) that (winter n = 148; summer:migratory n = 94; summer:resident; n = 34) to determine how wildfire, juniper canopy cover, and dominant vegetation patches influenced seasonal habitat selection for mule deer from a mixed migratory population. The RSF model for mule deer on winter ranges indicated that deer were selecting for areas that burned 1-10 years prior, areas with little juniper cover (< 10%), and areas dominated by exotic grasses. Both migratory and resident deer summer RSF models indicated deer selected strongly for burned areas (11-15 years prior). The resident summer RSF model indicated that deer were selecting for riparian areas (Jun-Aug) and the migratory summer RSF model indicated deer selected for montane meadows (Jun-Aug). The findings from this study elucidated how mule deer, as a landscape species, responded to multiple drivers of change by selecting for highly productive patches in the summer including riparian areas, montane meadows, and areas where vegetative growth has been stimulated by fire during the summer. During the winter, mule deer avoid areas where shrub cover is low including areas with dense juniper canopy cover and burned areas.
I used known-fate data for 111 adult female radio collared mule deer to estimate seasonal survival rates and investigate a variety of factors expected to affect these rates including time-since-fire, juniper canopy cover, and summer vegetation productivity. Differences in survival rates for this population was best explained by season and time since fire. Survival was positively influenced during the summer by 16-20 yr-old burns and negatively influenced in the winter by 11-15 yr-old burns. Survival was significantly higher in the summer (Jun-Aug), followed by spring (Mar-May), then fall (Sept-Nov), and lowest in the winter (Dec-Feb). The annual survival estimate for adult females in this population was 0.79 which is low when compared to survival rates in other parts of their range. The vast majority of studies that try to quantify how wildfire influences wildlife use a short (<5 yrs) temporal scale to measure effects. This research highlights the importance of considering a longer postfire response (>10 yrs) for wildlife when evaluating wildfire effects. I found that mule deer respond to burned areas very differently depending on season, with wildfire having a positive effect during the summer and negative effect during the winter.
I collected vegetation measurements on migratory (n = 19) and resident (n = 17) mule deer summer ranges to determine changes in forage dry-matter digestibility (DMD) at three different phenological stages (emergent, mature, and cured) for three forage classes (grass, shrub, forb). I also collected biomass and cover data to determine differences in forage quantity for forage classes in migratory and resident summer ranges. I revisited a subsample of these sites the following year to measure the magnitude of variation between growing seasons. Migratory seasonal ranges had higher overall DMD (i.e., quality) of forage classes throughout the summer, but resident summer ranges had higher shrub forage quality. Forage quality declined across the study area as summer progressed. Grasses had the highest cover, biomass, and bulk density for all forage classes. This study provides insight into how available forage drives mixed migration behaviors of a population of mule deer by demonstrating migratory deer have access to higher quality forage throughout the summer. Yet, the trade-off for resident deer is the access to higher quality shrub forage at lower elevations.
My research examined how major changes to the landscape, such as juniper expansion, wildfire, and invasive annual grasses influence a declining population of mule deer. These findings lend more detailed support to the theory that habitat degradation and loss are major contributing factors to population decline. Furthermore, my results provide critical information on key areas to consider regarding mule deer habitat, including a target threshold for juniper canopy cover, limiting the spread of exotic grasses, and the positive and negative attributes of wildfire.