Population Ecology and Summer Habitat Selection of Mule Deer in the White Mountains: Implications of Changing Landscapes and Variable Climate
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Changes in landscape composition have the potential to negatively influence animal populations through shifts in dominant plant communities, loss of important forage items, or changes in structural components of habitat. In the western United States, expansion of woodland vegetation into shrub dominated communities is of concern, particularly with regard to animal populations reliant on robust sagebrush and shrub vegetation. Once established, trees can out-compete shrubs and herbaceous plants resulting in declines in abundance and diversity of shrub-forb vegetation, which female mule deer are reliant on during summer months to meet nutritional demands and to provide hiding cover for young. As a result, shifts in the distribution of pinyon-juniper woodland and increases in tree densities could negatively affect mule deer population. The study had two primary objectives, (1) to determine summer habitat composition of female mule deer in the White Mountains of California and eastern Nevada, and assess implications of pinyon-juniper expansion on habitat availability, and (2) evaluate the status of the population relative to nutritional carrying capacity and determined the influence of habitat and precipitation on demographic rates. I used mixed-effects logistic regression to model summer resource selection and demographic rates of female mule deer from 2005 to 2008. Summer resource selection was modeled at two spatial scales and among three behavioral periods, related to foraging, resting, and parturition. Summer habitat consisted of sites with high productivity, greater shrub abundance, and greater proximity to riparian areas. Deer avoided high levels of tree cover at all spatial and temporal scales, but they selected areas with low to intermediate tree cover during resting periods and during parturition. Moreover, mule deer avoided areas of productive shrub-forb vegetation (riparian and shrub NDVI), when surrounded by stands of high level pinyon-juniper cover, otherwise those vegetation types were strongly selected. During parturition female mule deer selected habitat that maximized hiding cover for newborns (greater shrub densities and structural cover), while still providing foraging opportunities (greater NDVI and shrub cover). Females underutilized certain areas that contained optimal forage such as riparian corridors, high AET sites, higher elevation shrub communities, and selected areas with low to moderate tree cover, suggesting some trade-off between minimizing predation risk for offspring, and maximizing foraging opportunities.Demographic rates (body condition, survival, fetal rates, and index of recruitment) of female mule deer were sensitive to changes in resource availability resulting from variation in precipitation or habitat composition and suggestive of a population regulated to a greater degree by bottom-up processes, and likely nearing nutritional carrying capacity. Moreover, I identified a strong negative effect of pinyon-juniper cover on annual survival, only during periods of drought, otherwise individuals were able to maintain relatively high survival regardless of habitat composition. These results suggest that in productive years mule deer are able to inhabit areas of varying levels of pinyon-juniper cover with little effect on survival, and only during the drought years are negative effects evident. Results from this study emphasize the importance of productive shrub and forb vegetation to mule deer inhabiting semi-arid regions. Maintaining areas with low-to-intermediate tree cover, where there is still abundant shrub understory and sufficient concealment cover, may be beneficial to mule deer populations. Nevertheless, the strong influence of resource availability on the population suggests that conversion of sagebrush-steppe communities into large stands of PJ dominated woodlands would likely reduce the quality and abundance of available habitat for mule deer in the Great Basin.