Small mammals are a keystone guild in arid ecosystems; often exhibiting top-down control of the diversity and structure of plant communities. However, changing climate, shifting fire regimes, and the invasion of exotic plants are modifying the structure of arid systems. Environmental changes in these arid systems are likely altering small mammal communities, and therefore, their ecological role. We examined two aspects of the community composition of small mammals in the Great Basin: changes in community composition since large scale sampling of the region began in 1930, and the current population of a sensitive species of small mammal, the dark kangaroo mouse (Microdipodops megacephalus). In Chapter 1, we compared diversity and composition of present day small mammal communities to communities sampled between the years of 1930 and 1980. We sampled 234 historical locations across the eastern Great Basin region during the summers of 2014 and 2015. Our results indicated that diversity, richness, and evenness of small mammals in the Great Basin have declined significantly over the last century (P=0.002, P=0.03, P=0.002). The relative abundance of generalist species has increased, while specialist species have declined (P<0.001, P<0.001). Also, community composition at each site has changed significantly over the past century. Alterations in the community structure of small mammals may have cascading implications for the future of the Great Basin ecoregion. In Chapter 2, we conducted a region-wide survey for the dark kangaroo mouse in western Utah. Four teams sampled 232 locations across western Utah during the summers of 2014-2015. Of the 232 sites sampled, only 5 sites resulted in dark kangaroo mouse captures, totaling 15 individuals. These results could indicate a state-wide population decline for this species, both compared to historic population levels and to the populations surveyed less than ten years ago. The rapid decline may be a result of habitat degradation associated with invasive plant species and increasing fire frequency, the effects of which are exacerbated by the dark kangaroo mouse's life history as an ecological specialist. Unless large-scale habitat restoration and preservation is conducted for remaining populations, it is likely the dark kangaroo mouse will continue to decline within the state.