Small mammal response to weather and vegetation structure in an isolated rangeland

The grasslands of the Southern High Plains are being encroached upon by woody shrubs, specifically invasive honey mesquite. Shifts in the vegetation regime caused by shrub encroachment can affect feedback loops between small mammal populations and the ability of those populations to shape the environment around them through seed dispersal and other mechanisms. Restoration, therefore, will play an important role in conservation of the Southern High Plains and the small mammal assemblages therein. In Chapter I, I reviewed research on shrub encroachment in grasslands and the role that increased woody cover has on small mammal population sizes and individual activity. I also synthesized literature on how weather underlies shifts in vegetation over time, and the indirect effects on small mammal population size, activity, and competition. Focused on Sigmodon hispidus and other mammal species found in an isolated rangeland site in Lubbock, Texas, I discussed my approach to assessing long-term (ten years) and short-term (less than one year) patterns in small mammal population size and associations with specific aspects of their environment. Chapter II focused on Sigmodon hispidus population dynamics across ten years in an isolated rangeland site. I modeled S. hispidus spatial spread and annual population size as functions of weather, vegetation, and other small mammal species. I found that the number of S. hispidus individuals per site was positively associated with herbaceous cover and negatively associated with brighter areas and areas with small mammals of other species. I did not detect patterns between population size and annual measures of weather and vegetation. Ultimately, my study showed the importance of using long-term weather, vegetation, and small mammal assemblage data to assess small mammal population dynamics across multiple scales of time and space in historic grasslands undergoing long-term shifts in the vegetation regime. In Chapter III, I assessed changes in small mammal activity in response to restoring areas of a mesquite-invaded shrubland to grassland. I modeled the relationships between subsets of small mammals (all species detected, Sigmodon hispidus, Reithrodontomys megalotis, and Mus musculus) and herbaceous cover, a canopy-mediated measure of nighttime brightness, and aspects of microclimate during the first year post-restoration. There was less activity in restored plots than control plots for the combined small mammal species assemblage, and specifically for S. hispidus and R. megalotis. All species except M. musculus responded positively to increases in herbaceous cover; all species responded negatively to increased overnight temperature instability except R. megalotis, which responded positively; and S. hispidus and M. musculus were associated with brighter sites. These relationships expose some of the complexity underlying the short-term sampling of an assemblage recovering from an overarching population crash. This study highlighted the importance of continuing to observe restored areas for a minimum of several years post-treatment to account for changes in the successional state of the vegetation community after the initial disturbance of the treatment.