Fully understanding how species will respond to current and future climatic change is one of the greatest conservation challenges facing society today. Mammals can and have shown a variety of responses to climatic change throughout their history, from genetic change at local scales to biotic turnover at the largest spatial and temporal scales (Chapter 1). For my dissertation, I used the fossil record of the past 21,000 years as a model for understanding how mammals respond to climatic change, focusing on those species that survived the Late Pleistocene megafaunal extinction event. These survivors are important because they form the modern mammalian community and are now exposed to unprecedented pressure due to anthropogenic climatic change.

I first investigated how the body size of one species, the California ground squirrel (Spermophilus beecheyi), responsed to climate through space and time (Chapter 2). I found that precipitation was more important than temperature in explaining body-size variation, both across California in modern ground squirrels and over the past 21,000 years in northern California. Next, by excavating a new cave fossil deposit that spanned the Late Pleistocene and Holocene, I reconstructed mammalian population and community changes at one location in northern California in order to understand other ways species and communities may respond to climatic and community changes (Chapters 3, 4). Overall, local diversity declined and the structure of the community became more uneven due to climatic changes during the latest Pleistocene (Chapter 3). Finally, I investigated factors contributing to genetic diversity and population dynamics within one taxon, pocket gophers (Thomomys), which experienced a species turnover event across the Pleistocene-Holocene transition due to climatic changes at that time (Chapter 4). Overall, my dissertation has centered on understanding the full spectrum of responses that species have shown to past climatic change. I have shown that small mammals responded to climatic change in many different ways, including through morphologic, genetic, and abundance change, culminating in changes to the structure and diversity of the overall small mammal community. All of these responses will likely be manifested as mammals respond to current and future anthropogenic climatic change.