My dissertation research, in the Hadly lab at Stanford University, focused on examining how animals, particularly mammals, respond to environmental change. I integrated data from both modern and fossil populations of mammals to understand the connections between individual, population, species and community-level responses.
I used the Pleistocene-Holocene transition as a model for answering these questions in the mammalian community of northern California. Within this broad framework, I had three main projects- squirrel body size, mammalian community structure, and the genetics of species replacement. My end goal was to characterize biotic change from multiple perspectives. For example, how did the overall community change? Are particular species tied to particular climates? How did morphology and genetic diversity change in response to climatic change? Does this help us understand how today’s species will respond to future climatic change?
Mammalian community structure
The bulk of my research centered on examining community dynamics over the past 20,000 years using fossil and modern populations of small mammals in northern California.
Deposits from two caves near Lake Shasta (Samwell Cave and Potter Creek Caves) were excavated in the early 1900’s and contain numerous mammals. These two caves date to around 21,000 years ago and provide a snapshot of late Pleistocene communities. In 2007, I excavated a new deposit within Samwell Cave that spans the Pleistocene-Holocene transition. See here for more information on excavations in Samwell Cave and the Shasta-Trinity region. In 2008, I returned to the Lake Shasta region to conduct small mammal surveys. My goal was to obtain a snapshot of the modern mammalian community in the region to compare with the mammal communities of the past 20,000 years. We trapped in 6 locations around the McCloud River and caught lots of cool animals!
We found that the small mammal community changed significantly since the Last Glacial Maximum, in several important ways, so that the local community in the Shasta region became increasingly less diverse over the past 21,000 years: some species underwent replacement events and others shifted their ranges so that overall, there are fewer species in the local community. Additionally, the community became much less even over time so that the species that remain have a skewed abundance distribution and the modern community is dominated by one genus, Peromyscus mice. All of these events seem to be related to climatic change at the Pleistocene-Holocene transition, and the loss of evenness, in particular, seems to be correlated with periods of rapid climate change between 14,000 and 11,000 years ago. My main collaborators on this project are Jenny McGuire and Liz Hadly.
Genetics of species replacement
Gophers (genus Thomomys) were strongly affected by climatic changes since the Last Glacial Maximum and underwent a species replacement event at the end of the Pleistocene. I used a combination of morphology and ancient DNA from gopher teeth to determine that at least two species were present in the region around Samwell Cave in the late Pleistocene, but only one of them is there today. In the Pleistocene, the dominant species was T. mazama, whereas T. bottae was present but fairly rare. However, with climatic warming at the Pleistocene-Holocene transition, T. mazama contracted it’s range northward and became completely absent from the fossil deposit by the early Holocene. At the same time, T. bottae became much more abundant and today is the only species around the cave. I am currently investigating the population expansion and genetics of this species replacement event, using ancient and modern DNA techniques. In addition, we are using niche modeling of present gophers to understand the role of climate in determining gopher species distribution limits and facilitating species turnover. My main collaborators on this project are Liz Hadly, Lily Li, and Sam Veloz.
Squirrel body size
I also examined (with Bob Feranec and Liz Hadly) how body size varied through time and space in relation to climate within the California ground squirrel, Spermophilus beecheyi. I first determined which variables explained the most body-size variation using museum specimens from the past 100 years and found that, across space, precipitation explains the most variation in body size. Then, I tested whether body size change over the past 20,000 years conformed to our expectations from the modern specimens and found that it did. Overall, body-size variation seems to be best explained by precipitation and not temperature change in this species.
For more information, see: Blois, JL, Feranec, RS & EA Hadly (2008). Environmental influences on spatial and temporal patterns of body size variation in California ground squirrels (Spermophilus beecheyi). Journal of Biogeography 35:602-613