The fossil record not only allows the examination of shifts in the distributional patterns of organisms, but also provides information about the evolutionary adaptability of organisms when confronted with environmental perturbations.
We have studied the responses of a single mammalian genus (Neotoma, woodrats or packrats) to late Quaternary climate change within the southwestern United States. We focus on Neotoma not only because of their robust and well documented physiological response to temperature over space and time, but also because of the unique and incomparable historical record they inadvertently archive in their middens (debris piles, or paleomiddens).
Using woodrat paleomiddens (radiocarbon-dated debris piles that span the past 40,000 years) collected from caves across the western United States, we characterize how these animals responded to climate shifts since the last ice age (~25,000 years to present). The late Quaternary is the best proxy we have for anthropogenic climate change, and so our work yields information important for predicting the responses of modern ecosystems to ongoing environmental changes. Our studies document the entire gamut of potential responses to late Quaternary climate change including tolerance, local extirpation, and range shifts, as well as adaptive changes in genetics and/or morphology. As predicted by Bergmann's Rule, a negative correlation exists between animal body size and environmental temperature. Midden sequences plotted for each mountain range consistently demonstrate that colder climatic conditions select for larger body size and warmer conditions select for smaller body size. Thus, body size decreased rapidly at the Pleistocene/ Holocene boundary as glacial ice sheets melted and global temperatures increased, increased during the Younger Dryas cold episode, decreased during the warm conditions of the middle-Holocene, and was greater during the Little Ice Age. That changes in climate were the proximate driving force behind morphological changes has been demonstrated in our other field and lab work with contemporary animals.
Biodiversity loss in the fossil record
We are exploring the consequences of the terminal Pleistocene ‘trophic and body size downgrading’ (the catastrophic extinction of most large-bodied mammals in the Americas around 13,000 ybp) as a proxy for biodiversity loss today.
While scientists have hotly debated the cause of the late Pleistocene megafauna extinction for decades, only a handful of studies have moved the debate forward. We are examining the consequences of the loss of tens of millions of large-bodied mammals on the structure and functioning of ecosystems in the Americas. By employing stable isotopes, 2 and 3D imaging, geometric morphometrics, species distribution, functional diversity and occupancy modeling, we characterize abundance, distribution, diet and morphology in surviving mammals before and after the extinction. Our study employs continental-wide databases and focused work at a unique cave site in the Edward’s Plateau of Texas, which has a continuous 22,000 fossil record. Thus far, we demonstrate a fundamental shift in the structure/functioning of the community related to the extinction and climate shifts. For example, extinct species formed significantly more associations within the community than do modern animals. And, while larger-bodied surviving mammals mostly eat the same things, the medium-sized animals shifted their diets to incorporate newly available resources. We find the loss of significant functional diversity, which interestingly, has been partially regained by the introduction of exotic game species in Texas. We believe our work is establishing an important ecological baseline for the understanding of contemporary trophic downgrading.
The fusion of macroevolution and macroecology
I am heavily involved in projects that seek to address broad-scale ecological and evolutionary questions over space and time.
We ask questions like: Why are organisms the size they are? What are the ecological and evolutionary consequences of being a certain size? And, how do the complex and dynamic tradeoffs between physiology, life history, environment, phylogeny, and past history influence the ultimate size and fate of an organism? For example, certain invariant size-dependent scaling relationships are seen repeatedly over evolutionary time for mammals and other taxa. Do these arise because of common ancestry, because organisms exist in similar environments, or because they face similar design or life history constraints?
To address macroecological/macroevolutionary questions requires the compilation of large-scale temporal and spatial datasets. Unsurprisingly, I have long been involved in such efforts, beginning with a National Center for Ecological Analysis and Synthesis working group I organized and led as a postdoc. This group compiled the first synoptic electronic global database of late Quaternary mammals (MOM) with body size, taxonomy, diet, and geographic distribution. Our dataset has been heavily cited and incorporated into subsequent efforts (i.e., Pantheria, Phylacine (sp?).
In 2006, along with Kate Lyons and Morgan Ernest, I organized a NSF funded Research Coordination Network that constructed similar data for the Cenozoic. While the extinction of non-avian dinosaurs at the K/Pg boundary was arguably the seminal event in the history of mammals, opening the door for their subsequent diversification into a wide range of lifestyles and body sizes, we found remarkable congruence in the rate, trajectory and upper limit of size across continents, orders, and trophic guilds. This, despite differences in geological and climatic history, turnover of lineages, and variation in species composition. Our analysis suggested while the primary driver for mammal evolution over the Cenozoic was diversification to fill ecological niches after the dinosaur extinction, constraints on maximum size were set by environmental temperature and land area. These synthetic efforts have continued and have been particularly rewarding, leading to numerous papers.
Other working groups include a NESCent Working Group: Phanerozoic body size trends in time and space: Macroevolution and macroecology, which was organized by Jon Payne, Jennifer Stempien and Michal Kowalewski. Here, we explored the identification and explanation of long-term evolutionary trends in higher taxa and biological communities. And, I am a part of a newly initiated Research Coordination Network: E6: Ecological and Evolutionary Effects of Extinction and Ecosystem Engineers, organized by Kate Lyons, Simon Darroch, Cindy Looy and Pete Wagner.