What is likely to happen when the remaining megafauna in Eurasia and Africa go extinct? Today, these animals are highly vulnerable and estimates suggest most may be extinct within 100 years. Understanding the consequences of this contemporary 'trophic downgrading' is a pressing environmental issue. Yet, such large-scale changes in ecological systems have occurred in the recent past. For example, about 13,000 years ago, most species of large-bodied mammals were extirpated in the Americas. While scientists have hotly debated the cause of the megafauna extinction for decades, only a handful of studies have moved the debate forward. Here, we are working to do just that: with funding from NSF, 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. Specifically, we are characterizing patterns of abundance, distribution, diet and morphology in the surviving mammals both before and after the extinction event. Our study establishes an important ecological baseline for the understanding of contemporary trophic downgrading by characterizing how terrestrial ecosystems were influenced by humans at the very beginning of the Anthropocene; some 13,000 years ago.
The historical 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. Our paleoecological research program focuses on a synoptic examination of 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).
We measure the radiocarbon-dated fossil fecal pellets contained in large numbers within paleomiddens, and from them estimate body size, which leads in turn to quantitative predictions about life history, ecological and evolutionary responses to climate change. Since each cave site contains several to dozens of differentially dated middens, a comprehensive investigation can be made of the response of a single woodrat population to climate change occurring over many thousands of years. We find highly significant effects of past temperature fluctuations on Neotoma. As predicted by Bergmann's Rule, a negative correlation exists between woodrat body size and temperature. Body size decreased rapidly at the Pleistocene/Holocene boundary as glacial ice sheets melted and global temperatures increased. Our studies have documented the entire gamut of responses to late Quaternary climate change possible, including tolerance, local extirpation, and range shifts, as well as adaptive changes in genetics and/or morphology. We are currently collecting and examining paleomiddens from the mountains surrounding Death Valley. Another related project involves the extraction and analysis of aDNA from ancient woodrat middens to look at population-level processes.
Heat, death and sex: woodrats in Death Valley. Woodrats not only provide a rich fossil history of the late Quaternary, but because they are extant we can study them in situ. We use a combination of approaches to examine many of the same broad questions addressed by our paleomidden studies. By examining museum and field data for various Neotoma populations, for example, we have been able to firmly establish relationships between ambient high/low temperature and body size. We have found some intriguing and unexpected patterns that are now leading to investigation of the importance of body size and life history tradeoffs. Woodrats in Death Valley, for example, are twice as large as expected on the basis of ambient high temperature. We believe that these animals may have "solved" the problem of life in such an extreme environment (mean maximum July temperature > 46°C) by becoming annuals (i.e., evolving a semelparous reproductive strategy). We are now working on analyzing data from a population we live-trapped monthly for 4 years at Furnace Creek, Death Valley (-84m elevation).
Increasingly, I have been employing macroecological approaches to address broad-scale questions about body size over geographic space and evolutionary time. For example, over the past few years, colleagues and I have compiled and begun to analyze data on maximum body mass at the ordinal level for each sub-epoch for each major continent over the past 100 million years. Our analyses suggest that while the primary driver for the evolution of giant mammals over the Cenozoic was diversification to fill ecological niches subsequent to the dinosaur extinction, constraints on maximum size were set by environmental temperature and land area. I have been involved in several large-scale collaborations over the past few years. These are outlined below.
Research Coordination Network: IMPPS Working Group (Integrating Macroecologial Pattern and Processes across Scales). The body size of an organism reflects complex tradeoffs among numerous processes. Nevertheless, certain size-dependent relationships are repeatedly observed for mammals and other taxa. For example, the distribution of mammalian body sizes is remarkably similar across continents, despite little taxonomic overlap. Moreover, distributions appear to have been similar for the past 50 million years. Do patterns arise because of common ancestry, because organisms exist in similar environments, or because they face similar design or life history constraints? The broad goal of this project is to assess the generality of body size patterns and investigate general underlying processes.
Collaborators: James Brown (University of New Mexico), Alison Boyer (University of Tennessee ), Daniel Costa (University of California Santa Cruz), Morgan Ernest (Utah State University), Alistair Evans (Monash University, Australia), Mikael Fortelius (University of Helsinki), John Gittleman (University of Georgia), Kari Lintulaakso (University of Helsinki), Kate Lyons (Smithsonian Institute), Richard Sibly (University of Reading), Juha Saarinen (University of Helsinki), Patrick Stephens (University of Georgia), Jessica Theodor (University of Calgary), Mark Uhen (George Mason University).
Phanerozoic body size trends in time and space: Macroevolution and macroecology–NESCentWorking Group. The identification and explanation of long-term evolutionary trends in higher taxa and biological communities is an important goal of biological research. Body size is the single most important ecological characteristic of metazoa and the variable most easily applied to analysis of evolutionary trends across distantly related taxa. The working group will initiate a community-wide database of body sizes through the Phanerozoic, an effort that requires standardized data on body size across higher taxa. The working group will also catalyze collaborations between paleobiologists and biologists to develop the theory necessary to investigate long-term dynamics in body-size evolution across diverse living and extinct metazoan lineages.
PI's: Jonathan L. Payne (Stanford University), Jennifer Stempien (University of Colorado), Michal Kowalewski (Virginia Polytechnic Institute and State University).
Collaborators: Alison Boyer (University of Tennessee), James Brown (University of New Mexico), Seth Finnegan (Caltech), Richard Krause (Museum fürNaturkunde der Humboldt-Universitätzu Berlin), Kate Lyons (Smithsonian Insititute), Craig R. McClain (National Evolutionary Synthesis Center), Daniel W. McShea (Duke University), Philip M. Novack-Gottshall (Benedictine University), Steve Wang (Swarthmore College).