Anthony D Barnosky

University of California, Berkeley

Earth's climate is getting warmer, and it will probably continue to do so over the coming century. The emerging consensus is that human activities are stimulating an increase in global mean temperature that will amount to 1.4-5.8°C by the year 2100 (Houghton et al., 2001), with 90% probability that the change will amount to 1.7-4.9°C in the absence of climate mitigation policies (Wigley and Raper, 2001). Regionally, the changes will be even greater. Average warming for the United States is predicted to be at least 3°C and possibly as much as 6°C (National Assessment Synthesis Team, 2001). The effects of some of these changes are already apparent. For example, a warming of approximately 4°C in Alaska since the 1970s has led to vast expanses of spruce forests being killed by beetles that reproduce faster in warmer temperatures. Roads are buckling and houses are sinking, as what used to be permafrost thaws seasonally.

A growing number of scientists have recognized that global warming can be expected to affect the few remaining intact, naturally operating ecosystems on Earth in unpredictable ways. This issue came to widespread attention just over a decade ago, with the publication of a compendium of papers, edited by Peters and Lovejoy (1992), concerning the effects of global warming on biodiversity. The effects of climate change on biodiversity are a matter of concern because biodiversity is often associated with ecosystem health. Significant losses in biodiversity may be analogous to the death of the canary in the coal mine, which signals that the mine is no longer safe for humans. Though debate continues about whether "more is better" in terms of numbers of species in ecosystems (Norton, 1987; Grime, 1997; Tilman, 1997; McCann, 2000), available information suggests that larger numbers of species help buffer ecosystems in the face of changing environments (Loreau et al., 2001). Thus of key concern is the question of whether climatic warming will reduce biodiversity to the extent that a given ecosystem loses its ability to maintain the baseline functions that define it. Maintaining these baseline functions is, in fact, integral to an operational definition of ecosystem health. In the words of Haskell et al. (1992:9), "An ecological system is healthy ... if it is stable and sustainable—that is, if it is active and maintains its organization and autonomy over time and is resilient to stress." Put another way, the basic question is: at what point do disruptions to baseline diversity cause ecosystems to cross functional thresholds and catastrophically shift their dynamics (Sheffer et al., 2001)?

Adding to concerns about the effects of climate change on biodiversity is the fragmentation of previously widespread biota by human activities, which itself—probably more so than climate change—often leads to reduction in species richness. As Soule (1992:xiii) put it, it is simply the wrong time for climate change. "Even if species are able to move quickly enough to track their preferred climate, they will have to do so within a major obstacle course set by society's conversion of the landscape. . . . A species may be impelled to move, but Los Angeles will be in the way" (Peters and Lovejoy, 1992: xviii).

Over the past decade, researchers have continued to study how climate change affects biodiversity, and how biodiversity relates to the health of ecosystems. By necessity, most of these studies have been theoretical (Kerr and Packer, 1998; Ives et al., 1999) and/or focused on experiments at the level of study plots, which track diversity changes in response to environmental changes or treatments that take place over months, years, or at best decades (see, e.g., Brown et al., 1997; Chapin et al., 2000; Tilman, 2000; Reich et al., 2001; Tilman et al., 2001). Difficulties arise in scaling the results from small study plots up to the landscape, ecosystem, and biome levels (Loreau et al., 2001). A further difficulty lies in understanding how results obtained over short time scales compare with the natural baseline of variation inherent over ecologically long time scales: hundreds to thousands to millions of years. To study this question, other researchers have focused on tracking ecosystem changes across major climatic transitions, such as those at the Paleocene-Eocene boundary (Wing, 1998), in the early Oligocene (Prothero and Heaton, 1996; Barnosky and Carrasco, 2002), across the middle Miocene climatic optimum (Barnosky, 2001; Barnosky and Carrasco, 2002), and across the Pleistocene-Holocene transition (Graham and Grimm, 1990; Graham, 1992; Webb, 1992; FAUNMAP Working Group, 1996). To link across temporal scales, some studies have taken a comparative approach, which examines how flora and fauna responded to climate changes over varying time scales from years to decades to centuries to thousands or millions of years (Brown et al., 2001; Barnosky et al., 2003). A missing piece of the puzzle, however, has been data sets that allow scientists to track changes in biodiversity through multiple climatic fluctuations over hundreds of thousands of years in one geographic locality.

This book offers one such data set, in the form of more than 20,000 identified specimens of fossil vertebrates distributed over more than 200,000 years, spanning the time from approximately 1,000,000 to at least 780,000 years ago. The specimens come from more than 26 fossil localities within Porcupine Cave, in the high Rocky Mountains of South Park, Colorado (see chapter 2 for locality details). They span at least two glacial-interglacial transitions as well as smaller-scale climatic fluctuations within glacials and interglacials. The deposits also seem to bracket a major transition in the periodicity of glacial-interglacial cycles, from a 41,000-year rhythm in the early Pleistocene to a 100,000-year rhythm that was firmly in place by 600,000 years ago. Therefore it is possible to track a single ecosystem through climate changes of variable intensity and to assess the biodiversity response, which is one goal of this book. However, an equally important goal has been to make the data available to future researchers in a way that can facilitate additional analyses.

Part 1 provides relevant background information on Porcupine Cave, the fossil deposits themselves, and the modern environment of South Park. Part 2 provides the basis for species identifications (which are critical in assessing the quality of the data and what it can be used for) as well as summaries of actual numbers of specimens representing each species (which are necessary for many ecological analyses). Part 3 focuses on faunal dynamics and how the fossil information applies to understanding the effects of climatic warming on biodiversity. The nature of the data makes it possible to examine how climate change affected biodiversity in terms of trophic and size structure, species richness, species composition, and population change.

An overriding impetus for this effort has been the need to establish a baseline that will allow clear recognition of disruptions to natural biodiversity caused by human-induced global warming. An initial priority is to assess how global warming indicated by the middle Pleistocene glacial-interglacial transitions compares with rates of warming that are currently under way, those that are predicted, and those that have occurred throughout geological time.

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