Climate Change, Biodiversity, and Ecosystem Health
Climate Change, Biodiversity, and Ecosystem Health
The Past as a Key to the Future
Abstract and Keywords
Porcupine Cave has produced an astounding number of fossils. More than 20,000 identified specimens of fossil vertebrates were distributed over more than 200,000 years, spanning at least two glacial–interglacial transitions as well as smaller-scale climatic fluctuations within glacials and interglacials. This book discusses the role paleontology plays in understanding ecosystem dynamics, such as the maintenance of biodiversity, the effects of climate change on biodiversity, and how biodiversity relates to the health of ecosystems. The faunal dynamics that characterize Porcupine Cave climatic transitions typify how ecosystems respond to climatic warming episodes. The book suggests that faunal responses to climate change may be useful as an ecological baseline against which future changes can be measured.
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 Soulé (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 (p.4) 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.
Current warming rates have long been recognized to be very fast, and projected rates exceed rates inferred for at least the last 100,000 years (e.g., Schneider et al., 1992; Jackson and Overpeck, 2000). But exactly how anomalous are these fast modern rates in comparison with the many changes in warming rates that ecosystems have experienced and evolved within over the past thousands and millions of years? Determining this is not as straightforward as it sounds, because rates of change typically are computed over differing time intervals. This has been shown to be a problem in studies of evolutionary rates, for example, where there is an inverse relationship between rates of evolutionary change and the length of time over which the change is measured (Gingerich, 2001). Sedimentation rates show the opposite relationship: the thickness of sediments deposited over short time intervals under (p.5) estimates the total thickness that will accumulate over longer periods (Kirchner et al., 2001). How then do rates of climate change scale with the interval of time over which the climate change is measured?
Figure 1.1 answers this question. The data were compiled from paleotemperature proxies provided mainly by oxygen isotope curves (Barnosky et al., 2003). The shorter the interval of time over which the temperature is measured, the faster the per-hundred-year rate of change appears. Plotting these data in log-log space and highlighting the per-hundred-year temperature change indicated for various past, present, and predicted warming rates place both the middle Pleistocene and the current global warming crisis in perspective. It is clear that some of the major global warming events of the past 65 million years—the Paleocene Methane Event (Katz et al., 1999), the late Oligocene Warming Event (Zachos et al., 2001; Barnosky and Carrasco, 2002), the mid-Miocene Climatic Optimum (Barnosky, 2001; Zachos et al., 2001), middle Pleistocene glacial-interglacial transitions (Raymo, 1997; Schneider and Root, 1998), the Pleistocene-Holocene glacial-interglacial transition (Schneider and Root, 1998), and the Medieval Warm Period (Hughes and Diaz, 1994; Campbell et al., 1998; Broecker, 2001)—define the high end of what is normal for per-hundred-year rates of global warming. Rates of change measured since 1950 do not exceed the bounds of normalcy, although, as in past global warming events, they help define the high end of normal. However, if any but the lowest predictions for the anticipated temperature rise by 2100 come to pass, the rate of change would exceed any rates of change known for the past 65 million years. If the highest projections are borne out, the rate of change would be dramatic.
In view of this fact, the faunal dynamics that characterize Porcupine Cave climatic transitions probably typify how ecosystems respond to climatic warming episodes that are at the high end of “natural” warming rates, but nevertheless do not exceed the range of rates that is normal for Earth. Thus the faunal responses to climate change that are detailed in the following chapters are probably among the most pronounced that might be expected in naturally varying systems. Therefore they may be useful as an ecological baseline against which future changes can be measured. As global warming continues into the coming decades, changes in biodiversity and other faunal dynamics will undoubtedly occur—indeed are probably already occurring (Schneider and Root, 1998; Post et al., 1999; Pounds et al., 1999; Saether et al., 2000; Both and Visser, 2001; McCarty, 2001). Faunal responses comparable to those defined by the Porcupine Cave data do not necessarily imply that the bounds of ecological health have been exceeded. However, faunal responses that exceed those demonstrated by the Porcupine Cave data may well herald the death of the canary—a shift in the state of ecosystems that is unprecedented.
Preparation of this chapter was partially supported by NSF grant EAR-9909353. This chapter is University of California Museum of Paleontology contribution 1808.