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Humanity as Press and Pulse in the Modern Mass Extinction

 


 

Mass extinctions—moments in Earth’s history when a significant proportion of species become extinct over a short span of geologic time—are unusual. Paleontologists recognize only five such intervals of major dying in the Phanerozoic1, 2 (the last 542 million years of Earth’s history during which the fossil record is richest). There is debate among scientists both over whether these five mass extinction are truly exceptional3 and how many mass extinctions have occurred in Earth’s history.4 However, the fact that contemporary extinction rates are as much as 120,000 times higher than the normal or background rate5 qualifies the modern era as an episode of mass extinction potentially rivaling that at the end of the Cretaceous Period, when non-avian dinosaurs became extinct6-8. In our paper “Press-Pulse: A General Theory of Mass Extinction?” we concluded that the mass extinctions observed in Earth’s past required the coincidence of “press” (long term stress) and “pulse” (short term disturbance) events9. Yet if a combination of press and pulse mechanisms is required to generate a mass extinction, what accounts for the current biodiversity crisis? And how will understanding today’s presses and pulses help us to develop a better conservation policy?

The Press-Pulse Model of Mass Extinction

Arens and West9 borrowed the concept of press and pulse disturbances from community ecology10, and used them to categorize events that contribute to extinction11, 12. Press mechanisms exert long-term stress on biodiversity11, primarily by changing the physical environment. They may cause extinction by increasing mortality, but—more importantly—they curtail reproduction or reduce habitat, thus shrinking and endangering populations over multiple generations. Also, these mechanisms irreversibly change the ecological dynamic in ways that may render previously well-adapted species unfit. However, because press stresses occur over ecologically long time scales, adaptation to new conditions is possible. Examples of press mechanisms from the geologic past include climate change, sea level rise or fall, or changes in the ratio of gasses in the atmosphere. In contrast, pulse disturbances are geologically instantaneous events that cause significant mortality. Examples of pulse disturbances in the geologic record include bolide (e.g., asteroid or comet) impact and sudden marine anoxia. If too many individuals of a species die during the pulse disturbance, populations become unviable and the result is extinction. However, if enough individuals survive and endure the hard times following a pulse disturbance, survival is likely because the physical and biotic world will eventually recover to their pre-disturbance state9. The problem for many species, however, is that recovery may take hundreds of thousands or even millions of years13, 14. Within this framework, we concluded that to generate high levels of extinction characteristic of a mass extinction, a press stress must first endanger a wide range of populations. If a pulse disturbance follows, a major extinction is likely.

In our study of extinctions in the Earth’s history, we chose long-lived (one to two million years duration) volcanic eruptions that produced widespread flood basalts as our proxy for press stresses, and bolide impacts as a major pulse disturbance.9 While the rationale behind these choices is in a more technical paper, an in-depth discussion of that work and its methods is beyond the scope of this essay. However, neither phenomenon can be blamed for the modern biodiversity crisis. Instead, anthropogenic mechanisms such as over-harvesting, pollution, disease, invasive species, deforestation, land-use conversion, and other forms of habitat destruction are generally blamed for the current biodiversity crisis5, 6, 15. Under this argument, it is the cumulative effect of many individual human-caused species extinctions that result in the global biodiversity crisis. This assumption has led conservationists to adopt a “first responder” approach to modern extinction threats. Conservationists rush to sites of emergency—termed biodiversity hotspots6—to save endangered species and habitats. This deeply entrenched philosophy is further reflected in national and international legal frameworks for species conservation (e.g., the U.S. Endangered Species Act and the Convention on International Trade in Endangered Species of Wild Fauna and Flora)—that address endangered wild populations with a species-by-species approach. However, if our perception of the causes of contemporary mass extinction is flawed, our approach to solutions will be as well.

Humans and the Global Carbon Cycle

A basic understanding of the global carbon cycle is essential to recognizing the role humans play as press and pulse agents of extinction. The global carbon cycle (Figure 1) is the process by which the element carbon moves from one place (reservoir) to another on the globe. For example, carbon, in the form of carbon dioxide, moves from the atmosphere into the biosphere as green plants take it up and use it to build biological molecules, like sugars and starches for food, and structural molecules such as lignin and cellulose. When these plants are eaten by herbivores, this carbon moves through the biological food web. Some of it is respired back to the atmosphere as carbon dioxide, while some builds the organism’s body. When organisms die, their carbon may be food for decomposers and respired back into the atmosphere as carbon dioxide, or it may be incorporated as organic matter into soil. Carbon in the oceans is even more complex. In the surface ocean (Figure 1), carbon may exist as dissolved carbon dioxide, as bicarbonate ion, as living creatures, or as dissolved organic carbon (DOC). All of these materials are exchanged with the deep ocean, and some are mixed with sediments to enter the geologic reservoir. Organic carbon that escapes decay and is buried by sediment may also enter the geologic reservoir and be transformed into fossil fuels such as coal, petroleum and natural gas.

Humans interact with the global carbon cycle in a variety of ways. At the most basic level, we consume organic carbon from the biological reservoir (food), oxidize it to provide energy, and respire carbon dioxide back into the atmosphere. In this way, humans are like all aerobic organisms, cycling carbon from the biological to the atmospheric reservoirs. Humans also modify the environment in ways that move carbon from one reservoir to another. For example, burning to modify a landscape’s vegetation15, 16 and clearing land for timber production and agriculture17 transform forest into woodland, grassland, and crop monoculture which returns carbon contained in organic molecules stored in trees, undergrowth, and soil back to the atmosphere as carbon dioxide. In the late 18th Century, humans began tapping the geologic carbon reservoir (the largest of Earth’s carbon reservoirs) for fossil fuels. Burning fossil fuels returns carbon that had been sequestered in Earth’s crust for millions of years to the atmosphere as carbon dioxide. In this way, humans mobilized a new carbon reservoir and used it to power the Industrial Revolution and our energy-hungry modern lifestyle.

Humans as Press

The human family began to stress and alter biological communities even before the emergence of our own species, Homo sapiens, some 100,000 years ago. Human ancestors in Africa hunted large herbivores that were greater than 1000 kg as early as 3.9 million years ago. While this activity exerted only local influence, the loss or reduction of large grazing and browsing animals changed vegetation structure, increased fuel load and caused wildfires to become more common and destructive. Then, between 1.9 and 1.5 million years ago, hominids began to use fire deliberately to modify the landscape16. Changing the natural fire regime in this way permanently altered habitat structure, perhaps contributing—with global climate change—to the opening of the great African grasslands. However, hunting or fire-mediated habitat conversion appear to have caused few if any extinctions in Africa18, 19.

Fire as a tool for landscape management also represented humankind’s first experiment in tampering with Earth’s global carbon cycle. Coupled with Late Pleistocene and Holocene climate change, frequent burning at the hands of early humans transformed forests into woodlands and woodlands into grasslands. This released significant amounts of biological carbon stored in forests into the atmosphere. Estimates vary, but from 1,000 to 3,000 tons of carbon are transferred to the atmosphere for each square kilometer of forest cleared17.

As mankind emerged from Africa and migrated around the globe, isolated populations continued to have profound effects on local environments. Events such as the deforestation of Ain Ghazal in present-day Jordan, and the devastation of Easter Island in the Pacific transformed local habitats, caused local extinctions, and altered the local carbon cycle20. However, on a global scale, these changes had no discernable effect17, 20.

The human press became global with the emergence of large-scale agriculture around 6,500 years ago20. Field crops grown by early farmers required about three times more forest to be cleared than was necessary to support hunter-gatherers in the same climate. Intensive grazing by domestic livestock, particularly in marginal lands, similarly altered vegetation. Furthermore, tilling soil and diverting natural water courses for irrigation led to additional releases of carbon dioxide and methane, respectively. An anomalous rise in carbon dioxide and methane, two of today’s most powerful greenhouse gases, has been observed in Arctic and Antarctic ice layers deposited during this time. This demonstrates that, for the first time, humans were exerting a global influence on the carbon cycle. These human-mediated changes in greenhouse gasses likely contributed to climate change observed during the spread of early agriculture17. With the rise of agriculture, humans became firmly entrenched as agents of environmental press, both as instruments of habitat conversion and of climate change. However, in the areas where early agriculture began and flourished—Asia, Europe and Africa—there is little evidence of major extinction during this period19, supporting our conclusion that press stress alone is generally not sufficient to cause mass extinction9.

There is increasing evidence that the climate change resulting from human manipulation of the global carbon cycle acts as a press stress. For example, emperor penguins (Aptenodytes forsteri), the icons of the Antarctic, have suffered a 50 percent decline in population numbers over the last 50 years. Demographic studies of the Terre Adélie population show that the population decline can be linked directly to low adult survivorship during the late 1970s, when a warm interval reduced sea ice in the penguins’ foraging areas21. Animals and plants may respond to climate-induced stress by altering geographic range. In a study of 35 non-migratory butterfly species in Europe, all were observed to have shifted their ranges north by 35-240 km. Most species became locally extinct in the southern portions of their ranges22. A more extensive study including 1,700 species of land plants and animals shows that almost all species were migrating poleward23. Furthermore, the rate of species migration matches the global poleward advancement of spring events, such as the first thaw or last frost23, indicating that species are attempting to track their preferred climate. This strategy can be successful as long as habitat in higher latitudes is otherwise suitable. However, mountain ranges, human population centers, oceans, and other barriers that prevent the movement of species may thwart migration. In such cases, range contraction and population decline that lead to endangerment are inevitable.

Humans as Pulse

Humankind’s transition from solely a press agent of environmental change to a perpetrator of pulse disturbance involved two important shifts, one in attitude and a second in technology.

From prehistory into the late medieval period, humans generally regarded the Earth as a living being with animistic qualities24. Although human activity was already causing environmental stress, humans saw themselves as part of nature and merely victims of her25 capricious moods as drought, flood or fire destroyed crops and livestock. This image of Earth as Mother precluded the large-scale exploitation of natural resources, through activities such as mining or clear-cut forestry, which characterize the modern era. However, as Europe entered the Renaissance and became a world power, attitudes began to change. The elite rejected the previously held belief that the Earth was a living, breathing, female body in favor of the interpretation, buttressed by religious doctrine of the day: that Earth was inanimate and its creatures were flawed. This bred a disconnection between humans, created in the image of God and striving for spiritual perfection, and the fallen Earth. An ethic emerged in which humans were not dependent on and had no responsibility to the environment in which they lived. In fact, Judeo-Christian tradition, and a mercantile economy, promoted an interpretation of the closing verses of the first chapter of Genesis as a license to exploit creation for human gain24.

The religious view was bolstered by the introduction of empiricism, which cast the Earth and its biota as cogs in a great machine. This new mechanistic model of nature removed previously held ethical restrictions, and allowed the exploitation of the environment, such as mining, clear-cut forestry, hunting for the extermination of “pest” species, and over-harvest of others 24. This, in turn, promoted the rise of the industrial society in the 18th and 19th centuries.

The Industrial Revolution inaugurates humankind in its role as mechanistic pulse. The industrial transformation is powered by a new energy source; fossil fuel from Earth’s geologic carbon reservoir. As fossil fuels burn, they move carbon from long-term storage in Earth’s crust into the atmosphere as carbon dioxide (Figure 1). This transition is clearly visible in Arctic and Antarctic ice core records that chart changes in the composition of Earth’s atmosphere over the last 350,000 years17. Where humans had previously manipulated the terrestrial, oceanic and atmospheric carbon reservoirs through agriculture, burning fossil fuel introduced previously inaccessible carbon into Earth’s atmosphere leading to a litany of climate-related changes17, 26. While climate change is conventionally considered a press stress11, new evidence points to a climate system that does not change slowly and gradually, but jumps from equilibrium to equilibrium over very short time scales27. For example, a 123,000 year long record of temperature and atmospheric composition preserved in Greenland ice shows 24 abrupt climatic warmings, including one 115,000 years ago that occurred over as little as five years28. From the perspective of Earth’s biota, such rapid shifts—triggered by anthropogenic greenhouse emissions—become a pulse disturbance capable of generating significant mortality in animal and plant populations.

A growing body of evidence indicates that climate change can act as a pulse disturbance, either directly or indirectly, to promote mortality and extinction. In 1977, drought triggered by El Niño in the Galápagos Islands killed more than 85 percent of one of the archipelago’s native finches, Geospiza fortis29. This event is a classic pulse disturbance for this ecosystem. In another example, several species of harlequin frog (Atelopus sp.) and the iconic golden toad (Bufo periglenes) of Costa Rica both became extinct suddenly in the late 1980s. The culprit was eventually identified as a pathogenic chytrid fungus that spread into the region as a direct result of climatic warming30.

Implications for Conservation

Our analysis of mankind’s role as both press and pulse agents of extinction through the manipulation of the global carbon cycle and the resulting climate change has powerful implications for conservation. However, there is at present little popular or policy energy behind the link between climate and the modern biodiversity crisis. Al Gore’s Oscar-winning documentary, An Inconvenient Truth, detailed the case for anthropogenic climate change but virtually ignored climate as an agent of extinction, apart from the sad reference to polar bears that swim to exhaustion while searching for ice flows31. However, the scientific community is increasingly recognizing that Earth’s biota is already responding to climate change at local, regional, continental and global scales32,33. Some responses involve acclimation to new conditions, some involve adaptation, but a significant number of species are likely to become extinct. A 2004 study encompassing a wide range of habitat types and regions estimated that 15-37 percent of the 1103 land plants and animals studied would be committed to extinction by the year 2050, given conservative estimates of global climate change34. This has led some in the scientific community to call for conventional conservation approaches to be replaced with “global change managers”35. Ibisch and colleagues argued that even ecosystem-centered conservation approaches36 that focus on the interrelationship among species fail to acknowledge nature and magnitude of change facing species in the decades ahead.

Traditionally, conservation efforts, both governmental and non-governmental, have focused solely on the pulse component of the extinction equation. Furthermore, the range of pulse disturbances generally considered is limited to habitat loss and over-harvesting. At the federal level in the United States, laws such as the Endangered Species Act of 1973 and the Bald Eagle Protection Act of 1940 provide protection only for individual species under threat. Although ESA protection was to be extended to the “critical habitat” of endangered species in 1978, this provision was weakened by executive order in 1986. At the international level, the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) was also ratified in 1973. CITES provides varying levels of protection for approximately 33,000 species by prohibiting legal trade in them or their parts37. Outside of government, a wide range of organizations work for biodiversity conservation using a similar strategy. For example, the World Wildlife Fund (WWF)—a major non-governmental player—focuses its programs on the conservation of individual species by urging the creation of a protected area off the coast of Chile for blue whales, and lobbying Congress for the passage of the Great Cats and Rare Canids Act38 that would provide funding for the conservation for animals such as lions, snow leopards and African wild dogs. They also aim to conserve critical habitats by lobbying Congress against petroleum drilling in Bristol Bay, Alaska39. WWF also funds conservation research. Conservation International, another major U.S. non-governmental organization, chooses the ecosystem approach by promoting protection of biodiversity hotspots, high biodiversity wilderness areas and key marine regions through scientific research and local education/development partnerships40.

However, there are hopeful signs. Both WWF41 and Conservation International42 have identified climate change as a conservation priority and are developing response programs. And in a bold move, Governor Arnold Schwarzenegger signed the California Global Warming Solutions Act (AB 32) on 27 September 2006 that would mandate comprehensive reduction in greenhouse gas emissions by 25 percent by 2020. Policy-makers, organizations, and citizens alike will be first to recognize the central role of climate change in the biodiversity crisis, and work, both individually and as a society, to minimize humankind’s manipulation of the global carbon cycle. paJ

About the Author: Nan Crystal Arens is associate professor of geoscience at Hobart & William Smith. She has degrees in geology and English from Penn State and biology from Harvard. Her research explores the evolution of plants and the environment through the lens of the fossil record. Prior to joining the HWS faculty, she was curator of fossil plants at the University of California Museum of Paleontology.

Ian West has a degree in Environmental Studies from Hobart and William Smith, with a minor in Geoscience. He has recently completed his first year as a high school Science Teacher in New Hampshire and is completing Alternative IV certification for General Science. Currently he is enrolled in a Master’s program for Environmental Education at the University of New Hampshire.

 


 

(Notes)

  1. Raup, D. M. & Sepkoski, J. J, “Mass Extinctions in the Marine Fossil Record.” Science 215, no. 4539 (1982): 1501-1503.
  2. Taylor, P. D, “Extinction and the Fossil Record.” In Extinctions in the History of Life, ed. Taylor, P. D., 1-34. (Cambridge: Cambridge University Press, 2004).
  3. Wang, S. C, “On the Continuity of Background and Mass Extinction.” Paleobiology 29, no. 4 (2003): 455-467.
  4. Bambach, R. K, “Phanerozoic Biodiversity: Mass Extinctions.” Annual Review of Earth and Planetary Sciences 34 (2006): 127-155.
  5. Leakey, R. & Lewin, R, The Sixth Mass Extinction: Patterns of Life and the Future of Humankind. (New York: Anchor Books, 1995).
  6. Wilson, E. O, The Diversity of Life. (New York: W.W. Norton & Co., 1992).
  7. Stork, N. E, “Measuring Global Biodiversity and its Decline.” In Biodiversity II: Understanding and Protecting Our Biological Resources, ed. Reaka-Kudla, M. L., Wilson, D. E. & Wilson, E. O., 41-68. (Washington D.C.: Joseph Henry Press, 1997).
  8. May, R. M., Lawton, J. H. & Stork, N. E, “Assessing Extinction Rates.” In Extinction Rates, ed. Lawton, J. H. & May, R. M., 1-24. (Oxford, England: Oxford University Press, 1995).
  9. Arens, N. C. & West, I. D, “Press-pulse: A General Theory of Mass Extinction?” Paleobiology, (in press).
  10. Bender, E. A., Case, T. J. & Gilpin, M. E, “Perturbation Experiments in Community Ecology: Theory and Practice.” Ecology 65, no. 1 (1984): 1-13.
  11. Erwin, D. H, “Understanding Biotic Recoveries: Extinction, Survival and Preservation During the End-Permian Mass Extinction.” In Evolutionary Paleobiology, ed. Jablonski, D., Erwin, D. H. & Lipps, J., 398-418. (Chicago: Chicago University Press, 1996).
  12. Erwin, D. H, “Lessons from the Past: Biotic Recoveries from Mass Extinctions.” Proceedings of the National Academy of Sciences of the United States of America 98 (2001): 5399-4503.
  13. Beerling, D. J. et al, “Evidence for the Recovery of Terrestrial Ecosystems ahead of Marine Primary Production Following a Biotic Crisis at the Cretaceous-Tertiary Boundary.” Journal of the Geological Society 158 (2001): 737-740.
  14. Aberhan, M., Weidemeyer, S., Kiessling, W., Scasso, R. A. & Medina, F. A, “Faunal Evidence for Reduced Productivity and Uncoordinated Recovery in Southern Hemisphere Cretaceous-Paleogene Boundary Sections.” Geology no. 3 (2007): 227-230.
  15. Flannery, T, The Future Eaters: An Ecological History of the Australasian Lands and People. (New York: Grove Press, 2002).
  16. Burchard, I, “Anthropogenic Impact on the Climate Since Man Began to Hunt.” Palaeogeography, Palaeoclimatology, Palaeoecology 139, no. 1-2 (1998): 1-14.
  17. Ruddiman, W. F, Plows, Plagues and Petroleum: How Humans Took Control of Climate. (Princeton, NJ, USA: Princeton University Press, 2005).
  18. Barnosky, A. D, “The Late Pleistocene Event as a Paradigm for Widespread Mammal Extinction.” In Mass Extinctions: Processes and Evidence. ed. Donovan, S. K., 235-255. (New York: Columbia University Press, 1991).
  19. Barnosky, A. D., Koch, P. L., Feranec, R. S., Wing, S. L. & Shabel, A. B, “Assessing the Causes of Late Pleistocene Extinctions on the Continents.” Science 306, no. 5693 (2004): 70-75.
  20. Redman, C. L, Human Impact on Ancient Environments. (Tucson, AZ: University of Arizona Press, 1999).
  21. Barbraud, C. & Weimerskirch, H, “Emperor Penguins and Climate Change.” Nature 411 (2001): 183-186.
  22. Parmesan, C. et al, “Poleward Shifts in Geographical Ranges of Butterfly Species Associated with Regional Warming.” Nature 399 (1999): 579-583.
  23. Parmesan, C. & Yohe, G, “A Globally Coherent Fingerprint of Climate Change Impacts Across Natural Systems.” Nature 421 (2003): 37-42.
  24. Merchant, C, The Death of Nature: Women, Ecology and the Scientific Revolution. (San Francisco: Harper, 1990).
  25. Earth or the diety representing Earth is most commonly described as female. We follow that convention in our choice of pronoun.
  26. Fagan, B, The Long Summer: How Climate Changed Civilization. (New York: Basic Books, 2004).
  27. Overpeck, J. T. & Cole, J. E, “Abrupt Change in Earth’s Climate System.” Annual Review of Environment and Resources 31 (2006): 1-31.
  28. Andersen, K. K. et al, “High-Resolution Record of Northern Hemisphere Climate Extending into the Last Interglacial Period.” Nature 431, no. 7005 (2004): 147-151.
  29. Grant, P. R, Ecology and Evolution of Darwin’s Finches. (Princeton, NJ: Princeton University Press, 1999).
  30. Pounds, J. A. et al, “Widespread Amphibian Extinctions from Epidemic Disease Driven by Global Climate.” Nature 439, no. 7073 (2006): 161-167.
  31. Gore, A, An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. (New York: Bloomsbury Books, 2006).
  32. Root, T. L. et al, “Fingerprints of Global Warming on Wild Animals and Plants.” Nature 421 (2003): 57-60.
  33. Walther, G. R. et al, “Ecological Responses to Recent Climate Change.” Nature 416 (2002): 389-395.
  34. Thomas, C. D. et al, “Extinction Risk from Climate Change.” Nature 427 (2004): 145-148.
  35. Ibisch, P. L., Jennings, M. D. & Kreft, S, “Biodiversity Needs the Help of Global Change Managers, Not Museum-keepers.” Nature 438 (2005): 156.
  36. Kareiva, P. & Marvier, M, “Conserving Biodiversity Coldspots.” American Scientist 91 (2003): 344-351.
  37. www.cites.org
  38. H.R. 1464/S. 1033
  39. www.worldwildlife.org
  40. http://web.conservation.org
  41. www.worldwildlife.org/globalwarming/
  42. http://web.conservation.org/xp/CIWEB/regions/priorityareas/climatechange...
  43. www.pmel.noaa.gov
  44. One gigaton is equal to one million metric tons. A metric ton is equal to one thousand kilograms.

References

Aberhan, M., Weidemeyer, S., Kiessling, W., Scasso, R. A. & Medina, F. A. “Faunal Evidence for Reduced Productivity and Uncoordinated Recovery in Southern Hemisphere Cretaceous-Paleogene Boundary Sections.” Geology no. 3 (2007): 227-230.

Andersen, K. K. et al. “High-resolution Record of Northern Hemisphere Climate Extending into the Last Interglacial Period.” Nature 431, no. 7005 (2004): 147-151.

Arens, N. C. & West, I. D. “Press-pulse: A General Theory of Mass Extinction?” Paleobiology (in press).

Bambach, R. K. “Phanerozoic Biodiversity: Mass Extinctions.” Annual Review of Earth and Planetary Sciences 34 (2006): 127-155.

Barbraud, C. & Weimerskirch, H. “Emperor Penguins and Climate Change.” Nature 411 (2001): 183-186.

Barnosky, A. D. “The Late Pleistocene Event as a Paradigm for Widespread Mammal Extinction.” In Mass Extinctions: Processes and Evidence. ed. Donovan, S. K., 235-255. New York: Columbia University Press, 1991.

Barnosky, A. D., Koch, P. L., Feranec, R. S., Wing, S. L. & Shabel, A. B. “Assessing the Causes of Late Pleistocene Extinctions on the Continents.” Science 306, no. 5693 (2004): 70-75.

Beerling, D. J. et al. “Evidence for the Recovery of Terrestrial Ecosystems Ahead of Marine Primary Production Following a Biotic Crisis at the Cretaceous-Tertiary Boundary.” Journal of the Geological Society 158 (2001): 737-740.

Bender, E. A., Case, T. J. & Gilpin, M. E. “Perturbation Experiments in Community Ecology: Theory and Practice.” Ecology 65, no. 1 (1984): 1-13.

Burchard, I. “Anthropogenic Impact on the Climate Since Man Began to Hunt.” Palaeogeography, Palaeoclimatology, Palaeoecology 139, no. 1-2 (1998): 1-14.

Erwin, D. H. “Understanding Biotic Recoveries: Extinction, Survival and Preservation During the end-Permian Mass Extinction.” In Evolutionary Paleobiology, ed. Jablonski, D., Erwin, D. H. & Lipps, J., 398-418. Chicago: Chicago University Press, 1996.

Erwin, D. H. “Lessons from the Past: Biotic Recoveries from Mass Extinctions.” Proceedings of the National Academy of Sciences of the United States of America 98 (2001): 5399-4503.

Fagan, B. The Long Summer: How Climate Changed Civilization. New York: Basic Books, 2004.

Flannery, T. The Future Eaters: An Ecological History of the Australasian Lands and People. New York: Grove Press, 2002.

Gore, A. An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. New York: Bloomsbury Books, 2006.

Grant, P. R. Ecology and Evolution of Darwin’s Finches. Princeton, NJ: Princeton University Press, 1999.

Ibisch, P. L., Jennings, M. D. & Kreft, S. “Biodiversity Needs the Help of Global Change Managers, not Museum-keepers.” Nature 438 (2005): 156.

Kareiva, P. & Marvier, M. “Conserving Biodiversity Coldspots.” American Scientist 91 (2003): 344-351.

Leakey, R. & Lewin, R. The Sixth Mass Extinction: Patterns of Life and the Future of Humankind. New York: Anchor Books, 1995.

May, R. M., Lawton, J. H. & Stork, N. E. “Assessing Extinction Rates.” In Extinction Rates, ed. Lawton, J. H. & May, R. M., 1-24. Oxford, England: Oxford University Press, 1995.

Merchant, C. The Death of Nature: Women, Ecology and the Scientific Revolution. San Francisco: Harper, 1990.

Overpeck, J. T. & Cole, J. E. “Abrupt Change in Earth’s Climate System.” Annual Review of Environment and Resources 31 (2006): 1-31.

Parmesan, C. et al. “Poleward Shifts in Geographical Ranges of Butterfly Species Associated with Regional Warming.” Nature 399 (1999): 579-583.

Parmesan, C. & Yohe, G. “A Globally Coherent Fingerprint of Climate Change Impacts Across Natural Systems.” Nature 421 (2003): 37-42.

Pounds, J. A. et al. “Widespread Amphibian Extinctions from Epidemic Disease Driven by Global Climate.” Nature 439, no. 7073 (2006): 161-167.

Raup, D. M. & Sepkoski, J. J. “Mass Extinctions in the Marine Fossil Record.” Science 215, no. 4539 (1982): 1501-1503.

Redman, C. L. Human Impact on Ancient Environments. Tucson, AZ: University of Arizona Press, 1999.

Root, T. L. et al. “Fingerprints of Global Warming on Wild Animals and Plants.” Nature 421 (2003): 57-60.

Ruddiman, W. F. Plows, Plagues and Petroleum: How Humans Took Control of Climate. Princeton, NJ, USA: Princeton University Press, 2005.

Stork, N. E. “Measuring Global Biodiversity and its Decline.” In Biodiversity II: Understanding and Protecting Our Biological Resources, ed. Reaka-Kudla, M. L., Wilson, D. E. & Wilson, E. O., 41-68. Washington D.C.: Joseph Henry Press, 1997.

Taylor, P. D. “Extinction and the Fossil Record.” In Extinctions in the History of Life, ed.Taylor, P. D., 1-34. Cambridge: Cambridge University Press, 2004.

Thomas, C. D. et al. “Extinction Risk from Climate Change.” Nature 427 (2004): 145-148.

Walther, G. R. et al. “Ecological Responses to Recent Climate Change.” Nature 416 (2002): 389-395.

Wang, S. C. “On the Continuity of Background and Mass Extinction.” Paleobiology 29, no. 4 (2003): 455-467.

Wilson, E. O. The Diversity of Life. New York: W.W. Norton & Co., 1992.

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