Extinction

Introduction

Extinction is the complete disappearance of a species of plant or animal. Extinction can be natural; over 99% of all species that have existed in the history of Earth are now extinct. When many species become extinct at nearly the same time, the event is called a mass extinction. Earth is now experiencing a mass extinction caused by human beings, perhaps the first mass extinction ever caused by a single species rather than by natural forces like meteor impacts, volcanoes, or continental drift. After all past mass extinctions, biodiversity (number of species) recovered as new species evolved to fill the ecological niches left vacant by the dead species; however, this took millions of years. On a human time-scale, that is, over hundreds or even tens of thousands of years, extinction causes irreversible loss of biodiversity.

Climate change is only one means by which human beings are causing extinction, and is not yet the dominant one. Destruction of habitat, such as the cutting down of rainforests, has been the means of most human-caused extinctions to date. However, as global average temperatures increase by 3.6–5.4°F (2–3°C) above pre-industrial levels, which may well occur in this century, some 20 to 30% of all known species are likely to be put at higher risk of extinction. In some regions, the extinction-risk rate will be as low as 1%; in others, as high as 80%.

Historical Background and Scientific Foundations

Without extinction, human beings would not exist, as humans likely would not have evolved unless the dinosaurs had died. Even mass extinctions—the sudden or relatively sudden extinction of many species—have happened repeatedly. For example, 251 million years ago, the Permian-Triassic extinction event killed about 96% of all marine (sea-living) species and about 70% of all land-dwelling species. About 65 million years ago, the Cretaceous-Tertiary extinction event killed about half of all species, including the dinosaurs, which created the opportunity for larger mammals and, eventually, humans to evolve.

Today, most biologists accept that humans are in the midst of the first mass extinction since the Cretaceous-Tertiary event. This event is termed the Holocene extinction. (The Holocene is the most recent geographic epoch, stretching from about 10,000 years ago to the present.) This new wave of extinctions has been caused by human activities including hunting, agriculture, pollution, and habitat destruction. A century ago, some scientists advanced the theory that the mass extinction of large mammals in North America (giant ground sloths, giant bears, mammoths, camels, and dozens of other creatures) occurring about 10,500 years ago was caused by hunting done by early Americans. This view, called the overkill hypothesis, is supported by the fact that the extinction of animals on islands often occurs after colonization by humans. However, the hypothesis has been recently challenged by scientists who point to climate change at the end of the Ice Age as a more likely cause of the extinctions.

Ten thousand years ago, all climate changes were caused by natural forces. In the last several decades, however, climate change has been supplemented by anthropogenic (human-caused) greenhouse gases. Although anthropogenic global climate change has only recently become large enough to distinguish definitely from natural changes, it is happening rapidly. In the next century, it will probably become a major cause of extinctions. The United Nations' Intergovernmental Panel on Climate Change (IPCC) stated in 2007 that about 20– 30% of all plant and animal species are likely to be at increasingly high extinction risk as the average global temperature warms to 3.6–5.4°F (2–3°C) above the level it held before the Industrial Revolution, when human beings first began to burn large amounts of fossil fuels. This much warming by 2100 is likely even for conservative scenarios for global warming, in the absence of effective actions to reduce greenhouse-gas emissions and other causes of global climate change; warming by up to 8.1°F (4.5°C) by 2100 is possible.

A 2004 study by Chris D. Thomas and colleagues calculated that, out of a sample of 1,103 animal and plant species, 15–37% would be committed to extinction by 2050 as a result of climate change—that is, that the number of living individuals of each species would be so small that the species' extinction some time after 2050 would become highly likely. Thomas and his colleagues estimated that in many if not most ecological regions, climate change would become the greatest threat to biodiversity by 2050.

One tool that scientists use to estimate extinction risk from climate change is the climate envelope. A species' climate envelope is the range of climate conditions under which populations of that species tend to survive in spite of natural enemies and competitors. As climate changes, the areas where a species' climate envelope exists will shift or, in some cases, shrink. For example, as climate warms, the highest altitude at which trees can grow on mountains, the treeline, will ascend. As there is a limited amount of land on a mountain range, as the treeline ascends, the area above the treeline will shrink. Species that depend on the mountain-tundra environment above the treeline will therefore, have less room in which to live; the area where their climate envelope exists will decrease.

A well-known relationship in biology exists between area and the number of species of plants and animals that can live in that area: the smaller the area, the fewer the species. This is often expressed as a mathematical equation called the species-area relationship. Using S to stand for the number of species, A for area, and c and z for some fixed numbers or constants that depend on the ecological setting, the species-area relationship is S = cA^z. For example, if c =1and z =2, then S = A². Halving area, A, willin this case reduce the number of species, S, toone-fourthits previous value. This type of rule is usually a good predictor of how many species will go extinct when habitat is lost.

The rule does not work in reverse—that is, increasing area does not bring more biodiversity (more species) into being. Extinction can happen overnight, but the evolution of new species is usually, on the timescale of habitat destruction, too slow to detect.

By sampling species habitats around the world, using projections of likely climate change to estimate how much various habitats will shrink or grow, and using the species-area relationship to predict about how many species will become extinct when habitats shrink, scientists can make an educated guess about how many extinctions are likely to be caused by a given amount of global warming.

Impacts and Issues

It is usually estimated that there are 5 to 15 million species of creatures on Earth. If 20% of these are committed to extinction by climate change, then from 1 million to 3 million species will eventually become extinct as a result of global climate change. (Most of these would be species of insects.)

WORDS TO KNOW

BIODIVERSITY: Literally, “life diversity”: the number of different kinds of living things. The wide range of organisms—plants and animals—that exist within any given geographical region.

CONTINENTAL DRIFT: A theory that explains the relative positions and shapes of the continents, and other geologic phenomena, by lateral movement of the continents. This was the precursor to plate tectonic theory.

EPOCH: Unit of geological time. From longest to shortest, the geological system of time units is eon, era, period, epoch, and stage. Epochs are generally about 500 million years in length.

HOLOCENE EXTINCTION EVENT: The Holocene is the geological period from 10,000 years ago to the present; the Holocene extinction is the worldwide mass extinction of animal and plant species being caused by human activity. Global warming may accelerate the ongoing Holocene extinction event, possibly driving a fourth of all terrestrial plant and animal species to extinction.

ICE AGE: Period of glacial advance.

INDUSTRIAL REVOLUTION: The period, beginning about the middle of the eighteenth century, during which humans began to use steam engines as a major source of power.

SPECIES-AREA RELATIONSHIP: In biology, the relationship between the size of an area and the number of species of plants and animals that can live in that area: the smaller the area, the fewer the species. Expressed algebraically as S = cA^z, where S stands for number of species, A for area, and c and z for fixed numbers (constants) that depend on the ecological setting.

However, there are many uncertainties in guessing how many extinctions will occur because of anthropogenic climate change. First, there is no certainty about how much climate change is going to occur. This depends on natural feedbacks, tipping points, and human choices. Second, the range of uncertainty around climate-change extinction estimates, both in timing and quantity, is large. The species-area relationship is only an approximation (other methods are also used), and it is not known how many ecosystems will respond to climate change.

The 2004 study by Thomas and colleagues mentioned earlier was widely misreported in the media as saying that a million species would be extinct within 50 years. This was not, however, what the scientists said: they emphasized the uncertainties in their predictions and said that many species would be “committed to extinction,” not extinct, by 2050. They also said that reducing greenhouse-gas emissions would change their projections, and that reducing global temperatures would eventually bring back some species from the edge of extinction. The study's basic message—that anthropogenic global climate change may very well threaten thousands or millions of species with extinction within the next century or more—was viewed as sufficiently alarming, even without exaggeration.

Climate-related extinctions have already begun. A 2006 study found that global warming almost certainly played a key role in the recent extinction of about 67% of the 110 or so species of the Monteverde harlequin tree frog of the mountains of Costa Rica, making the frogs more vulnerable to a deadly fungal infection.

Primary Source Connection

Scientist are in the midst of an argument over the cause of the mass extinction of large mammals in North America over 10,000 years ago. Since the 1960s the overkill theory has been the prevailing theory on this mass extinction. The overkill theory holds that the arrival of humans in North America about 11,000 years ago led to the extinction of these mammals through hunting. A more recent theory, though, suggests that climate change was responsible for the mass extinction. This article from National Geographic News presents views from proponents of both theories.

CLIMATE CHANGE CAUSED EXTINCTION OF BIG ICE AGE MAMMALS, SCIENTIST SAYS

A renewed assault is being made on the popular idea that the mass extinction of large mammals in North America around 10,500 years ago was the result of human hunting.

The overkill hypothesis was first put forward more than a century ago and has been widely accepted for the past 30 years. But it does not square with the known facts and has become more a faith-based credo than good science, said Donald Grayson, an archaeologist at the University of Washington.

Understanding what caused the extinction has implications for conservation biology.

Grayson, who specializes in vertebrate paleontology and archaeology, argues that a call by some environmental-ists to return Ice Age mammals—elephants, camels, llamas, and other large herbivores—to the southwestern United States is based on bad science.

“Overkill proponents have argued that these animals would still be around if people hadn't killed them and that ecological niches still exist for them,” said Grayson. “Those niches do not exist. Otherwise the herbivores would still be there.”

Grayson points to climate shifts during the late Pleistocene and related changes in weather and vegetation patterns as the likely culprits in the demise of North America's megafauna.

Islands and Continents

The number of large mammals that became extinct in North America at the end of the late Pleistocene, about 10,500 years ago, is staggering. Among some 35 different kinds of animals that disappeared from the fossil record were mammoths, mastodons, camels, horses, giant ground sloths, and bears.

A leading proponent of the overkill theory, Paul S. Martin, believes the Ice Age megafauna disappeared not because they lost their food supply but because of human hunting.

The extinction of animals as a result of human colonization in island settings has been well documented and the causes widely agreed on. New Zealand is offered as a classic example of human impacts on island animal populations. Before it was colonized, New Zealand was home to 11 species of moas, a large flightless bird species weighing from 45 to more than 400 pounds (20 to 200 kilograms). Within a few hundred years after human settlement, moas were extinct.

IN CONTEXT: IMPACTS ON ANIMAL POPULATIONS

According to the National Academy of Sciences: “Even within a single regional ecosystem, there will be winners and losers. For example, the population of Adélie penguins has decreased 22% during the last 25 years, while the Chinstrap penguin population increased by 400%. The two species depend on different habitats for survival: Adélies inhabit the winter ice pack, whereas Chinstraps remain in close association with open water. A 7-9°F rise in midwinter temperatures on the western Antarctic Peninsula during the past 50 years and associated receding sea-ice pack is reflected in their changing populations.”

SOURCE: Staudt, Amanda, Nancy Huddleston, and Sandi Rudenstein. Understanding and Responding to Climate Change. National Academy of Sciences, 2006.

The wave of extinctions that followed human colonization of New Zealand occurred as a result of several factors. Humans arriving on the islands brought with them rats, dogs, and other non-indigenous animals that competed for food or preyed on native species. Humans

hunted the native animals extensively and destroyed much habitat by burning down forests.

A similar pattern of events occurred on other islands around the world. Proponents of the overkill theory argue that the same process—human colonization followed by massive extinction—also occurred in North America.

But Grayson rejects that idea. “The fossil record simply doesn't stand up to the theory, and comparing continents to islands is simply inappropriate,” he said.

Arguing Against Overkill

The overkill hypothesis, Grayson says, rests on five tenets: human colonization can lead to the extinction of island species; the Clovis people were the first humans to arrive in North America, around 11,000 years ago; the Clovis people hunted a wide range of large mammals; the extinction of many species of North American mega-fauna occurred 11,000 years ago; and therefore, Clovis hunting caused those extinctions.

Grayson disputes several of these tenets.

There is no proof, he said, that the late Pleistocene extinctions occurred in conjunction with the arrival of the Clovis people. “Of the 35 genera to have become extinct beginning around 20,000 years ago, only 15 can be shown to have survived beyond 12,000 years ago,” Grayson said. “The Clovis peoples didn't arrive until shortly before 11,000 years ago. That leaves 20 ‘genera’ unaccounted for.”

There is also no evidence that the Clovis people hunted anything other than mammoths, he said. Although numerous sites where large numbers of mammoths were killed have been uncovered, no similar sites for any other large mammals have been found in North America.

And while there is no evidence of widespread human-caused environmental change similar to that seen on island settings, there is evidence that animal populations in Siberia and Western Europe, as well as North America, were affected during the same period by climate changes and glacial retreat.

Martin, who in 1967 wrote the seminal book proposing the overkill hypothesis, disagrees that climate change could have caused the extensive extinctions.

“The climate had been changing over a million-year period, with swings from cold to warm, and back again—some much more severe than the one that occurred at the end of the Pleistocene,” he said. “It doesn't make sense that just one more [climate shift swing] would make all the difference in the world.”

Martin holds that the “dreadful syncopation”—humans arrive, animals disappear—seen in the islands of Oceania, Australia, New Zealand, Madagascar, and other islands fits with what happened in North America.

He suggests that because humans were responsible for the demise of large animals in the desert southwest, these animals should be reintroduced.

Grayson thinks this is dangerous thinking.

“One of the reasons people have glommed on to the overkill hypothesis is ‘green’ politics,” said Grayson. “It plays to the Judeo-Christian theme that human beings are all-powerful and responsible for negative impacts on the environment.”

“The hypothesis made a lot of sense 30 years ago,” Grayson said, “but now it can be compared to the empirical record, and it just doesn't hold up.”

His work appears in the current issue of the Journal of World Prehistory and an upcoming issue of the Bulletin of the Florida Museum of Natural History.

mayell, hillary. “climate change caused extinction of big ice age mammals, scientist says.” national geographic news, november 12, 2001.

See Also Biodiversity; Polar Bears.

BIBLIOGRAPHY

Books

Parry, M. L., et al, eds. Climate Change 2007: Impacts, Adaptation and Vulnerability: Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Periodicals

Eilperin, Juliet. “188 More Species Listed as Near Extinction.” The Washington Post (September 13, 2007).

Higgins, Paul A. T. “Biodiversity Loss Under Existing Land Use and Climate Change: An Illustration Using Northern South America.” Global Ecology and Biogeography 16 (March 2007): 197–204.

Pounds, J. Alan, et al. “Widespread Amphibian Extinctions from Epidemic Disease Driven by Global Warming.” Nature 439 (January 12, 2006): 161–167.

Thomas, Chris D. “Extinction Risk from Climate Change.” Nature 427 (January 8, 2004): 145–148.

Web Sites

Ladle, Richard J., et al. “Crying Wolf on Climate Change and Extinction.” Oxford University, School of Geography, 2004. < http://www.geog.ox.ac.uk/research/biodiversity/crywolf.pdf> (accessed October 10, 2007).

Mayell, Hillary. “Climate Change Caused Extinction of Big Ice Age Mammals, Scientist Says.” National Geographic News, November 12, 2001. < http://news.nationalgeographic.com/news/2001/11/1112_overkill.html> (accessed October 10, 2007).

Larry Gilman