Extinction

Humans are the only species aware of not only of their own eventual personal deaths but of their collective demise as well. This latter insight came late in human history—a product of evolutionary theory and of paleontological and archaeological research.

The West still deals with the twin "cognitive shocks" from the mid-nineteenth-century discoveries that extinction was the fate both of creatures very much like human beings and of cultures as articulate as those in the West, whose people can still speak directly to Western civilization because of their writing systems. On top of these are the twentieth century's scientific end-of-world scenarios, predicting the inevitable extinction of not only all life forms on Earth when the sun goes supernova, but of the entire universe, when entropy is complete and the last nuclear fires flicker out.

Since WWII—particularly following the publications of Rachel Carson's Silent Spring (1962) and Paul Ehrlich's Population Bomb (1968)—sensitivities toward extinction have heightened with cold war fears of nuclear war, the obvious diminishment and degradation of the natural order owing to industrialization and burgeoning human numbers, and with millennial apocalyptic fears. With increasing regularity the news brings scientific findings of global warming, of a rapidly growing list of endangered and newly extinct life forms, of "earth-killer" asteroids lurking nearby, and of lethal pollutants in the air, soil, and water. The polar ice caps are melting as are the famed snows of Kilimanjaro. The blubber of orcas from the American Pacific Northwest found to contain PCB (polychlorinated biphenyls) concentrations up to 500 times greater than those found in humans. In the extreme, such phenomena are interpreted as symptoms of the beginning of the end of not only the planet's entire ecosystem but human beings as well.

Extinction as the consequence of natural selection and the fate of the "unfit" was to become the metaphor for understanding not only natural but social phenomena as well, whether it be the success or failures of businesses or of entire cultural orders. It also found expressions in the justifications for war, exploitation, and forced sterilizations and genocides of human populations.

Scientific Perspectives

The law of life from scientific perspectives holds that all life forms, ecosystems, planets, stars, and galaxies come and go. Personal death and collective extinction are the debts for life—on the individual, species, genus, family, order, class, and phyla levels. Since life first appeared on Earth 3.5 billion years ago, over 10 billion species have come into existence (and this figure is the roughest of estimates). Somewhere between 10 million and 30 million species (of which scientists have counted only about one-eighth) currently reside on the planet. In other words, for every 1,000 species that have ever existed probably fewer than 10 are alive in the twenty-first century.

Challenging the conventional idea that "species were immutable productions," the naturalist Charles Darwin wrote On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, explaining how life is a continuous process of modification and current life forms are descendants of extinct ancestral species. Natural selection is "the process by which genes change their sequences" (Ridley 2000, p. 35), by which those organisms best adapted to their environment survive to pass on their genetic code while those (previously "successful") life forms unable to make adaptive modifications to varying conditions perish. Species' billing on the stage of life is relatively short in terms of geologic time, with most becoming extinct within a few million years after their evolution. Most leave no related descendant species but rather become evolutionary dead ends.

Life is a continuous process of extinction and diversification, where only the fittest life forms survive in a world where, according to Darwin, they are "bound together by a web of complex relations" (1963, p. 54). Central to these "relations" are the ecological niches, the millions of different "fits" and functional interdependencies that plants and animals have with each other in their ecosystem. While various creatures may share a habitat, generally a niche can only be occupied by one species of animal or plant. When two different organisms compete for a particular niche, one will invariably lose out. If a niche should become empty, such as through extinction, other life forms will rush in and compete to fill the vacuum. In the case of a mass die-off, evolutionary explosions of new life forms occur as plants and animals adapt to take advantage of the abundance of newly opened niches.

Another factor affecting the rate of extinction and differentiation of life forms is the extent of biodiversity in an ecosystem. These natural systems comprising the biosphere do such things as cycle oxygen and carbon, purify the water, and decompose waste. Diminish the genetic variation in individual species and they become more vulnerable to extinction. Homogenize the biotas in human-dominated ecosystems and biodisparity declines.

The diversity of life used to be considerably richer than is the case in the twenty-first century. For instance, based on the fossil record of the Cambrian Period over 500 million years ago, there may have been four to five times the number of phyla as exist today. Given present trends, this multiplier may be even greater in the near future as the site of the planet's greatest biodiversity, the tropical forests where between 50 and 90 percent of species reside, are rapidly disappearing. Worldwide, tropical forests four times the size of Switzerland are cut down annually, replaced with fields for cash crops and to make room for burgeoning human populations—which, in turn, pollute the water and land, further diminishing the variety and size of natural habitats.

Though humans may be one of the only species to have adapted to nearly all environments on Earth, their existence nevertheless remains dependent upon the complex systems of inter-dependent life forms.

Periodic Mass Extinctions

The rate of extinction is far from constant. In addition to the routine processes of Darwin's natural selection, various terrestrial and extraterrestrial catastrophes have profoundly altered the course of evolution. Paleontologists report historic cycles of mass death, dooming or marginalizing a portion of previously well-adapted species, followed by steep increases in the diversity of new life forms.

At least five periods of mass extinction have been identified, eliminating over half of the world's species at the time of their occurrence (although, in total, causing less than 5% of all extinctions). Scientific debate centers not on whether these mass die-offs have occurred but rather if they happened quickly, such as due to asteroid impact or a nearby star going supernova, or more slowly, such as due to glaciations, volcanic activity, changes in seawater oxygen or salinity, or epidemics. The time required for the species diversification to rebound to pre-catastrophe levels is estimated to be roughly 25 million years.

Asteroids are suspected in at least the three most recent mass die-offs. Most publicized is the suspected 10-kilometer-wide meteorite that slammed into what's now the Gulf of Mexico and the Yucatán Peninsula 65 million years ago, leaving the 180-kilometer-wide Chicxulub Crater, wiping out the dinosaurs, and ending the Cretaceous period. Another is believed to have concluded the Permian period with even more devastating results, when 250 million years ago some 96 percent of marine and 70 percent of land fauna disappeared. Ironically, shortly after Jurassic Park 's release, the world witnessed over twenty parts of Comet Shoemaker-Levy collide with Jupiter. At least nine of these could have been Earth-killers, one forming a crater the planet could easily fit within. As of 2000, according to the British National Space Center, 284 asteroids have been located with the size and orbits to be deemed "potentially hazardous" to Earth.

There is evidence that mass extinctions may number in the dozens, with up to three-quarters of all species disappearing every 26 to 30 million years. One explanation for this cyclical regularity is the possible existence of a companion star to the sun. Called Nemesis, when this star reaches its perigee every 26 million years, it shakes loose and hurls a comet storm from the Oort cloud on the fringe of Earth's solar system. Every 50,000 years or so, for over a period of 2 million years, one of these comets collides with Earth, producing ecological catastrophes that lead to periodic global deaths.

Among biologists there is broad consensus that human beings are witnessing the sixth mass extinction, with anywhere from 10,000 to 20,000 species of animals and plants disappearing annually—a rate conservationists estimate to be 1,000 to 10,000 times greater than would be the case under natural conditions. A 1998 American Museum of Natural History survey of 400 experts in the biological sciences found approximately 70 percent expecting up to one in five of all living species on the planet disappearing within thirty years. What makes this different from the five other known periods of mass extinction—which were linked with asteroids, ice ages, and volcanoes—is the human complicity involved. We are death and are responsible for this "species holocaust" (Day 1981). In a sense, all creatures have become our miner's canaries and their disappearance means that the

health of the ecosystem is endangered and that we are running out of biological room

Human complicity in the Pleistocene extinction of large game animals remains a subject of debate, as there is a coincidence of receding glaciers, human migrations, and the extinction of many large game animals. The moa, a 400-pound flightless bird that thrived in New Zealand until about the year 1250, totally disappeared within 60 to 120 years of the first human arrival. Using fire as a weapon and tool, these settlers also burned into extinction an entire forest that was to become grassland. Similarly, woolly mammoths, camels, horses, sabertoothed tigers, and more than 120 other Pleistocene mammalian species all disappeared within a few hundred years after humans arrived in the New World roughly 11,000 to 13,000 years ago.

Perhaps the most poignant of recent extinctions is the passenger pigeon, which was to be hunted into oblivion. At the beginning of the American Civil War, this was one of the most successful bird species in North America, comprising an estimated 40 percent of the entire bird population. In 1870, a single flock one mile wide and 320 miles long flew over Cincinnati. In 1974 the last surviving pigeon died in that city's zoo. As of 2001, conservationists estimated that one in every six species of birds is in decline on the continent and could wane by half by the year 2030.

But the major cause of animal demise is the destruction of natural habitats as human populations exploded, tripling just between 1930 to 2000, leading to urban sprawl, overfishing, overgrazing, deforestation, overuse of pesticides and herbicides, strip mining, and pollution of fresh water systems. Human introduction of non-native "invasive" species has also taken its toll. Fungus carried by North American ships, for instance, led to the Irish potato famine. Of those who fled migrated to Australia with rabbits and cats, many of which went wild and decimated indigenous plants and animals.

In Africa, the deserts annually expand thousands of square miles as a rapidly growing human population strips its indigenous vegetation for fuel (wood remains the chief energy source for many of its inhabitants) and to make room for profitable crops or livestock. Here the removal of ground cover leads to a greater runoff of rain, reducing evaporation into the clouds and thereby contributing to the severity of the region's routine droughts.

The loss is not confined to wild, undomesticated plants and animals but includes domesticated ones as well. During the twentieth century, because of the rise of cash crops and farmers' cultivation for maximum yield, three-quarters of the genetic diversity of the world's agricultural crops was lost. Roughly half of all domestic animal breeds in Europe became extinct. Gone are about 6,000 apple varieties that grew on U.S. farms 100 years ago. In the mid-1990s, the United Nations Food and Agriculture Organization reported that nearly onequarter of the 3,882 breeds of 28 species of farm animals around the world were "at risk"—meaning fewer than 1,000 females or 20 breeding males.

What worries experts most is the prospect that farmers will have a shrinking pool of breeds to draw upon to keep up with changing soil conditions, pests, and new diseases.

In other words, concerns are for preserving biodiversity, particularly in light of cash crops and humanity putting its proverbial eggs in fewer and fewer baskets.

Two groups of extinctions have intrigued humans over the past two centuries: the demise of the dinosaurs and of human ancestors. Over the final two decades of the twentieth century there was considerable research and public interest in the instantaneous (in geological time) demise 65 million years ago of dinosaurs, among the most successful (in terms of their length of reign on Earth) animals known. This interest coincided with the cold war, which was not coincidental. Humanity's ability to equally quickly make itself extinct through thermonuclear devices became a widely accepted fact. Another factor heightening cultural receptivity was the rapid disappearance of its own social past—for example, old crafts, buildings, social etiquette—producing a sense of irretrievable loss. Modernity had come to mean impermanence in a culture of obsolescence where the only certainty had become change.

Unearthed in a quarry by Germany's river Neander in 1856 as an unusual humanlike skeleton with a beetled browed skull. This discovery of the Neanderthals, a tool-making creature very similar to Homo sapien, ultimately led to the unsettling insight that extinction has also been the fate of all branches of the hominid family and of the Homo genus except for human sapiens. Modern humans' complicity in the demise of these archaic humans remains a matter of speculation, with evidence indicating that humans shared the planet with at least the Neanderthals as recently as 26,000 years ago in Europe.

Social Reactions to Extinctions of the Natural Order

Only in modern times has there been the realization that the planet's living resources are neither infinite nor always replenishable—and the scientific understanding of how precarious the interdependencies between all life forms are. People were surprised in the 1860s when human artifacts were first found with extinct species in Europe. Severely challenged were biblical allusions to some "steady-state" or regenerative natural order, the fauna descendents of passengers on Noah's ark, all of which were to be dominated by humans. Only later did people come to learn how the overkilling of "keystone species" could lead to an environmental collapse and the extinction of even nonhunted creatures.

For those living within largely manmade environments and within a highly individualistic culture, appreciation of life's thorough interdependencies comes hard. Further, Cartesian conceptions of human's biological essence—the body-as-machine metaphor that has long predominated in medicine—complicates more holistic understandings.

Limited is any acknowledgment of human complicity in the sixth mass extinction. According to 1993 and 1994 surveys of the American adult public conducted by the National Opinion Research Center, only 16 percent of American adults believe it is "definitely true" that "human beings are the main cause of plant and animal species dying out." Four in ten agree with the statement, "We worry too much about the future of the environment and not enough about prices and jobs today." And only one in three was "very" or "fairly" willing to accept cuts in his or her standard of living in order to protect the environment.

The perspective from the scientific community is less benign. Several scientists have likened human effects on the ecosystem to cancer. If people take the Gaia perspective of Earth functioning like one mega living organism, aerial views can show striking similarity to melanomas attacking healthy tissue.

The Economics of Extinction

The rise of the world economic order and its capitalist structure has accelerated the decimation of species. If, as Martin Luther King Jr. observed in 1967, capitalism has a tendency to treat profit motives and property rights as more important than people, one can easily surmise the importance it gives to the natural order. The system produces a bias toward short-term gains at the cost of long-term consequences. As mentioned, cash crops in the international marketplace have accelerated the end of many species. The price tag of the resultant reduction of biodiversity rarely enters into the market system's cost-benefit equation. This absence is due, in part, to the fact that people really don't know how to calculate the worth of such endangered species as snail darters or white spotted owls.

In 1976 Daniel Bell wrote of the cultural contradictions of capitalism, perceiving them as seeds for change—or the death knell for entire social orders. One contradiction of relevance here is the economics of extinction, how the growing scarcity of a life form can increase its value and thereby accelerate its disappearance. For instance, in the 1980s when the market value of rhino horns exceeded the per ounce value of silver by ten times, this reduced the number of black rhinos in the Tanzanian Ngorongoro Crater from seventy-six to twenty-six in a single year. Pathetically, as their numbers declined the value of their parts hyperinflated, encouraging further exploitation. Another contradiction is how farmers will have a shrinking pool of breeds to draw upon to keep up with changing soil conditions, pests, and new diseases.

With the remnants of endangered species increasingly likely to only be preserved within zoos, research institutes, and parks, matters of cost and utility enter. Consider, for instance, the perspective of zoos. To garner public support, animal collections must attract audiences. The most popular animals are often those that are "cute," furry, and large-eyed. Consequently, with time, Darwin's thesis of the survival of the fittest may have to be modified to be the survival of the cutest, like the black and white colobus or the arctic seal. Also saved will be those creatures that enhance humankind's own longevity, such as apes or pigs for xenotransplantations. Concerns over the loss of the tropical rain-forests are often expressed in terms of human needs. One-quarter of prescription drugs are extracted from plants, including 70 percent of anti-cancer agents, and most of these come from tropical forests—where less than 1 percent of the flora has been examined for its pharmacological utility.

The Politics of Extinction

At the beginning of the twenty-first century, decisions as to whether the needs of humans ultimately outweigh those of endangered species are largely the monopoly of political regimes. However, the planet's greatest biodiversity largely exists within developing nations where fertility rates are the highest, people the poorest, population pressures on nature the greatest, and unstable political regimes the least likely to resist powerful corporate and population encroachments on natural habitats.

Even in developed nations the accelerating rates of extinction have proven difficult to resist. The Green movement remains a minority voice. In the United States, awarenesses have become politicized. National surveys of 1993 and 1994 show Republicans are, for instance, 50 percent more likely than Democrats to disagree that "human beings are the main cause of plant and animal species dying out."

And just when environmental causes have entered the public consciousness, ecoterrorism (or threats thereof) has become a new weapon of disenfranchised and antiestablishment groups. During the summer of 2000, soon-to-be-laid-off chemical workers in Northern France dumped 3,000 liters of sulfuric acid into a tributary of the Meuse River.

Religion and Extinction

The notion that humans are programmed by only 40,000 or so genes and descended from a chain of earlier life forms profoundly challenged traditional religious beliefs that life and death are acts of divine will. From the scientific method came the possibility that the cosmos was not divinely constructed around humanity but rather is the product of random chance, and that the same natural forces controlling the fates (and inevitable extinctions) of animals shape human fate as well. Also assaulting people's sense of specialness is the evidence that human beings and their fellow thinking mammals are not the "fittest," as 95 percent of all animal species are invertebrates.

According to the Bible, God made human beings in his image and gave them dominion over all other creatures, whose existence was to serve their needs. To this day religion maintains this subordination of the natural order. When a random sample of Americans were asked whether "animals should have the same moral rights that human beings do," their agreement decreased with increasingly religiosity. When asked if "it is right to use animals for medical testing if it might save human lives," agreement increased with increasing religiosity.

Accused of causing "speciesism," Christianity has played a limited role in the war against extinction. Instead, the scientifically prompted ecological movement has come to assume some quasi-religious characteristics of its own.

The Battle against Extinction As Part of the War against Death

So why the extensive attempts to save the California condor, whose diet at its peak 1 million years ago consisted of such currently extinct creatures as dead mastodons, or the Kemp's Ridley turtle that, with a brain cavity the size of a pencil-width hole, is unable to learn a thing from experience and whose existence is totally hardwired? The reason may be largely symbolic.

The cultural war against death has expanded beyond the medical front to include battles against extinction in the natural order—and even resurrection of the extinct, as when in 2000 scientists awakened dormant 250-million-year-old bacteria encased in a salt crystal. Through human intervention, creatures extinct in the wild, like the Arabian oryx, are preserved in species preservation programs in zoos, ideally to be reintroduced to their native habitats. (In fact, there are cases where animals presumed extinct have been "discovered" in zoos, such as the Cape Lion, which was rediscovered in zoos in Novosibirsk, Serbia, and in Addis Ababa, Ethiopia.) In addition, genetic repositories, the contemporary "Noah's Arks," have been established by the Museum of Natural Science at Louisiana State University and the American Museum of Natural History to preserve in liquid nitrogen tissue samples from thousands of different creatures. In 2001, tissue frozen for eight years was used to successfully clone a gaur (a large type of cattle).

Perhaps in reaction to the extinction of the passenger pigeon, considerable press has been given to attempts to rescue the whooping crane from oblivion. This cause célèbre is a white crane with a seven-foot wingspan, mistakenly believed to be extinct in 1923. The victim of overhunting, its numbers had dwindled to about fifteen in 1941—this, despite the Migratory Bird Treaty Act of 1918, which brought market hunting of the cranes and other migratory birds to an end, and the 1937 creation of the Arkansas National Wildlife Refuge on the Texas coast to protect the crane's last wintering ground. The U.S. Fish and Wildlife Service began a whooping crane recovery program in 1967, relying initially on captive breeding to build up the population. As of 2002, there are roughly 300 of the birds.

The Environmental Movement

In the broader cultural war against death, battles against extinction have assumed numerous fronts, such as conservation biology, gene banks, attempts to clone endangered species, and habitat restoration. Numerous new social solidarities have arisen, crosscutting the stratifications of class, gender, and age, in such groups as Friends of the Sea Otter, Sea Turtle Restoration Project, and Trees for the Future.

Growing concern over "ecocide" and "biological meltdown" has given rise to increasingly politicized environmental groups (Greenpeace, the Sierra Club, the National Wildlife Federation, Friends of the Earth, and various animal rights organizations). This social movement has quasi-religious trappings in its demands for ecojustice and concerns for leaving a legacy for future generations.

On the extreme fringe of this movement are the so-called ecoterrorists, such as the Earth Liberation Front and the Animal Liberation Front. Radical environmentalists, such as Finland's Pentti Linkola, see mass human death as the only way to save the planet's fragile ecosystem. This Voluntary Human Extinction Movement has the slogan "May We Live Long and Die Out." In addition to his hope for war, Linkola recommends ending capitalism, abolishing aid to the third world, and requiring mandatory abortions.

Cultural Extinction

Analogous to people's interest in extinct creatures is their curiosity about extinct civilizations, such as the mythical Atlantis or the Anasazis and Mayans of the New World, and the causes of these cultures' doom. As paleontologists study life forms of the past so archaeologists study past cultural orders, investigating whether their demise was due to avoidable factors with lessons that can be applied to the present. The archaeologist Richard Hansen, for instance, argues that deforestation produced an ecological disaster precipitating the collapse of the Maya civilization in approximately 800 C.E.To produce the stucco for their huge limestone pyramids, the Mayans leveled forests to fuel the hot fires required for transforming limestone into lime. Or was the fate of these doomed cultures sealed by unforeseen and uncontrollable calamities, such as epidemics, earthquake, or drought?

The human war against death extends from battles against the extinction of biological to cultural systems. Memes are to anthropologists what genes are to biologists—carriers of life's organizing principles, ways of doing this, that are passed down generation to generation. And as biologists worry about diminishing biodiversity, cultural scientists worry about the abatement of cultural diversity and endangered languages. Of the 6,800 languages spoken worldwide in 2001—cultural DNA, if you will—only 600 are robust enough to be in existence at the end of the twenty-first century. The last speaker of Ubykh, for instance, died in Turkey in 1992 at the age of eighty-eight.

As the fossilized remains of extinct creatures are the rage of affluent collectors so too are artifacts of extinct cultures. So lucrative is the market and so environmentally damaging are the attempts to satisfy demands that laws have had to be passed to ensure that remnants of the extinct themselves do not disappear.

See also: Apocalypse; Darwin, Charles; Disasters; Nuclear Destruction

Bibliography

American Museum of Natural History. "National Survey Reveals Biodiversity Crisis-Scientific Experts Believe We Are in Midst of Fastest Mass Extinction in Earth's History." Press release, April 20, 1998.

Bell, Daniel. The Cultural Contradictions of Capitalism. New York: Basic Books, 1976.

Browne, Malcolm W. "New Clues to Agent of Life's Worst Extinction." New York Times, 15 December 1992, C1.

Carson, Rachel. Silent Spring Boston: Houghton Mifflin, 1962.

Chang, Kenneth. "Creating a Modern Ark of Genetic Samples." New York Times, 8 May 2001.

Darwin, Charles. On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. 1869. Reprint, New York: The Heritage Press 1963.

Davis, James A., and Tom A. Smith. General Social Surveys, 1972–1998. Investigator, James A. Davis; directed by Tom W. Smith; produced by National Opinion Research Center. The Roper Center for Public Opinion Research, University of Connecticut, 1998. [machine-readable data file].

Day, David. The Doomsday Book of Animals. New York: Viking Press, 1981.

Ehrlich, Paul. The Population Bomb. New York: Ballantine Books, 1968.

Erwin, Douglas H. "Lessons from the Past: Biotic Recoveries from Mass Extinctions." Proceedings of the National Academy of Sciences 98 (2001):5399–5403.

Food and Agriculture Organization. "Up to 1,500 Animal Breeds Are at Risk of Extinction, FAO Warns." United Nations Press Release 3628, 7 December 1995.

Gould, Stephen J. Wonderful Life: The Burgess Shale and the Nature of History. New York: W. W. Norton, 1989.

Hallam, Anthony, and P. B. Wignall. Mass Extinctions and Their Aftermath. New York: Oxford University Press, 1997.

Hern, Warren. "Is Human Culture Carcinogenic for Uncontrolled Population Growth and Ecological Destruction?" BioScience 43, no. 11 (1993):768–773.

Holdaway, Richard, and C. Jacomb. "Rapid Extinction of the Moas (Aves: Dinornithiformes): Model, Test, and Implications." Science 287, no. 5461 (2000):2250–2254.

Leakey, Richard, and Roger Lewin. The Sixth Extinction. New York: Doubleday, 1995.

Lewis, Paul. "Too Late to Say 'Extinct' in Ubykh, Eyak or Ona." New York Times, 15 August 1998, B7.

Lowe, Sarah, Michael Browne, and Souyad Boudjelas. 100 of the World's Worst Invasive Alien Species. Auckland, New Zealand: Invasive Species Specialist Group, 2001.

Miller, George T. Living in the Environment: Principles, Connections and Solutions, 8th edition. Belmont, CA: Wadsworth, 1994.

Montalbano, William D. "Agriculture's Gene Pool Shrinks Drastically Under Human Selection." The Wichita Eagle, 27 December 1993, 1A, 4A.

Muller, Richard, Marc Davis, and Piet Hut. "Cometary Showers and Unseen Solar Companions." Nature 312, no. 5992 (1984):380–381.

Osborne, Lawrence. "The Fossil Frenzy." New York Times Magazine, 29 October 2000, V1:70.

Perlin, John. A Forest Journey: The Role of Wood in the Development of Civilization. Cambridge, MA: Harvard University Press, 1991.

Raup, David M., and John J. Sepkosi Jr. "Periodic Extinctions of Families and Genera." Science 231 (1986):833–836.

Sampat, Payal. "Last Words: The Dying of Languages." World-Watch 14, no. 3 (2001):34–40.

Sepkoski, John J., Jr., and David Raup. "Mass Extinctions in the Fossil Record." Science 219, no. 4589 (1983):1240–1241.

Ridley, Matt. Genome: The Autobiography of a Species in 23 Chapters. New York: HarperCollins, 2000.

Singer, Peter. Animal Liberation. New York: Avon, 1991.

Smith, Jonathan Z. "A Slip in Time Saves Nine: Prestigious Origins Again." In John Bender and David E. Wellbery eds., Chronotypes: The Construction of Time. Stanford, CA: Stanford University Press, 1991.

United Nations Food and Agriculture Organization and the U.N. Environmental Program. World Watch List for Domestic Animal Diversity, edited by Beate D. Scherf. 3rd edition. Rome: FAO, Viale delle Terme di Caracalla, 2000.

Internet Resources

British National Space Center. "Report of the UK Task Force on Near Earth Objects." In the Near Earth Objects [web site]. Available from www.nearearthobjects.co.uk/neo_report.cfm

MacDougall, A. Kent. "Humans as Cancer." In the Church of Euthanasia [web site]. Available from www.churchofeuthanasia.org/e-sermons/humcan.html.

Pickover, Clifford A. "Immortalization of Humanity." In the Edge: The Third Culture [web site]. Available from www.edge.org/3rd_culture/story/27.html.

Siegel, Lee. "The Five Worst Extinctions in Earth's History" In the Space.com [web site]. Available from www.space.com/scienceastronomy/planetearth/extinction_sidebar_000907.html.

MICHAEL C. KEARL

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

Extinction

The asteroid-impact theory

The Great Ice Age

The current mass extinction

Resources

Extinction is the death of all members of a species and so of the species itself. Extinction may occur as a result of natural or human-caused environmental changes or of competition from other organisms. A species confronted by environmental change or competitors may (1) adapt behaviorally (for example, shift to a different diet), (2) adapt by evolving new characteristics, or (3) die out. At the present time, human impact on the environment is the leading cause of extinction. Habitat destruction by development and resource extraction, hunting, pollution, and the introduction of alien species into environments formerly inaccessible to them are causing the greatest burst of extinctions in at least 65 million years.

Extinction as such is a normal event; over 99.9% of all plant and animal species that have ever lived are now extinct. Extinction has always occurred at a fluctuating background rate. However, the fossil record also reveals a number of exceptional mass-extinction events, each involving the rapid disappearance of thousands of species of animals, plants, and microorganisms: five major mass extinctions, and about 20 minor ones, have occurred over the past 540 million years. The extinctions that occurred some 225 million years ago, at the end of the Permian period (the most severe of all mass extinctions, wiping out, for example, over 90% of shallow-water marine species), and some 65 million years ago, at the end of the Cretaceous period, are particularly well known. Almost half of all known species disappeared in the Cretaceous extinction, including all remaining dinosaurs and many marine animals.

The asteroid-impact theory

The primary cause of the Cretaceous mass extinction was a mystery for decades, until geologists discovered a thin layer of rock that marks the boundary between the Cretaceous period and following Tertiary period; this layer of sediments is termed the K-T boundary, and gave rise to the asteroid-impact theory of the Cretaceous extinction.

The asteroid-impact theory was first proposed in detail in 1978, by a team led by American geologist Walter Alvarez (1940–) and physicist Luis Alvarez (1911–1988). The Alvarez team analyzed sediment collected in the 1970s from the K-T layer near the town of Gubbio, Italy. The samples showed a high concentration of the element iridium, a substance rare on earth but relatively abundant in meteorites. Other samples of K-T boundary strata from around the world were also analyzed; excess iridium was found in these samples as well. Using the average thickness of the sediment as a guide, they calculated that a meteorite about 6 mi (10 km) in diameter would be required to spread that much iridium over the whole earth.

If a meteorite that size had hit Earth, the dust lofted into the air would have produced an enormous cloud of dust that would have encircled the world and blocked out the sunlight for months, possibly years. This climactic change would have severely depressed photosynthesis, resulting in the death of many plants, subsequent deaths of herbivores, and finally the death of their predators as well. (This chain of events would have occurred so rapidly that there would have been no chance for evolutionary adaptation to the new environment, which requires thousands of years at minimum.) A major problem with the theory, however, was that a 6-mi (10-km) meteorite would leave a very large crater, 93–124 mi (150–200 km) in diameter—and while earth has many impact craters on its surface, few are even close to this size, and none of the right age was known.

Because 65 million years had passed since the hypothetical impact, scientists shifted the search underground. A crater that old would almost certainly have been filled in by now. In 1992, an impact crater was discovered under the surface near the village of Chicxulub (pronounced CHIX-uh-loob) on Mexico’s Yucatan Peninsula. When core samples raised by drilling were analyzed, they showed the crater to be about 112 mi (80 km) in diameter and 65 million years old— the smoking gun that validated the Alvarez asteroid-impact theory.

The asteroid impact theory is now widely accepted as the most probable explanation of the K-T iridium anomaly, but many geologists still debate whether the impact of this large meteorite was the sole cause of the mass extinction of the dinosaurs and other life forms at that time, as the fossil record seems to show an above-average rate of extinctions in the time leading up to the K-T boundary. A number of gradual causes can accelerate extinction: falling ocean levels, for example, expose continental shelves, shrinking shallow marine environments and causing drier continental interiors, both changes that encourage extinction. Further, very large volcanic eruptions may stress the global environment. The asteroid that caused the Chicxulub crater may have coincidentally amplified or punctuated an independent extinction process that had already begun. There is no reason why many different causes cannot have acted, independently or in concert, to produce extinction events.

The asteroid-impact theory has been applied to many mass extinctions since the discovery of Chicxulub. Most of the five major mass extinctions of the last 540 million years, and several of the smaller ones, have been shown to coincide in time with large impact craters or iridium spikes (layers of heightened iridium concentration) in the geological column.

The Great Ice Age

The Great Ice Age that occurred during the Pleistocene era (which began about 2 million years ago and ended 10,000 years ago) also caused the extinction of many plants and animal species. This period is of particular interest because it coincides with the evolution of the human species. This was a time of diverse animal life, and mammals were the dominant large surface forms (i.e., in contrast to the reptiles of previous periods; as always, insects and bacteria dominated the animal world in absolute numbers, numbers of species, and total biomass). In the late Pleistocene (50,000–10,000 years ago), several other extinctions occurred. These wiped out several species known collectively as the megafauna (“big animals”), including mammoths, mastodons, ground sloths, and giant beavers. The late Pleistocene extinctions of megafauna did not occur all at once, nor were they of equal magnitude throughout the world. However, the continents of Africa, Australia, Asia, and North America were all affected. Recent work has shown that these extinctions did not, as long thought, single out megafauna as such, but rather all vertebrate species with slow reproduction rates (e.g., one offspring per female per year or less)— megafauna merely happen to be members of this larger class. It has been speculated that human beings caused these late-Pleistocene extinctions, hunting slow-reproducing species to extinction over centuries. Paleontologists continue to debate the pros and cons of different versions of this “overkill” theory.

The causes of the extinction events of the late Pleistocene are still debated, as are those of the larger mass extinctions that have punctuated earlier geological history; most likely several factors were involved, of which the global spread of human beings seems to have been one. Indeed, the past 10,000 years have seen dramatic changes in the biosphere caused by human beings. The invention of agriculture and animal husbandry and the eventual spread of these practices throughout the world have allowed humans to utilize a large portion of the available resources of earth—often in an essentially destructive and nonsustainable way. For example, in pursuit of lumber, farmland, and living room, human beings have reduced earth’s forest area from over one half of total land area less than one third.

The current mass extinction

Eventual extinction awaits all species of plants and animals, but the rate at which extinctions are taking place globally has been greatly accelerated by habitat destruction, pollution, global climate change, and over-harvesting to 1,000–10,000 times the normal background rate, equaling or exceeding rates of extinction during great mass extinctions of Earth’s geological history. The present mass extinction is unique in that it is being caused by a single species— ours—rather than by natural events; furthermore, biologists agree that the effects may be so profound as to threaten the human species itself.

Many countries, including the United States, have passing laws to protect species that are threatened or in danger of extinction, preserving some wild areas from exploitation and some individual species from hunting and harassment. Furthermore, several private groups, such as the Nature Conservancy, seek to purchase or obtain easements on large areas of land in order to protect the life they harbor. In the case of species whose numbers have been reduced to a very low level, the role of the zoological park (or “zoo”) has changed from one of simple exhibition to a more activist, wildlife-conservation stand.

Despite conservation efforts, however, the outlook is decidedly bleak. According to a statement from the World Conservation Union issued in 2000, over 11,000 species of plants and animals are already close to extinction—and the pace is accelerating. Since only 1.75 million species out of an estimated 14 million have been documented by biologists, these estimates may actually understate the magnitude of the developing disaster. Although Earth has restocked its species diversity after previous mass extinctions, this process invariably takes many millions of years; human beings are unlikely to ever see the Earth recover from the wave of extinctions now occurring due to human activity.

See also Biodiversity; Catastrophism; Evolutionary change, rate of; Evolutionary mechanisms; Punctuated equilibrium.

Resources

BOOKS

Gould, Stephen Jay. The Structure of Evolutionary Theory. Cambridge, MA: Harvard University Press, 2002.

Hallam, Tony. Catastrophes and Lesser Calamities: The Causes of Mass Extinctions. New York: Oxford University Press USA, 2005.

PERIODICALS

Cardillo, Marcel and Adrian Lister, “Death in the Slow Lane.” Nature. (October 3, 2002): 440–441.

Raffaelli, David. “How Extinction Patterns Affect Ecosystems.” Science. 306 (2004): 1141-1142.

Thomas, J.A., et al. “Comparative Losses of British Butterflies, Birds, and Plants and the Global Extinction Crisis.” Science. 303 (2004): 1879-1881.

Larry Gilman

Extinction

Extinction is the irreversible disappearance of all signs of life pertaining to a particular group of organisms. This group can be any of the accepted categories of taxonomic nomenclature, including kingdom, phylum, class, order, family, genus, species, and even subspecies. Most often, extinction is used in reference to the species level organism. The organism may have gone extinct at any time within the past 600 million years of the fossil record or may be in the process of going extinct today. Furthermore, this phenomenon does not pertain only to animals but is also applicable to plants, fungi, protists, bacteria, and archaebacteria.

Many situations can theoretically lead to the extinction of an organism. Determining the cause of extinction is very difficult, especially when information from the fossil record must be used to reconstruct a sequence of events. In many cases, a direct explanation is not forthcoming, and many hypotheses must be examined. Unfortunately, present-day cases of extinction are becoming increasingly common, leading to the need to predict and counteract the disappearance of currently living organisms. Extinction is a natural occurrence well documented in prehuman fossil deposits, and there is evidence that it may allow opportunistic evolution of species that had previously been overshadowed. Human domination of the biosphere, however, is accelerating the extinction rate. For many reasons, human lives are negatively affected by mass extinctions, and humans are struggling to oppose further loss.

Causes of Extinction

Many mechanisms account for extinction. Although organisms depend on other species in their ecosystem for food and population control, this delicate balance may be upset through the pressure of interspecies interactions. If an organism serves as the food source for another, it may be overpredated or overharvested, leading to the death of all members of the taxonomic group. This can occur if the predator/herbivore evolves keener sensory or cognitive capabilities, making it a more efficient consumer. It can also be the result of a food web disruption, such as when an environmental change suddenly favors the propagation of the predator/herbivore without offering such a benefit to the prey. Similarly, when two species occupy the same environmental niche, meaning that they prefer similar habitats and food types, they can be in direct competition for limited resources. If the competing species is better able to inhabit that niche, by gaining more territory and consuming more food, it can drive its competitors to extinction. Humans sometimes hunt animals for sport. This is not exactly predation or competition, but it can also result in the death of an entire species, for example, the dodo bird.

Environmental alteration can also lead to extinction. This may be part of a global climate change, one of Earth's natural climate fluctuations. In this case, one might expect to find a high number of global extinctions, especially among animals in regions greatly affected by the temperature and weather pattern differences. The microenvironment also undergoes change over time. For example, a new mountain range may form along a plain, an island may sink into the ocean, and a river may divert its course. Each of these examples would strongly affect the organisms that depend on those particular microenvironments for sustenance. In some cases, this local regional change can be called habitat loss. This term describes the destruction of a particular type of habitat, such as decline of biodiversity in the North American natural plains, beginning in the nineteenth century.

Evolution can also be considered a direct mode of extinction. Taxonomic boundaries are sometimes very arbitrary, meaning that the distinction between groups is unclear. The lines between groups are drawn using evidence about the organism's habitat, body shape (or morphology), and living habits; most often, however, only the morphology can be relied upon. Some species in the fossil record show gradual change in morphology over evolutionary time. Paleontologists, scientists who research extinct organisms, must sometimes examine a progression from one body type to another and determine at what point the original species can be called a new species. Nonetheless, the original species can no longer exist when the new species begins, and this in itself is a kind of extinction.

Factors that Can Contribute to Extinction

Several factors influence the likelihood that an organism will go extinct. Some species have a very small range, so that they are very susceptible to small-scale changes in the environment. This is an extremely common explanation for the extinction of subspecies. Subspecies sometimes evolve when isolated populations of a species living at the boundaries of its natural range develop distinct behaviors and appearances that distinguish them from the parent species. Given a long enough period of separation from the parent species, these subspecies would develop into an entirely novel organism. Despite this, they often are subject to extinction because their small population size and range cause them to either die out or to be reassimilated into the parent species.

A particular type of feeding pattern is another extinction factor. Whereas generalist feeders, which rely on many sources of nutrition, can switch to a different diet if their food source were to disappear, specialist feeders, which consume only one particular food source, cannot and are therefore highly susceptible to extinction.

Some organisms can be said to have a very delicate niche dynamic for a combination of the above reasons. For example, olive ridley sea turtles return to the same beach year after year to lay eggs. The eggs are highly predated by local animals, and the beaches are often brightly lit and filled with human tourists, factors that decrease the fitness of the mothers and the young. The turtles, however, seem physiologically incapable of choosing other sites for nesting because of their dependence on a particular sort of fine-grained sand and on particular temperature patterns at the chosen beaches.

Once species numbers decline to a small amount of individuals, extinction speed is increased by a factor known as inbreeding depression. This occurs when so few individuals remain in a species that they are forced to mate with members of their own families out of necessity. The smaller gene pool causes increased incidence of genetic disorders, general poor health, and increased susceptibility to disease, all of which increase the species' decline.

Difficulties Determining When an Extinction Has Occurred

Extinctions are not always easy to pinpoint or determine. When a fossil organism stops appearing in the fossil record, this may be explained by a variety of reasons. It may be that environmental conditions during the following period did not favor fossil formation, so that the species survived but was no longer fossilized. Another possibility is that only a small population of a widely dispersed species may live in the region that allows fossilization; if this local population goes extinct, the entire species will disappear from the fossil record, but that does not necessarily mean that the entire species has also gone extinct. Another explanation is that the species may have merely migrated from a fossil-friendly environment to a fossil-unfriendly habitat, or perhaps to a region for which the fossil bed has not yet been found. A confounding factor in determining extinction can be as simple as researcher bias. Organisms seem to go extinct at the boundaries between geological periods of Earth's history, but this may be due to a quirk of the field of paleontology: Paleontologists often study only one period in the rock strata. They categorize all organisms that appear at the beginning of their period, and may not realize that the scientists researching an earlier period have already named the same species. Thus although it appears that an organism has become extinct, in reality it has just been mistakenly renamed.

Paleontologists try to account for all of these sources of error through thorough study and the use of probability equations, but the fossil record is irregular and difficult to interpret. For example, in the mid-twentieth century, the coelacanth, a species of bony fish that scientists had falsely declared extinct, was rediscovered as a living species. Coelacanths were thought to have gone extinct 70 million years ago because they disappeared from the fossil record. This mistake was remedied in 1938 when a fisherman caught a living coelacanth off the coast of southeastern Africa.

Reasons for Preventing Extinctions

Although extinction is a natural occurrence, there are many reasons to actively protect existing species from extinction. More and more, ecosystems are viewed as integrated modules, so that the extinction of one species in the ecosystem can disrupt all of the other species' population dynamics. If humans ignore the extinction of a seemingly uninteresting species, this could cause widespread extinctions in many other organisms. In general, biodiversity (the presence of a high number of species in an environment) is equated with a healthy ecosystem, and high extinction rates decrease biodiversity. Often, an environment of low biodiversity reveals what humans consider to be pests. These are merely organisms that are able to survive in an ecosystem reduced and dominated by humans. People naturally prefer a healthier ecosystem, with clean breathing air, green surroundings, and a wide variety of species. There is a value to the beauty of the environment, which is damaged by high extinctions. Furthermore, humans directly depend on some species for medical benefits, such as medicine, tissue and organ donations, and animal models of human disease. Organisms that go extinct can no longer be researched as possible cures for human ailments or be studied by engineers as models for building computers, machinery, and vehicles.

Preventing extinction is a political as well as a biological priority. The needs of humans often overshadow the decline of endangered species. For example, the economy of many Third World countries depends on agriculture. If natural ecosystems such as rain forests must be destroyed to support the agricultural needs of these nations, many species are put at risk and forced into extinction. Without that source of income, however, the peoples of these countries could languish and starve. A balance between preserving the rain forest habitat and ensuring the well-being of the humans must be sought.

Methods of Preventing Extinction

There are several contemporary means of preserving a particular species. Active breeding programs at nature reserves and at zoos attempt to maintain a sizable population of individuals to avoid inbreeding depression. When animals such as cheetahs are the victims of inbreeding depression, specialized veterinarians and behaviorists monitor their health and attempt to circumvent the danger of disease. Hunting and fishing regulations attempt to predict population flux and disallow overharvesting of game animals. Specialized herbariums and eco-landscaping firms are working to restore lost habitats within depleted landscapes.

Despite these endeavors, there is still an acute danger that many of the world's habitats will be thrown into disarray by human intervention. The shrinkage and parceling of natural habitats is especially damaging to animals with large ranges, such as Siberian tigers, bald eagles, and buffalo. Nature preserves are under constant study to improve the efficiency of the surroundings in order to prevent extinction. span>

see also Feeding Strategies; Fossil Record; Habitat Loss; Habitat Restoration; Hunting; Paleontology; Zoological Parks.

Rebecca M. Steinberg

Bibliography

Courtillot, Vincent. Evolutionary Catastrophes: The Science of Mass Extinction. Cambridge, U.K.: Cambridge University Press, 1999.

Glen, William, ed. The Mass-Extinction Debates: How Science Works in a Crisis. Stanford, CA: Stanford University Press, 1994.

Hallam, Anthony, and Paul B. Wignall. Mass Extinctions and Their Aftermath. Oxford,

U.K.: Oxford University Press, 1997.

Raup, David M. Extinction: Bad Genes or Bad Luck? New York: Norton, 1992.

Schneiderman, Jill S. The Earth Around Us: Maintaining a Livable Planet. New York:

W. H. Freeman, 2000.

Snyder, Noel F., and Helen Snyder. The California Condor: A Saga of Natural History and Conservation. San Diego: Academic Press, 2000.

Weinberg, Samantha. A Fish Caught in Time: The Search for the Coelacanth. New York:

Harper Collins, 2000.

CALIFORNIA CONDOR

California condors are the largest birds in North America, weighing up to 11.5 kilograms (25 pounds), with a wingspan of more than 2.7 meters (9 feet). In 1967 condors had declined to approximately thirty individuals. In 2000, eighty-seven condors were living, but only three were in the wild. While captive breeding programs have improved the outlook for condors, their future now depends entirely on human intervention.

Extinction

Extinction is the permanent disappearance of an entire species. (A species is a group of closely related, physically similar living organisms that can breed with one another.) A species goes extinct when every single one of its kind has died. Extinctions have been happening since life first began on Earth. However, human activity has greatly accelerated the pace of extinction, and many believe that a great extinction may be taking place today.

While some use the word extinction to describe what might better be described as "local or regional extinction," to most it means the complete and total elimination of a certain species from the face of Earth. Others describe extinction as the loss of a species and its replacement by an evolved version. However, although a particular species may no longer exist, the fact that it adapted or changed so much that it became a distinctly different species means that it never died out completely. Scientists know from fossils that the earliest ancestor of today's horse stood less than 2 feet (0.61 meters) high. Although that species of horse no longer exists, in a way it does, because it slowly and gradually evolved into the very large horse that is common today. If the smaller horse had died out completely and never had a chance to adapt and change and to pass on those changes to its descendants, then it would have gone extinct.

A NATURAL PART OF EARTH'S LIFE CYCLE

Extinction is an ongoing feature of the Earth's ever-changing ecosystem. It is usually caused by major environmental or climate changes. Humans also indirectly cause extinction by the effects their activities have on the environment. Hunting, habitat destruction, and pollution are some of the ways that humans have driven species to extinction. Also, a phenomenon known as "coextinction" can take place when the disappearance of one species of plant or animal dooms the existence of others that are entirely dependent upon it. It is believed that the saber-toothed tiger of prehistoric times died out completely when its sole prey, the mastodon, went extinct.

Throughout the enormously long time that life has existed on Earth, millions of species have simply come and gone. It is estimated that more than 90 percent of all the species that ever existed during Earth's history are now extinct. This is known from the fossil record, which reveals that a number of mass extinctions took place; huge events that resulted in the destruction of large numbers of different species. (A fossil is the remains of a living thing that has tuned into rock and therefore has been preserved.) The different layers, or strata, of Earth's crust represent different times in Earth's history. By this record, we know that the earliest mass extinction was about 650,000,000 years ago during a time called the Precambrian period when life on Earth consisted mostly of algae (plants or plantlike organisms that contain chlorophyll and other pigments that trap light from the Sun) floating on oceans. Scientists think that about 70 percent of the algae species were killed by a major ice age (a period where glaciers covered much of the Earth). Another mass extinction, probably caused by a climate change and considered the largest ever, occurred about 250,000,000 years ago, killing about 90 percent of all land and sea creatures. However, the best-known extinction killed the dinosaurs and many other species during the Cretaceous period about 65,000,000 years ago. Scientists are now fairly certain that this event was caused by a meteorite that crashed into the Earth, sending a vast cloud of dust into the atmosphere and blotting out all sunlight. Most think that the food chain was disrupted since plants could not grow without light, resulting in the death of herbivores (plant-eaters) and the eventual death of carnivores (flesheaters). Scientists do not agree as to whether the dense cloud would have cooled the atmosphere or warmed it, but either scenario would have changed the environment so quickly that plant and animal life would have had no time to adapt. Finally, the Ice Age that occurred during the Pleistocene era (between 1,600,000 and 10,000 years ago) caused the destruction of many plants and animals. This was a time of extremely diverse animal life, and mammals rather than reptiles were the dominant species.

HUMANS INCREASE EXTINCTION RATE

Since the evolution (the process by which gradual genetic change occurs) of humans about 10,000 years ago, there have been dramatic changes in Earth's biosphere (all the parts of Earth that make up the living world). With the invention of agriculture (farming) and the domestication (taming) of some animals, humans spread throughout the world and began to use its resources. Some ecologists argue that early humans hunted certain species into extinction, but probably more have been simply crowded out by countless human populations who have taken over the species' habitat. For example, it is known that at least 200 plant species that were native to North America have become extinct only in the past few thousand years. Worldwide, it is estimated that as many as 20 percent of all bird species have disappeared since the evolution of humans.

LUIS WALTER ALVAREZ

American physicist (a person specializing in the study of physics) Luis Alvarez (1911–1988) was a physicist who had wide-ranging interests and abilities that led him to suggest that the extinction of dinosaurs was caused indirectly by an asteroid that struck Earth. During World War II (1939–45), he was involved with radar and the atomic bomb, and in 1968 he won the Nobel Prize for Physics.

Luis Walter Alvarez was born in San Francisco, California, the son of a medical researcher and physician. His paternal grandfather was originally from Spain, but had run away to Cuba before making his fortune in real estate in California. Young Alvarez attended school in San Francisco, but when his father accepted a position at the Mayo Clinic in Rochester, Minnesota, he attended high school there. After enrolling in the University of Chicago in 1928 to study chemistry, he soon came to love physics and switched his major. Alvarez stayed at Chicago through his bachelor's, master's, and the doctorate degree he received in 1936. He then joined the faculty at the University of California at Berkeley, where he remained until his retirement in 1978.

At Berkeley, Alvarez soon was given the title "prize wild idea man" by his colleagues because of his involvement in such a wide variety of research activities. His earliest research was in the area of nuclear physics and cosmic rays, and when World War II broke out in Europe he worked at Massachusetts Institute of Technology's radiation laboratory on radar (a method of detecting distinct objects) and helped develop a narrow beam radar system that allowed airplanes to land in bad weather. He was also involved in the Manhattan Project to develop the world's first nuclear weapons. Since he helped develop the bomb's detonating device, Alvarez flew aboard a B-29 airplane that followed the Enola Gay when it dropped the first atomic bomb on Hiroshima, Japan. Returning to Berkeley after the war, Alvarez built a huge bubble chamber that could track extremely short-lived particles. It was for this invention that he won the Nobel Prize for Physics in 1968.

In 1980, he and his son, Walter, who was a professor of geology at Berkeley, accidentally discovered a band of sedimentary rock in Italy that contained an unusually high level of the rare metal iridium. Dating techniques set the age of this layer at about 65,000,000 years old. Similar iridium-rich layers were later found in other parts of the world. This led the father-son team to propose a theory regarding the extinction of dinosaurs that occurred 65,000,000 years ago. They then proposed that the iridium had come from an asteroid that struck Earth and sent huge volumes of smoke and dust (including the iridium) into Earth's atmosphere. They suggested that the cloud produced by the asteroid's impact covered Earth for so long that it blocked out sunlight, causing widespread death of plant life. This loss of plant life in turn brought about the extinction of dinosaurs who fed on the plants. While this overall theory has found favor among many scientists and has been confirmed to some extent by additional findings, it is still the subject of much debate, although most now agree with the first half of the hypothesis (the asteroid impact).

While the threat of extinction has always existed in nature, the actual time it takes to happen has rapidly speeded up in modern times. The difference today is the overwhelming pressure placed upon the environment by human activity. Habitat destruction, pollution, and over-harvesting are some of the reasons why today, some ecologists argue that we are driving species to extinction at a rate between 1,000 and 10,000 times faster than has ever happened before. The American biologist Edward O. Wilson (1929– ) currently estimates that the world may have already lost one-fifth of its present species. Today's human interference in nature is probably the most destructive since much of it is the result of chemical pollutants that easily enter an organism's reproductive system and cause a wide range of birth defects. Habitat destruction also accounts for a great amount of loss as the rain forests of the world, which contains a vast variety of plant and animals species, are steadily being cut down.

COUNTRIES TRY TO CURB EXTINCTION RATES

Many countries are trying to control the effects of human activity, which can be most devastating but is also most manageable. Laws have been passed in many countries that protect certain habitats from destruction and individual species from being hunted. Zoos are changing from places that exhibit animals to places that keep an endangered species alive. One such success story is the resurgence of the American bison. Where it once dominated the plains from Alaska and western Canada to the United States and northern Mexico, the American bison was so ruthlessly hunted that by 1899 there were fewer than 1,000 left. When guarded preserves were established and breeding programs begun, the numbers of plains bison rose to more than 50,000 and the species is no longer threatened by extinction. The bison resurgence however, required an enormous and expensive effort on a national scale, and would be impossible to duplicate for every valued species that is threatened. Instead, it is easier and more efficient to regulate the activities that threaten the diversity of life on Earth in the first place. While this may sound simple however, it often involves very sensitive issues of economics, which many times takes priority over the protection of plant and animal species.

[See alsoEndangered Species; Habitat; Pollution ]