Extinction | Research & Encyclopedia Articles

Extinction is the complete disappearance of a species, when all of its members have died or been killed. As a part of natural selection, the extinction of species has been ongoing throughout the earth's history. However, with modern human strains on the environment, plants, animals, and invertebrates are becoming extinct at an unprecedented rate of thousands of species per year, especially in tropical rain forests. Many thousands more are threatened and endangered.

Scientists have determined that mass extinctions have occurred periodically in prehistory, coming about every 50 million years or so. The greatest of these came at the end of the Permian period, some 250 million years ago, when up to 96% of all species on the earth may have died off. Dinosaurs and many ocean species disappeared during a well-documented mass extinction at the end of the Cretaceous period (about 65 million years ago). It is estimated that of the billions of species that have lived on the earth during the last 3.5 billion years, 99.9% are now extinct.

It is thought that most prehistoric extinctions occurred because of climatological changes, loss of food sources, destruction of habitat, massive volcanic eruptions, or asteroids or meteors striking the earth. Extinctions, however, have never been as rapid and massive as they have been in the modern era. During the last two centuries, more than 75 species of mammals and over 50 species of birds have been lost, along with countless other species that had not yet been identified. James Fisher has estimated that since 1600, including species and subspecies, the world has lost at least 100 types of mammals and 258 kinds of birds.

The first extinction in recorded history was the European lion, which disappeared around A.D. 80. In 1534, seamen first began slaughtering the great auk, a large, flightless bird once found on rocky North Atlantic islands, for food and oil. The last two known auks were killed in 1844 by an Icelandic fisherman motivated by rewards offered by scientists and museum collectors for specimens. Humans have also caused the extinction of many species of marine mammals. Steller's sea cow, once found on the Aleutian Islands off Alaska, disappeared by 1768. The sea mink, once abundant along the coast and islands of Maine, was hunted for its fur until about 1880, when none could be found. The Caribbean monk seal, hunted by sailors and fishermen, has not been found since 1962.

The early European settlers of America succeeded in wiping out several species, including the Carolina parakeet and the passenger pigeon. The pigeon was one of most plentiful birds in the world's history, and accounts from the early 1800s describe flocks of the birds blackening the sky for days at a time as they passed overhead. By the 1860s and 1870s tens of millions of them were being killed every year. As a result of this overhunting, the last passenger pigeon, Martha, died in the Cincinnati Zoo in 1914. The pioneers who settled the West were equally destructive, causing the disappearance of 16 separate types of grizzly bear, six of wolves, one type of fox, and one cougar. Since the Pilgrims arrived in North America in 1620, over 500 types of native American animals and plants have disappeared.

In the last decade of the twentieth century, the rate of species loss was unprecedented and accelerating. Up to 50 million species could be extinct by 2050, with a rate of three per day. Most of these species extinctions will occur—and are occurring—in tropical rain forests, the richest biological areas on the earth. Rain forests are being cut down at a rate of one to two acres per second.

In 1988, Harvard professor and biologist Edward O. Wilson estimated the current annual rate of extinction at up to 17,500 species, including many unknown rain forest plants and animals that have never been studied or even seen, by humans. Botanist Peter Raven, director of the Missouri Botanical Garden, calculated that a total of one-quarter the world's species could be gone by 2010. A study by the World Resources Institute pointed out that humans have accelerated the extinction rate to 100 to 1,000 times its natural level.

While it is impossible to predict the magnitude of these losses or the impact they will have on the earth and its future generations, it is clear that the results will be profound, possibly catastrophic. In his book, Disappearing Species: The Social Challenge, Eric Eckholm of the Worldwatch Institute observed that humans, in their ignorance, have changed the natural course of evolution with current mass-extinction rates. "Should this biological massacre take place, evolution will no doubt continue, but in a grossly distorted manner. Such a multitude of species losses would constitute a basic and irreversible alteration in the nature of the biosphere even before we understand its workings..."

Eckholm further notes that when a plant species is wiped out, some 10 to 30 dependent species, such as insects and even other plants, can also be jeopardized. An example of the complex relationship that has evolved between many tropical species is the 40 different kinds of fig trees native to Central America, each of which has a specific insect pollinator. Other insects, including pollinators of other plants, depend on these trees for food. Thus, the extinction of one species can set off a chain reaction, the ultimate effects of which cannot be foreseen.

Although scientists know that human life will be harmed by these losses, the weight of the impact is unclear. As the Council on Environmental Quality states in its book The Global Environment and Basic Human Needs, over the next decade or two, "unique ecosystems populated by thousands of unrecorded plant and animal species face rapid destruction—irreversible genetic losses that will profoundly alter the course of evolution." This report also cautions that species extinction entails the loss of many useful products. Perhaps the greatest industrial, agricultural and medical costs of species reduction will stem from future opportunities unknowingly lost. Only about 5% of the world's plant species have yet been screened for pharmacologically active ingredients. Ninety percent of the food that humans eat comes from just 12 crops, but scores of thousands of plants are edible, and some will undoubtedly prove useful in meeting human food needs.

Biodiversity; Climate; Dodo; Endangered Species

Etheredge, N. The Miner's Canary: A Paleontologist Unravels the Mysteries of Extinction. Englewood Cliffs, NJ: Prentice-Hall, 1991.

Raup, D. M. Extinction: Bad Genes or Bad Luck? New York: Norton, 1991.

Tudge, C. Last Animals at the Zoo: How Mass Extinction Can Be Stopped. Washington, DC: Island Press, 1992.

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