Extinction—the total loss of a species or higher taxonomic grouping—occurs for a wide vari ety of physical and sometimes biological reasons. Extinction should be regarded as a normal process in the history of life: much like the death of individual organisms, species are destined eventually to become extinct. Some species become extinct shortly after they have evolved; others may last for thousands, hundreds of thousands, or in many cases even millions of years (see Species). The vast majority of species that have ever lived are now extinct. Many biologists and paleontologists are beginning to think that extinction actually plays a vital role in the evolutionary process: in many instances, new species tend to appear rapidly after extinction has claimed species that had been formerly living in a region (see Evolution).
Scientists distinguish between what is called background extinction and events when extinctions of many species occur more or less at the same time in a pronounced extinction "event." The term background extinction simply means that a certain percentage of species are likely to fall prey to extinction as time goes by. For example, if the favorite food source of a species begins to disappear, the size of the species will diminish—and if it shrinks too far, the entire species may disappear—a victim of extinction.
In general, however, species become extinct through environmental change that occurs too fast, or too strongly, for species to survive through habitat tracking or evolution. And when such changes occur, they tend to affect many species within the regional, or even global, ecosystems at the same time. Thus it has become clear in recent years that most extinction is not background but rather is concentrated in regional or global events—often (at least in the case of the global events) called mass extinctions.
Habitat destruction is the single leading cause of extinction. When habitat disappears, the animals, plants, fungi, and microbes dependent upon it for nourishment and shelter will also tend to disappear—to become extinct.
There are many ways that habitats can become disrupted or destroyed. Climate change is a leading cause of habitat destruction (see also Habitat Tracking). For example, in one well-documented example from eastern and southern Africa, global climate change starting around 2.8 million years ago radically transformed the environment. From warm, moist tropical woodlands, within the space of 300,000 years, grasslands, indicative of a cooler, drier climate, replaced the woodlands in most places, driving many woodland-adapted species to extinction.
Climate change alone, however, is often insufficient to cause extinction: many of the Pleistocene (Ice Age) animals and plants— including those living on land, as well as marine invertebrate species—simply migrated southward, ahead of the advancing glaciers; when the climate warmed up and the glaciers retreated northward, so did the species living in the various ecosystems that had been displaced to the south. Mere loss of habitat in one region is not enough to cause extinction—if similar habitat occurs elsewhere, and species are able to migrate to it.
Yet global climate change is thought to have played a role in at least some of the five major mass extinctions of the past—most notably, perhaps, the Late Ordovician and Late Devonian mass extinction events.
Global cooling has another related effect on habitats: when seawater is locked up into huge, expanding sheets of continental glaciers a half-mile thick or more, sea level drops. The ice caps today are relatively large—not as great, of course, as when glaciers came as far south as Illinois and southern New York state (as recently as 18,000 years ago), but nonethe less greater than they were for most of the past half-billion years. This means that, for most of the past half-billion years, shallow seaways covered substantial parts of the interiors of North America, Eurasia, and, to a somewhat lesser extent, the continents of the Southern Hemisphere.
These shallow epicontinental (epeiric) seaways usually teemed with life—species of bra-chiopods, corals, rooted echinoderms like crinoids, corals, and mollusks, as well as many kinds of fish in the Paleozoic; mollusks (clams, snails, and ammonoids) as well as mosasaurs (marine lizards) and fishes in the Mesozoic and Cenozoic. But when the seas dried up and their natural habitats disappeared, species abruptly became extinct—all at about the same time.
Perhaps the most famous cause of mass extinction is the collision between the earth and one or more comets or asteroids from outer space. Extraterrestrial impacts are well documented as the probable cause of the fifth major mass extinction—the one at the end of the Cretaceous Period that ended the Meso-zoic Era, some 65 million years ago; it took out the nonflying dinosaurs as well as the last of the ammonites and many other terrestrial and marine species. It is thought that bolide impacts may have been involved in the great Permo-Triassic extinction of 245 million years ago, as well, perhaps, as the extinction event at the close of the Triassic Period some 210 millions of years ago.
Collision of major extraterrestrial objects with the earth would have had the force of many hydrogen bombs—obviously obliterating all life forms within hundreds of miles of impact. But how could such an impact cause a wave of extinctions that engulfs the entire planet—as apparently happened 65 million years ago? The general theory of extinction-through-impact is profoundly ecological. We know from large volcanic eruptions in the past 200 years (such as the great explosion of Krakatoa in the strait between Malaysia and eastern Java in 1883) that large amounts of fine dust particles can be sent up into the highest reaches of the earth's atmosphere—coloring sunsets and, more important, blocking enough sunlight to lower the earth's temperature by 1 or 2 degrees centigrade. The Krakatoa eruption not only colored sunsets but is also thought to have triggered the great blizzard of 1888 in New York, and to have prompted snow to fall in the summertime in the northeastern United States; 1888 became known as the "year without summer."
These volcanic events provide a clue as to what might have happened immediately after a major impact event. Huge amounts of par-ticulate matter and vapors would be sent up into the atmosphere, blocking sunlight sufficiently to interfere severely with photosynthesis in land plants—and in the planktonic marine microorganisms that photosynthesize near the surface of the oceans. Photosynthesis provides the very base of the food chain on land and in the sea, so a die-off of photosynthesizers would soon be followed by the loss of many of the species that depend upon them. Extinction comes about, then, by major disturbance to the flow of matter and energy through the world's ecosystems. Moreover, wildfires broke out after impact, followed by a major cooling event. Habitats were severely altered or destroyed, causing the great loss of species that we have come to call global mass extinction.
Biologists, including Charles Darwin, have speculated that biological factors can also cause extinction. Although difficult to prove, it seems likely that, especially in times when species become able to invade new habitat, very often resident species—species already adapted to and living in a particular region—
will become extinct. For example, when the Isthmus of Panama was raised to form a direct land bridge between North and South America—an event that was completed by about 2.5 million years ago—some animal species (many of which were marsupials, like opossums) from South America were able to move north, while placental mammals from the north were able to move south. Although still a matter of study and some dispute, it appears that, on the whole, more species in South America were driven to extinction by the arrival of alien species than vice versa. In all instances, the cause of extinction is thought to be failure to survive given the competition with alien species.
The current Sixth Extinction is, in one sense, very different from the extinction events of the past—especially those that happened through physical causes. The difference is that the current extinction event (the "biodiversity crisis") currently gripping the word's species and ecosystems is caused virtually entirely by the behavior of a single species: Homo sapiens. On the other hand, the current extinction event is very like those mass extinctions of the past, because humanity is disrupting habitats and driving species extinct in a manner very reminiscent of extraterrestrial impacts and other physical causes of mass extinction.
Humans drive other species extinct in several ways. One is the conversion of land for agriculture. Over the past 10,000 years, since the invention of agriculture, people have cleared land—forests and grasslands—to plant one or two crops where normally many species of plants and animals would exist. We have drained swamps and marsh lands for cultivation, made deserts bloom through irrigation, and even regained farmland from the sea—as in the Netherlands. The negative effects (that is, in terms of driving species extinct) of such
human-caused habitat transformation are more pronounced in the tropics than in the higher latitudes, simply because there are more species in the tropics and many of them are restricted to smaller regions than is the usual case in the higher latitudes. In other words, there is a far greater chance of driving one or more species extinct by chopping down ten acres of forest in the tropics than there is in clearing ten acres in Ohio. But it all adds up, and conversion of natural habitat to farmland has been a leading cause of extinction in recent times.
But people destroy habitat for reasons other than the purely agricultural (which includes raising livestock on range land as well as growing crops). Especially in the past few hundred years, as human population has skyrocketed, cities and suburbs, with their malls, streets, and tract housing, have destroyed an enormous amount not only of natural habitat but also, increasingly, of the farmland we need to feed ourselves.
Warfare, too, has contributed increasingly to the damage that humans have caused to the landscape. We continue to cut down forests for the wood itself—for building, or even, in many instances especially in the Third World, simply for firewood to provide fuel for cooking and heat. Overharvesting of timber—where far more trees are cut than are being replaced in a sustainable manner by reforestation— remains a major source of habitat destruction.
Overharvesting of certain animal species by humans also has driven animal species to extinction. Many of the large mammalian species of the great Ice Age are thought to have been driven extinct by overhunting by humans. Twelve of the thirteen major oceanic fisheries, by the same token, are now so severely depleted by overfishing and harmful fishing practices that many fish species are now on the very brink of extinction. Pollution, too, poses a major threat to habitats—and directly to individual species of plants and animals.
Finally, the introduction of alien species (see Alien Species) is a major cause of extinction—just as it seems to have been in certain instances in the geological past. Humans bring domesticated animals (not to mention parasites and disease-causing micro-organisms) as they spread around the planet. Possibly a contributor to the first wave of the Sixth Extinction, the spread of alien species has reached shocking proportions in recent centuries, as opportunities for travel and trade have expanded. The brown tree snake, for example, is responsible for driving nine species of native birds extinct on the island of Guam in the South Pacific. The snakes arrived on airplanes as troops moved about during World War II, and they remain a major menace to this day. Aircraft in Hawaii are inspected daily, and some brown tree snakes have already been detected. It is felt that, should this aggressive snake ever become established on Hawaii, it would be the final death knell for many of Hawaii's endemic birds and other species already hard hit by extinction since the arrival of humans, human pets, and domestic animals, as well as by the ravages of cultivation.
See also: Agriculture, Origin of; Alien Species; Darwin, Charles; Evolution; Habitat Tracking; Pollution; Sixth Extinction; Species; Urbanization
Alvarez, Walter. 1997. T. rex and the Crater of Doom. Princeton: Princeton University Press; Dobson, Andrew P. 1996. Conservation and Biodiversity. New York: Scientific American Library; Eldredge, Niles. 1991. The Miner's Canary: Unraveling the Mysteries of Extinction. New York: Prentice Hall; Eldredge, Niles. 1998. Life in the Balance: Humanity and the Biodiversity Crisis. Princeton: Princeton University Press; Stanley, Steven M. 1987. Extinction. New York: Scientific American Books.
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