Extinction

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Extinction

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.

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

The Gale Encyclopedia of Science