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Extinction (biology)
I INTRODUCTION
Extinction (biology), the end of existence of a group of organisms, caused by their inability to adapt to changing environmental conditions. Extinction affects individual species—that is, groups of interbreeding organisms—as well as collections of related species, such as members of the same family, order, or class (see Classification). The dodo, for example, a species of flightless pigeon formerly living on the island of Mauritius, became extinct in 1665. About 10,000 to 12,000 years ago, the most of the woolly mammoths and the last of the mastodons, both members of the elephant family, died. And about 245 million years ago at the end of the Paleozoic Era, an entire class of primitive marine animals called trilobites disappeared forever.
Fossils, the remains of prehistoric plants and animals buried and preserved in sedimentary rock or trapped in amber or other deposits of ancient organic matter, provide a record of the history of life on Earth. Scientists who study this fossil record, called paleontologists, have learned that extinction is a natural and ongoing phenomenon. In fact, of the hundreds of millions of species that have lived on Earth over the past 3.8 billion years, more than 99 percent are already extinct. Some of this happens as the natural result of competition between species and is known as natural selection. According to natural selection, living things must compete for food and space. They must evade the ravages of predators and disease while dealing with unpredictable shifts in their environment. Those species incapable of adapting are faced with imminent extinction. This constant rate of extinction, sometimes called background extinction, is like a slowly ticking clock. First one species, then another becomes extinct, and new species appear almost at random as geological time goes by. Normal rates of background extinction are usually about five families of organisms lost per million years.
More recently, paleontologists have discovered that not all extinction is slow and gradual. At various times in the fossil record, many different, unrelated species became extinct at nearly the same time. The cause of these large-scale extinctions is always dramatic environmental change that produces conditions too severe for organisms to endure. Environmental changes of this caliber result from extreme climatic change, such as the global cooling observed during the ice ages, or from catastrophic events, such as meteorite impacts or widespread volcanic activity. Whatever their causes, these events dramatically alter the composition of life on Earth, as entire groups of organisms disappear and entirely new groups rise to take their place.
II MASS EXTINCTIONS
In its most general sense, the term mass extinction refers to any episode of multiple loss of species. But the term is generally reserved for truly global extinction events—events in which extensive species loss occurs in all ecosystems on land and in the sea, affecting every part of the Earth's surface. Scientists recognize five such mass extinctions in the past 500 million years. The first occurred around 438 million years ago in the Ordovician Period. At this time, more than 85 percent of the species on Earth became extinct. The second took place 367 million years ago, near the end of the Devonian Period, when 82 percent of all species were lost. The third and greatest mass extinction to date occurred 245 million years ago at the end of the Permian Period. In this mass extinction, as many as 96 percent of all species on Earth were lost. The devastation was so great that paleontologists use this event to mark the end of the ancient, or Paleozoic Era, and the beginning of the middle, or Mesozoic Era, when many new groups of animals evolved.
About 208 million years ago near the end of the Triassic Period, the fourth mass extinction claimed 76 percent of the species alive at the time, including many species of amphibians and reptiles. The fifth and most recent mass extinction occurred about 65 million years ago at the end of the Cretaceous Period and resulted in the loss of 76 percent of all species, most notably the dinosaurs.
Many geologists and paleontologists speculate that this fifth mass extinction occurred when one or more meteorites struck the Earth. They believe the impact created a dust cloud that blocked much of the sunlight—seriously altering global temperatures and disrupting photosynthesis, the process by which plants derive energy. As plants died, organisms that relied on them for food also disappeared. Supporting evidence for this theory comes from a buried impact crater in the Yucat?n Peninsula of Mexico. Measured at 200 km (124 mi) in diameter, this huge crater is thought to be the result of a large meteorite striking the Earth. A layer of the element iridium in the geologic sediment from this time provides additional evidence. Unusual in such quantities on Earth, iridium is common in extraterrestrial bodies, and theory supporters suggest iridium traveled to Earth on a meteorite.
Other scientists suspect that widespread volcanic activity in what is now India and the Indian Ocean may have been the source of the atmospheric gases and dust that blocked sunlight. Ancient volcanoes could have been the source of the unusually high levels of iridium, and advocates of this theory point out that iridium is still being released today by at least one volcano in the Indian Ocean. No matter what the cause, the extinction at the end of the Cretaceous Period was so great that scientists use this point in time to divide the Mesozoic Era (also called the Age of Reptiles) from the Cenozoic Era (otherwise known as the Age of Mammals).
III ROLE OF MASS EXTINCTION IN EVOLUTION
Historically biologists—most famous among them British naturalist Charles Darwin—assumed that extinction is the natural outcome of competition between newly evolved, adaptively superior species and their older, more primitive ancestors. These scientists believed that newer, more highly evolved species simply drove less well-adapted species to extinction. That is, historically, extinction was thought to result from evolution. It was also thought that this process happens in a slow and regular manner and occurs at different times in different groups of organisms.
In the case of background extinction, this holds true. An average of three species becomes extinct every million years, usually as a result of the forces of natural selection. When this happens, new species—differing only slightly from the organisms that disappeared—rise to take their places, creating evolutionary lineages of related species. The modern horse, for example, comes from a long evolutionary lineage of related, but now extinct, species. The earliest known horse had four toes on its front feet, three toes on its rear feet, and weighed just 36 kg (80 lb). About 45 million years ago, this horse became extinct. It was succeeded by other types of horses with slightly different characteristics, such as teeth better shaped for eating different plants, which made them better suited to their environments. This pattern of extinction and the ensuing rise of related species continued over the course of 55 million years, ultimately resulting in the modern horse and its relatives the zebras and asses.
In mass extinctions, entire groups of species—such as families, orders, and classes—die out, creating opportunities for the survivors to exploit new habitats. In their new niches, the survivors evolve new characteristics and habits and, consequently, develop into entirely new species. What this course of events means is that mass extinctions are not the result of the evolution of new species, but actually a cause of evolution. Fossils from periods of mass extinction indicate that most new species evolve after waves of extinction. Mass extinctions cause periodic spurts of evolutionary change that shake up the dynamics of life on Earth.
This is perhaps best demonstrated in the development of our own ancestors, the early mammals. Before the fall of the dinosaurs, which had dominated Earth for more than 150 million years, mammals were small, nocturnal, and secretive. They devoted much of their time and energy to evading meat-eating dinosaurs. With the extinction of dinosaurs, the remaining mammals moved into habitats and ecological niches previously dominated by the dinosaurs. Over the next 200 million years, those early mammals evolved into a wide variety of species, assuming many ecological roles and rising to dominate the Earth as the dinosaurs had before them.
IV THE CURRENT EXTINCTION CRISIS
Most scientists agree that life on Earth is now faced with the most severe extinction episode since the event that drove the dinosaurs extinct. No one knows exactly how many species are being lost because no one knows exactly how many species exist on Earth. Estimates vary, but the most widely accepted figure lies between 10 and 13 million species. Of these, biologists estimate that as many as 27,000 species are becoming extinct each year. This translates into an astounding 3 species every hour.
Instead of global climate change, humans are the cause of this latest mass extinction. With the invention of agriculture some 10,000 years ago, humans began destroying the world's terrestrial ecosystems to produce farmland. Today pollution destroys ecosystems even in remote deserts and in the world's deepest oceans. In addition, we have cleared forests for lumber, pulp, and firewood. We have harvested the fish and shellfish of the world's largest lakes and oceans in volumes that make it impossible for populations to recover fast enough to meet our harvesting needs. And everywhere we go, whether on purpose or by accident, we have brought along species that disrupt local ecosystems and, in many cases, drive native species extinct. For instance, Nile perch were intentionally introduced to Lake Victoria for commercial fishing in 1959. This fish proved to be an efficient predator, driving 200 rare species of cichlid fishes to extinction.
This sixth extinction, as it has come to be known, poses a great threat to our continued existence on the planet. As the sum of all species living in the world's ecosystems, known as biodiversity, dwindles, so too go many of the resources on which we depend. Humans use at least 40,000 different plant, animal, fungi, bacteria, and virus species for food, clothing, shelter, and medicines. In addition, the fresh air we breathe, the water we drink, cook, and wash with, and the many chemical cycles—including the nitrogen cycle and the carbon cycle, so vital to sustain life—depend on the continued health of ecosystems and the species within them.
The list of victims of the sixth extinction grows by the year. Forever lost are the penguinlike great auk, the passenger pigeon, the zebralike quagga, the thylacine, the Balinese tiger, the ostrichlike moa, and the tarpan, a small species of wild horse, to name but a few. More than 15,000 plants and animals are threatened by extinction. Each of these organisms has unique attributes—some of which may hold the secrets to increasing world food supplies, eradicating water pollution, or curing disease. A subspecies of the endangered chimpanzee, for example, has recently been identified as the probable origin of the human immunodeficiency virus, the virus that causes acquired immunodeficiency syndrome (AIDS). But these animals are widely hunted in their west African habitat, and just as researchers learn of their significance to the AIDS epidemic, the animals face extinction. If they become extinct, they will take with them many of the secrets surrounding this devastating disease.
V SPECIES CONSERVATION
In the United States, legislation to protect endangered species from impending extinction includes the Endangered Species Act of 1973. The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), established in 1975, enforces the prohibition of trading of threatened plants and animals between countries. The Convention on Biological Diversity, an international treaty developed in 1992 at the United Nations Conference on the Environment and Development, obligates more than 160 countries to take action to protect plant and animal species.
Scientists meanwhile are intensifying their efforts to describe the species of the world. So far biologists have identified and named around 1.75 million species—a mere fraction of the species believed to exist today. Of those identified, special attention is given to species at or near the brink of extinction. The World Conservation Union (IUCN) maintains an active list of endangered plants and animals called the Red List. In addition, captive breeding programs at zoos and private laboratories are dedicated to the preservation of endangered species. Participants in these programs breed members of different populations of endangered species in an effort to increase their genetic diversity, thus enabling the species to better cope with further threats to their numbers.
All these programs together have had some notable successes. The peregrine falcon, nearly extinct in the United States due to the widespread use of the pesticide DDT, rebounded strongly after DDT was banned in 1973. The brown pelican and the bald eagle offer similar success stories. The California condor, a victim of habitat destruction, was bred in captivity, and small numbers of them are now being released back into the wild.
Growing numbers of legislators and conservation biologists, scientists who specialize in preserving and nurturing biodiversity, are realizing that the primary cause of the current wave of extinction is habitat destruction. Efforts have accelerated to identify ecosystems at greatest risk, including those with high numbers of critically endangered species. Programs to set aside large tracts of habitats, often interconnected by narrow zones or corridors, offer the best hope yet of sustaining ecosystems, and thereby the majority of the world's species (see National Parks and Preserves).
Reviewed By:
Eugenie C. Scott
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Moa
This reconstruction of an extinct South Island giant moa is on display in a New Zealand museum. At one time over 20 species of this large, flightless bird inhabited the wilds of New Zealand. Already rare by the time Europeans arrived in the 18th century, moas became extinct within the next 100 years.
Photo Researchers, Inc./Tom McHugh
