Extinction of Species Essay

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As long as members of a species survive and reproduce themselves, they perpetuate themselves. However, if all the members of a species die, then the species becomes extinct. Extinction is local if a species disappears from a part of its range, but still exists elsewhere. A global extinction is the total disappearance of all the members of a species so that none are left alive on the earth.

Paleontologists have discovered millions of species of plants and animals that experienced extinction in the approximately 550-600 million years that life has existed on earth. The fossil remains of living creatures show clearly that life was teeming on the earth during the Cambrian Era. Many scientists believe that life probably emerged in the chapter of the biography of the earth called the Precambrian Era. However, these were probably soft bodied-fauna and flora. In the absence of shells, skeletons, or hard body parts, they have been lost to the fossil record. What is certain is that in the Cambrian Era, life forms exploded in number. New species seemed to have developed very rapidly.

Despite the presence of life and of numerous species, it is now known from the fossil record that massive deaths of whole species also occurred. Paleontologist, biologists, and other life scientists have estimated that extinction is a fact of biological life. It is estimated that at least 99.7 percent of all the species that have ever lived on earth are now extinct. The law of life is extinction.

Mass Extinctions

There have been an estimated five mass extinctions in the history of life on earth. Fossil evidence strongly suggests that massive dying of species occurred in the Ordovician, Devonian, and Permian geological eras. The most obvious example is the disappearance of the dinosaurs. These mass extinctions have been uncovered by paleontologists as they have examined the fossil record. The pattern has been the development of a few species, then an explosion in the numbers of new species, followed by a period of little change, followed by a deep dip in the number of species. The extinction-causing event is followed by a new period of at first slow development and then an explosion in numerous new species and then another massive loss.

Mass extinctions have been best seen in the fossil record of marine animals. The sedimentary record is clearer because better fossil specimens have been deposited in marine sediment than in other kinds of sediment, and show that the Paleozoic Era of the Cambrian Period was a time of rapid expansion, which was followed by massive extinctions in the Ordovician Era. Approximately 50 percent of the animal families disappeared. This massive die-off included many trilobites.

Species continued to diversify between 500 and 350 million years ago when the Ordovician extinction was followed by a Devonian extinction. At that time, about 30 percent of the species disappeared in animal families. Again many trilobites disappeared, along with many agnathan and placoderm species of fish.

In the Permian Era, about half of all animal families vanished. These included many of the new species that had not been found in the fossil record prior to the Devonian extinction. Swept away were 95 percent of marine species, great numbers of trees, amphibians, most bryozoans and brachiopods and all trilobites. The Triassic extinction occurred about 180 million years ago. It destroyed many reptiles, animal families and many marine mollusks. It is estimated that 35 percent of the animal families disappeared.

The fifth of the natural mass extinctions was the Cretaceous extinction. It was the most destructive and to date most widely known by the public. In the Cretaceous Era, which lasted for 165 million years, the dinosaurs were the rulers of the earth. Then, not slowly or gradually-but suddenly-all of the dinosaurs and most reptiles vanished into the Cretaceous Night. Gone were Tyrannosaurs Rex and all of his prey. Along with the dinosaurs, numerous other species aslo disappeared.

At Odds with Evolutionary Theory

The disappearance of the dinosaurs from the fossil record was disturbing to many geologists, biologists, paleontologists, and others because it violated the idea of evolutionary uniformitarianism. This philosophy of science views evolution as a process in which development of species proceeds very slowly through the workings of natural selection. New species come as each generation breeds its offspring and then allows them to find ways to adapt to nature’s changes.

In the history of geology and biology, the idea of evolution as stimulated by catastrophes became repugnant very early. Eventually, the prevailing scientific orthodoxy became the evolutionary theory of uniformitarianism. The fact of a sudden disappearance suggested a catastrophe for evolution theory, which was resisted by many who felt they had much to lose. Scientific revolutions occur when the anomalies, or data that does not fit the accepted model, become so great that a new model of explanation is needed.

The earth has enormous biodiversity. However, many scientists, biologists, oceanographers, zoologists, and numerous others who work in other scientific disciplines are very concerned that the earth may soon experience a catastrophic lost of great number of species. There are an estimated 10 million plant and animal species known today. Some scientists maintain that as many as 50 times that number may exist. Thus, there is a great danger is that numerous species may disappear before they are even discovered.

Massive Extinction Looming

Polls of biologists, environmentalist, naturalists, environmentalists, and other scientist report great concern that numerous species are not only threatened with extinction, but that a massive extinction is under way. Some have estimated that 20 percent of all species could disappear by 2040, with some estimtes as high as 50,000 each year. Many biologists expect species extinction rates to remain high for at least the next 100 years. Estimates are that 20 percent of the birds, reptiles, and mammals will disappear by the year 2100.

The single major cause of massive species extinction is widely believed to be human activities. These activities include the thinning of the ozone layer, global warming, hunting, farming, mining, pollution from industry, deforestation, logging, the introduction of invasive species, habitat loss, and degradation. Critics charge that the claims of massive extinction are exaggerated and alarmist overestimations derived from extrapolations based on the destruction of rainforests or other rich habitats. When asked in opinion polls, great numbers of lay people do not believe that mass extinction is likely to occur.

The relationship between extinction and human activity goes back thousands of years. At the end of the last Ice Age, about 30,000 years ago, the number of species was the greatest it has ever been in the history of the earth. Among the vast number of species were insects, vertebrates, and flowering plants that were more adapted to the environment than in previous geologic eras. About 10,000 years ago, as continental glaciation ended, there was a massive die-off of large birds and mammals. Smaller mammals were not affected.

This die-off occurred as human became more numerous. Since the rise of humans to global domination there has been an increase in the extinction of species, which began with prehistoric peoples. In the Americas mega(auna (mammals weighing more than 100 pounds) such as wooly mammoths, camels, horses, and saber tooth tigers disappeared after humans arrived on the scene. Speculation suggests that these extinctions were caused by human hunting parties. Also of great importance is the human use of fire for clearing grasslands and forests. In fact, the slash-and-burn method of farming is still practiced by subsistence groups of people in Central and South America.

Not all human activity is immediately destructive. The vast forests of North America were logged over several centuries. However, the logging was always in local areas so that probably half of the forest was standing at one time. As a result, most species have been able to recover as farming and timber operations have changed.

Human activity can also increase the opportunities for survival of some species. Deer and doves prefer broken country where open ground provides cover and a variety of foods. Farming of cereal grains promotes opportunities for both. However, the reforestation in areas that were marginal farmlands has reduced deer and dove habitats, but increased them again for other species.

Studies have shown that the extinction of species began to increase with the European expansion to the New World, Australia, and elsewhere. Some of the cases of extinction were the direct result of the enormous settlements of new lands. Others were due to older population stocks in Europe, Asia, and Africa.

A feature of current extinction rates is that they differ from extinction rates in previous geologic eras. Usually before massive extinctions in the past, there was a great increase in the number of new species. However, the human-caused extinctions are not being matched by the development of new species. In fact, the present rate of human-caused extinctions exceeds by a large margin the natural extinctions of previous geologic eras.

Comparisons of background extinction rates and the present calamitous extinction rates is a revealing exercise. Natural extinction occurs even in the absence of human interference. So it is natural to ask the question: What is the natural extinction rate? How many species will disappear without human involvement?

Examination of the fossil record show that most of the individual species lasted from one to ten million years. With ten million species alive on earth today, the estimate of loss would be one in ten years, a natural background rate of 0.00001 percent per year. Analysis of marine animals has supported this analysis. However, the observed extinction rate among bird and mammals is about 1 percent per century. This currently observed extinction rate is 100-1000 times greater than the background rate. Some scientists have argued against the validity of these estimates of extinction, while others have used more conservative methods and have arrived at extinction rates ranging from 36-75 times those of the natural background rate.

The graver danger to endemic species (those found in one location and nowhere else) is the arrival of invasive species and of humans. Endemic species have a high risk of extinction in the face of invasive species. For example, island species have evolved in a limited location against a limited set of challenges. Small numbers of a species can easily be exterminated or decimated by invasive species from the mainland.

National parks and nature reserves are really habitat islands surrounded by hostile seas of unsuitable habitat. If the areas around a national park remain undeveloped, then the habitat space is widened. However, if suburban development engulfs a national park, even those kept as battlefield monuments, the species inside are squeezed into smaller habitats. Inevitably, species losses will occur.

Fragmenting forests or other wild areas into small island habitats is eventually destructive of species. Studies of extinction rates on islands have shown that when 50 percent of the habitat is destroyed, 10 percent of the island’s species will also die out. However, increasing natural habitat to a state of 90 percent destroyed will result in the loss of 50 percent of the original native species.

Very vulnerable to habitat loss and species destruction are the world tropical forests. Plants and insects account for a large percentage of tropical forest species. Estimating their survival is difficult, but estimates are that at present rate of deforestation, up to 15 percent of plant species will become extinct by 2000. In Brazil, the Amazon is being cleared at a staggering rate. In some places, virtual warfare exists between those attacking the forests in order to clear land for cattle ranches and those seeking to preserve the natural habitat. Estimates are that 12 percent of the birds will go extinct in the coming decades.

Bibliography:

  1. Michael Benton, When Life Nearly Died: The Greatest Mass Extinction of All Time (Thames & Hudson, 2005);
  2. Tim W. Clark, Averting Extinction: Reconstructing Endangered Species (Yale University Press, 1997);
  3. Douglas H. Erwin, Extinction: How Life on Earth Nearly Ended 250 Million Years Ago (Princeton University Press, 2006);
  4. Stephen Jay Gould,Wonderful Life: The Burgess Shale and the Nature of History (W. W. Norton, 1999);
  5. Tony Hallam and Hallam, Catastrophes and Lesser Calamities: The Causes of Mass Extinctions (Oxford University Press, 2005);
  6. John Charles Kunich, Killing Our Oceans: Dealing with the Mass Extinction of Marine Life (Greenwood Publishing, 2006);
  7. Lawrence George Lux, Extinction of Species (Xlibris Corporation, 2001);
  8. Paul S. Martin, Twilight of the Mammoths: Ice Age Extinctions and the Rewilding of America (University of California Press, 2005);
  9. George McGavin, and David Burnie, Endangered: Wildlife on the Brink of Extinction (Firefly Books, 2006);
  10. Beverly Peterson, Watching, from the Edge of Extinction (Yale University Press, 1999);
  11. James Lawrence Powell, Night Comes to the Cretaceous: Comets, Craters, Controversy and the Last Days of the Dinosaurs (Harcourt Brace & Company, 1998);
  12. Richard B. Primack, Essential of Conservation Biology (Sinauer Associates, 2002);
  13. David Quammen and Kris Ellingsen, Song of the Dodo: Island Biogeography in the Age of Extinctions (Simon & Schuster, 1997);
  14. William Schopf, Cradle of Life: The Discovery of Earth’s Earliest Fossils (Princeton University Press, 1999);
  15. David W. Steadman, Extinction and Biography of Tropical Pacific Birds (University of Chicago Press, 2006);
  16. Malcolm Tate, Going, Going, Gone: Animals on the Brink of Extinction and How to Turn the Tide (Think Books, 2006);
  17. Peter Ward, Out of Thin Air: Dinosaurs, Birds and Earth’s Ancient Atmosphere (National Academies Press, 2006);
  18. Peter Douglas Ward, Rivers in Time: The Search for Clues to Earth’s Mass Extinctions (Columbia University Press, 2002).

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