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Principles of Sustainability

Extinctions and Species Loss

by Toby Stewart, Dion Stewart, Karen N. Kähler

This species of pigeon numbered in the millions in the late 1800s. Market hunting drove the pigeon to extinction. The meat was sold to restaurants and their feathers used for hats. (US Fish and Wildlife Service)

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Summary

Extinction is the complete local, regional, or global die-off of a species. The reduction in biodiversity due to extinction is known as species loss. Although extinctions occur naturally, human activities have accelerated the extinctions of some species. Early humans have been implicated in the complete disappearance of some prehistoric species, and in recent centuries, the increasing influences of human population growth and technology on the biological ecosystems of the world have caused extinction rates to skyrocket to thousands of times the background rate.

Fields of Study

Biology, Life Sciences, Ecosystems; Ecology, Environment, Environmentalism; Oceans, Seas, Lakes, Bodies of Water; Pollution, Emissions, Toxic Waste, Hazardous Materials; Water Resources

In general, the term “extinction” refers to global extinction, the elimination of a species from the earth, but species can also be said to be extinct locally or regionally. The term “extirpation” is sometimes used to describe local or regional extinction. By some estimates, before humans arrived on the scene animal and plant species disappeared from the planet at an average rate of about one per year. According to the International Union for Conservation of Nature and Natural Resources (IUCN), the modern extinction rate is anywhere from one thousand to ten thousand or more times higher than the expected natural rate.

The fossil record indicates that throughout the history of life on the earth extinctions have resulted primarily from the planet’s changing surface and climate. The rates of speciation (the creation of new species) and extinction have remained fairly constant over millions of years. However, five mass extinctions have occurred over the course of the earth’s history. The most famous, if not the largest, extinction event occurred 65 million years ago at the end of the Mesozoic era, when a large number of land and marine animals, including the dinosaurs, suddenly disappeared from the fossil record. Some scientists credit this extinction to a large meteor that hit the earth and subsequently caused climate changes that devastated some life-forms. Other scientists credit the demise of dinosaurs to the evolutionary superiority of mammals. The current accelerated loss of species is sometimes called the “sixth extinction crisis.” It is the only mass extinction attributable to a single species: human beings.

Causes of Recent Extinctions

As human populations grow, people clear forests and land at increasing rates to build homes, businesses, farms, and infrastructure. This process disrupts the habitats that are home to microbes, plants, and animals, which all have unique ecological relationships to one another. Habitat destruction is one of the most prominent causes of extinctions in recent history. With the destruction of one species, there is the potential for the secondary extinction of other species that are dependent on the first species for reproduction or food. For instance, a bird species may disperse seeds by eating the fruit of a plant and expelling the seeds in its droppings as it travels through its home range. If the bird dies off, the plant loses an important means of propagation. If a prey species becomes extinct, predators may not be able to find enough food to survive. If a major predator becomes extinct, there may be a population explosion among prey species, followed by a catastrophic depletion of food sources for those species and a subsequent die-off.

Highway construction and other development can cause fragmentation of a habitat, preventing migration between the two fragments. This can cause a species to die out if neither area is capable of supporting a viable population of the species. This is also true of habitats that are decreasing in size. In the United States at the close of the twentieth century, 99.8 percent of the nation’s tallgrass prairies had been destroyed, 50 percent of the wetlands were gone, and 98 percent of virgin and old-growth forests had been cut. At least five hundred native species had become extinct, with tremendous losses in the populations of animals such as wolves, black bears, bison, and cougars.

Tropical rain forests are home to anywhere from one-half to two-thirds of the world’s plants and animals. By some calculations, tropical rain forests lose approximately four to six thousand or more species per year because of deforestation. Many of the lost species are unique to the tropical forests. This destruction has also contributed to a serious decline in the numbers of many migratory songbirds that winter in Central and South America. The songbirds’ northern summer habitats are becoming fragmented as well, causing more losses in their populations.

Even when a species is not completely obliterated by development, extinction can occur if the population size becomes too small to recover. Such a population is said to fall into an extinction vortex. This problem may occur if there are too few females left in the population to breed, or, if the habitat is too fragmented, if individuals are not able to locate partners with which to mate. If enough mates cannot be found, genetic inbreeding can destroy the viability of the species. For instance, a 2009 study of moss carder bumblebees living on the Hebridean islands off the west coast of Scotland found that many generations of inbreeding had left the insects more susceptible to parasitic disease than their mainland counterparts. Inbreeding among these bumblebees had also resulted in an increase in infertile males.

Small populations that are vulnerable to environmental fluctuations may also fall into an extinction vortex. For example, extremely harsh winter weather can wipe out an entire species that has already been reduced to a small population. Larger groups, by contrast, are better able to survive adversity. The chance of being destroyed by environmental fluctuations increases exponentially with decreasing population size.

The smallest population of a species that is able to stay above the extinction vortex is often called the minimum viable population (MVP). If a population declines below this size, it is usually only a matter of time before breeding problems and climatic fluctuations will destroy the whole population. Likewise, if a habitat is reduced to a point where it is unable to support the MVP in an adverse year, the population will vanish.

Pollution, Overuse, and Overhunting

Pollution can kill many plants and animals and at the same time alter and destroy habitats. Acid rain and air pollution are detrimental to forests and forest animals. Sediment and excessive nutrients that run off into lakes, rivers, and bays often have adverse effects on aquatic life. Pesticides that persist in the environment, such as Dichloro-diphenyl-trichloroethane (DDT), have caused large losses and near extinction among some birds, notably meat- and fish-eating species such as bald eagles, ospreys, peregrine falcons, and brown pelicans. These species were particularly susceptible to DDT because the effects of chemicals are amplified as they become increasingly concentrated in the fatty tissue of organisms at higher trophic levels in the food chain.

Exotic species are animals or plants that have been introduced into an area to which they are not native. It has been estimated that the introduction of exotic species has contributed to approximately 40 percent of all animal extinctions worldwide since 1600. Because the new exotic species may not have any natural predators or competitors in its new habitat, it can dominate the new ecosystem and reduce the populations of many native species. Islands are particularly vulnerable to invasive exotic species. The brown tree snake, a mildly venomous reptile, was introduced to the Pacific island of Guam during World War II, probably as an accidental stowaway on a military cargo ship from Papua New Guinea. By the end of the century, the snakes had decimated most of Guam’s native species of birds and small mammals and reptiles. Among the island’s native fauna, nine of eighteen bird species, two of three bat species, and four of ten lizard species had been extirpated, and more had become uncommon or rare. Because the snake has no natural predator on Guam, and overall lizard populations have remained high enough to provide the snakes with an ample food supply, in some areas the snake has maintained population densities of nearly thirteen thousand individuals per square mile.

During the nineteenth century whales were harvested at a rate of fewer than one hundred every three years. With the development of faster boats and more efficient weapons, however, by 1933 the whaling industry was killing thirty thousand whales per year; by 1967 that number had more than doubled. It is interesting to note that 2.5 million barrels of whale oil were harvested in 1933, as opposed to only 1.5 million barrels in 1967. This is because the larger whales, the blues and fins, had been hunted to the brink of extinction by 1967. An international whaling moratorium observed by many nations since 1986 has allowed the partial recovery of some species. However, many may lack the genetic diversity to thrive in their pollution-stressed environment.

In the United States, overhunting has led to the extinction or near extinction of many species, such as the American bison. Early in the nineteenth century the passenger pigeon was one of the most abundant birds on earth. Alexander Wilson, a renowned ornithologist, is said to have observed a flock of passenger pigeons that took several hours to fly by him. He estimated that the flock, which appeared to be approximately 1.6 kilometers (1 mile) wide and 386 kilometers (240 miles) long, was composed of two billion birds. The passenger pigeon became extinct in 1914, largely because of overhunting by market hunters. The hunters used nets, guns, and even dynamite to trap the birds, which were viewed as a culinary delicacy.

Many species are under government and international protection but are still hunted because the economic incentive of selling skins, horns, or bone outweighs the risk of a small fine or short prison sentence. In the early twenty-first century, a Bengal tiger skin can fetch $35,000 on the black market, and rhinoceros horns used in traditional medicines and aphrodisiacs can sell for as much as $1,200 per kilogram. Bushmeat, often from endangered primates, provides protein to isolated populations in Central Africa, but the demand for illegal bushmeat extends to large cities in Africa, Europe, and the United States, where thousands of pounds of bushmeat are confiscated every year. A poacher can earn millions of dollars annually.

Pest Control, Monocultures, and Wild Plants

Species can become endangered or even extinct if they must compete with the human population for food. Farmers in Africa have killed elephants to prevent the elephants from eating or trampling food crops. In the United States the Carolina parakeet was exterminated by farmers in 1914 because it fed on fruit crops, and 98 percent of the nation’s prairie dog population has been exterminated with poisons so that horses and cattle do not break their legs stepping into the rodents’ burrows. In turn, the prairie dog’s primary predator, the black-footed ferret, has come close to extinction with the massive reduction of the population of its food source.

Extinctions and species losses are critical problems for the human population because human life is dependent on the biodiversity of species. One area in which this is apparent is agriculture. Of the world’s estimated seventy-five hundred species of edible plants, farmers have developed high-yielding monocultures of only about thirty species, each with a minimum of genetic variation. These monocultures lack the vigor of wild plants, which are constantly developing new ways to adapt to adverse conditions and fend off the animals and microorganisms that attack them. When a monoculture crop fails because of disease or other problems, plant breeders must go back to wild species to find the traits that their crop needs to thrive and breed these characteristics into the crop species. If wild species are not protected, their gene pools will dwindle and the species will eventually disappear.

Wild plants, especially in the tropical rain forests, have important uses as medicine. In 1960 the rosy periwinkle, a shrub that grows in Madagascar, was found to contain two chemicals that revolutionized the treatment of childhood leukemia and Hodgkin’s disease. The use of these drugs brought about a 95 percent remission rate in children who previously had little chance of surviving leukemia. Paclitaxel (originally known as taxol), a drug extracted from the bark of the Pacific yew tree, has been valuable in the treatment of Kaposi’s sarcoma and breast, ovarian, and small-cell cancers. A plant related to the periwinkle, rauwolfia, provides an alkaloid that has been widely used in the control of high blood pressure. Digitalis, which comes from the foxglove plant, is a highly effective drug used for centuries in the treatment of various heart conditions. Of the world’s estimated 400,000 plant species, only a few thousand have been studied for medicinal use. It is believed that the world’s tropical forests may be home to thousands of plants that have cancer-fighting properties. Some 25 percent of the active ingredients in the anticancer drugs currently in use come from organisms native to rain forests.

Bibliography

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Citation Types

Type
Format
MLA 9th
Stewart, Toby, and Dion Stewart, and Karen N. Kähler. "Extinctions And Species Loss." Principles of Sustainability, edited by Lerner Media Group, Salem Press, 2017. Salem Online, online.salempress.com/articleDetails.do?articleName=POSustain_0066.
APA 7th
Stewart, T., & Stewart, D., & Kähler, K. N. (2017). Extinctions and Species Loss. In L. M. Group (Ed.), Principles of Sustainability. Salem Press. online.salempress.com.
CMOS 17th
Stewart, Toby and Stewart, Dion and Kähler, Karen N. "Extinctions And Species Loss." Edited by Lerner Media Group. Principles of Sustainability. Hackensack: Salem Press, 2017. Accessed December 14, 2025. online.salempress.com.