Extinction

Related Topics:
Extinction

In biology and ecology, extinction is the ceasing of existence of a species or group of taxa. The moment of extinction is generally considered to be the death of the last individual of that species. Prior to the dominance of the earth by humans, extinction was a natural phenomenon involving a long term timing cycle; Starting about 100,000 years ago, an unprecedented rate of species extinctions commenced. A mass extinction summary lecture from the University of North Carolina at Chapel Hill estimates that living creations are drawn from only fifty billion species, but that fifty billion species may have lived on the planet. It estimates a background extinction rate (aside from the mass extinctions) at two to four families per million years. The American Museum of Natural History says that scientists estimate that "at least" 99.9% of all species of plants and animals that have ever lived are now extinct. The Permian-Triassic extinction alone killed off about 90 percent of marine species and 70 percent of the terrestrial vertebrate species then alive. Through evolution, new species are created by speciation — where new organisms arise and thrive when they are able to find and exploit an ecological niche — and species become extinct when they are no longer able to survive in changing conditions or against superior competition. A typical species becomes extinct within 10 million years of its first appearance, although some species survive virtually unchanged for hundreds of millions of years.

Descendants may or may not exist for extinct species. Daughter species that evolve from a parent species carry on most of the parent species' genetic information, and even though the parent species may become extinct, the daughter species lives on. In other cases, species have produced no new variants, or none that are able to survive the parent species' extinction. Extinction of a parent species where daughter species or subspecies are still alive is also called pseudoextinction. However, pseudoextinction is difficult to demonstrate unless one has a strong chain of evidence linking a living species to members of a pre-existing species. For example, it is sometimes claimed that the extinct Hyracotherium, which was an ancient animal similar to the horse, is pseudoextinct, rather than extinct, because there are several extant species of horse, including zebra and donkeys. However, as fossil species typically leave no genetic material behind, it's not possible to say whether Hyracotherium actually evolved into more modern horse species or simply evolved from a common ancestor with modern horses. Pseudoextinction is much easier to demonstrate for larger taxonomic groups. For example, it could be said that dinosaurs are pseudoextinct, because some of their descendants, the birds, survive today.

Currently, environmental groups and some governments are concerned with the extinction of species due to human intervention, and are attempting to combat further extinctions. Humans can cause extinction of a species through overharvesting, pollution, destruction of habitat, introduction of new predators and food competitors, and other influences. According to the World Conservation Union (WCU, also known as IUCN), 784 extinctions have been recorded since the year 1500, the arbitrary date selected to define "modern" extinctions, with many more likely to have gone unnoticed. Most of these modern extinctions can be attributed directly or indirectly to human effects. Endangered species are species that are in danger of becoming extinct; several organizations attempt to preserve recognized endangered species through a variety of conservation programs.

Species which are not extinct are termed extant.

Definition

A species becomes extinct when the last existing member of that species dies. Extinction therefore becomes a certainty when no surviving specimens are able to reproduce and create a new generation. A species may become functionally extinct when only a handful of individuals survive, which are unable to reproduce due to health, age, lack of both sexes (in species that reproduce sexually), or other reasons.

In addition to actual extinction, human attempts to preserve critically endangered species have caused the creation of the conservation status extinct in the wild. Species listed under this status by the WCU are not known to have any living specimens in the wild, and are maintained only in zoos or other artificial environments. Some of these species are functionally extinct. When possible, modern zoological institutions attempt to maintain a viable population for species preservation and possible future reintroduction to the wild through use of carefully planned breeding programs.

Pinpointing the extinction or pseudoextinction of a species requires a clear definition of that species. The species in question must be identified uniquely from any daughter species, as well as its ancestor species or other closely related populations, if it is to be declared extinct. For further discussion, see definition of species.

Extinction (or replacement) of species by a daughter species plays a key role in the punctuated equilibrium hypothesis of Stephen Jay Gould and Niles Eldredge. See: Niles Eldredge, Time Frames: Rethinking of Darwinian Evolution and the Theory of Punctuated Equilibria, 1986, Heinemann ISBN 0434226106

In ecology, extinction is often used informally to refer to local extinction, in which a species ceases to exist in the chosen area of study, but still exists elsewhere. This phenomenon is also known as extirpation.

Permanence

Until recently, it had been universally accepted that the extinction of a species meant the end of its time on Earth. However, recent technological advances have encouraged the hypothesis that through the process of cloning, extinct species may be "brought back to life." Proposed targets for cloning include the mammoth and thylacine. In order for such a program to succeed, a sufficient number of individuals would need to be cloned (in the case of sexually reproducing organisms) to create a viable population size. The cloning of an extinct species has not yet been attempted, due to technological limitations, as well as ethical and philosophical questions.

This concept was fictionalized in the popular novel and movie Jurassic Park.

Causes

There are a variety of causes that can contribute directly or indirectly to the extinction of a species or group of species. Most simply, any species that is unable to survive or reproduce in its environment, and unable to move to a new environment where it can do so, dies out and becomes extinct. Extinction of a species may come suddenly when an otherwise healthy species is wiped out completely, as when toxic pollution renders its entire habitat unlivable; or may occur gradually over thousands or millions of years, such as when a species gradually loses out competition for food to newer, better adapted competitors. Around three species of birds die out every year due to competition.

Genetic and demographic causes

Genetic and demographic phenomena affect the evolution, and therefore extinction, of species. Regarding the possibility of extinction, small populations which represent an entire species are much more vulnerable to these types of effects.

Natural selection acts to propagate beneficial genetic traits and eliminate weaknesses. However, it is sometimes possible for a deleterious mutation to be spread throughout a population through the effect of genetic drift.

A diverse or "deep" gene pool gives a population a higher chance of surviving an adverse change in conditions. Effects that cause or reward a loss in genetic diversity can increase the chances of extinction of a species. Population bottlenecks can dramatically reduce genetic diversity by severely limiting the number of reproducing individuals and make inbreeding more frequent. The founder effect can cause rapid, individual-based speciation and is the most dramatic example of a population bottleneck.

Habitat degradation

The degradation of a species' habitat may alter the fitness landscape to such an extent that the species is no longer able to survive and becomes extinct. This may occur by direct effects, such as the environment becoming toxic, or indirectly, by limiting a species' ability to compete effectively for diminished resources or against new competitor species.

Habitat degradation through toxicity can kill off a species very rapidly, by killing all living members through contamination or sterilizing them. It can also occur over longer periods at lower toxicity levels by affecting life span, reproductive capacity, or competitiveness.

Habitat degradation can also take the form of a physical destruction of niche habitats. The widespread destruction of tropical rainforests and replacement with open pastureland is widely cited as an example of this; elimination of the dense forest eliminated the infrastructure needed by many species to survive. For example, a fern that depends on dense shade for protection from direct sunlight can no longer survive with no forest to house it.

Diminished resources or introduction of new competitor species also often accompany habitat degradation. Global warming has allowed some species to expand their range, bringing unwelcome competition to other species that previously occupied that area. Sometimes these new competitors are predators and directly affect prey species, while at other times they may merely outcompete vulnerable species for limited resources.

Vital resources including water and food can also be limited during habitat degradation, causing some species to become extinct.

Predation, competition, and disease

Humans have been transporting animals and plants from one part of the world to another for thousands of years, sometimes deliberately (e.g., livestock released by sailors onto islands as a source of food) and sometimes accidentally (e.g., rats escaping from boats). In most cases, such introductions are unsuccessful, but when they do become established as an invasive alien species, the consequences can be catastrophic. Invasive alien species can affect native species directly by eating them, competing with them, and introducing pathogens or parasites that sicken or kill them or, indirectly, by destroying or degrading their habitat.

Coextinction

Effects

Mass extinctions

There have been at least five mass extinctions in the history of life, and four in the last 3.5 billion years in which many species have disappeared in a relatively short period of geological time. These are covered in more detail in the article on extinction events. The most recent of these, the K-T extinction 65 million years ago at the end of the Cretaceous period, is best known for having wiped out the non-avian dinosaurs, among many other species.

According to a 1998 survey of 400 biologists conducted by New York's American Museum of Natural History, nearly 70 percent of biologists believe that we are currently in the early stages of a human-caused mass extinction, known as the Holocene extinction event. In that survey, the same proportion of respondents agreed with the prediction that up to 20 percent of all living species could become extinct within 30 years (by 2028). Biologist E.O. Wilson estimated Biologist E.O. Wilson, Professor Emeritus and honorary curator of entomology at Harvard University, in his 2002 book The Future of Life (ISBN 0679768114). See also: The Sixth Extinction : Patterns of Life and the Future of Humankind by Richard Leakey ( ISBN 0385468091 ). in 2002 that if current rates of human destruction of the biosphere continue, one-half of all species of life on earth will be extinct in 100 years.

Human extinction

Human extinction refers to the possibility that the human species may become extinct. Although humans are relatively intelligent and adaptive, they could cause an extinction through their own actions, or allow a natural extinction event to occur through negligence. Humans can also possibly die out because of an event that they are incapable of preventing, such as another meteor strike.

Human attitudes on extinction

Planned extinction

Olivia Judson is one of few modern scientists to have advocated the deliberate extinction of any species. Her controversial 2003 NY Times article advocates "specicide" of thirty mosquito species through the introduction of recessive "knockout genes". Her defense of such an extreme measure rests on:

Anopheles mosquitoes and Aedes mosquito represent only 30 species; eradicating these would save at least one million human lives per annum at a cost of reducing the genetic diversity of the family Culicidae by only 1%.
She writes that since species go extinct "all the time" the disappearance of a few more will not destroy the ecosystem: "We're not left with a wasteland every time a species vanishes. Removing one species sometimes causes shifts in the populations of other species -- but different need not mean worse."
Anti-malarial & mosquito control programs offer little realistic hope to the 300 million people in developing nations who will be infected with acute illnesses this year; although trials are ongoing she writes that if they fail: "We should consider the ultimate swatting." The pro-specicide article's full fair use text is available on the animana pages (a socioecology blog).

Humans have aggressively worked towards the extinction of many species of virus and bacterium in the cause of disease eradication. For example, the smallpox virus is now essentially extinct in the wild - although samples are retained in laboratory settings, the polio virus is now confined to small parts of the world as a result of human efforts to cure the disease it causes.

Scientists

Although scientists are generally opposed to future extinctions they have found historic extinctions very useful for research; in the early nineteenth century Georges Cuvier's observations of fossil bones convinced him that they did not originate in extant animals. This discovery was critical for the spread of uniformitarianism The prologue of Ideas: A History from Fire to Freud (Peter Watson Weidenfeld & Nicolson ISBN 029760726X) makes this connection (on page 16), and says that there had been the hope that some of these extinct species would be found in undiscovered parts of the earth before the huge variety of prehistoric life was uncovered. and lead to the first book publicizing the idea of evolution. Robert Chambers, 1844, Vestiges of the Natural History of Creation, 1994 reprint: University of Chicago Press ISBN 0226100731

Commercial and industrial interests

When commercial technologies are tested the testing tends to concentrate on human effects. However, some technologies with no proven harmful effects on Homo sapiens can be devastating to wildlife (most famously DDT). In extreme case these new processes can in themselves cause unintended extinctions as a side-effect of business operations. Although most companies were formerly more concerned with bottom-line profits than corporate image, a move began (under campaign pressure) to account for corporate reputational risk from such environmental catastrophes.

Governments and international organizations

Governments sometimes see the loss of native species as a loss to ecotourism, and can enact laws with severe punishment against the trade in native species (in an effort to prevent extinction in the wild). Some endangered species are considered symbolically important.

Indigenous populations

People who live close to nature can be dependent on the survival of all the species in their environment and are some of the people who should be most concerned about extinction risks. However with the human overpopulation in tropical lesser developed countries, there has been enormous pressure on forests due to subsistence agriculture and imprudent use of slash-and-burn agricultural techniques. As a result the indigenous populations have placed day to day survival before species conservationPaul Ehrlich and Anne Ehrlich, Extinction, Random House, New York (1981) ISBN 0-394-51312-6

Endangered species

References

Notes

External links

A mathematical model for mass extinction
Species disappearing at an alarming rate (MSNBC)
Red List of Threatened Species
The Wildlands Project takes action on wildlife conservation to address the extinction crisis in the Americas.
The Extinction Website
Extinction forum
Committee on recently extinct organisms
ZMA - Threatened and Extinct Birds in the collection of the Zoological Museum Amsterdam (Netherlands). Also includes 3D images of the type specimens.
Naturalis - Extinct Birds: 3D images of extinct bird species in the collection of the National Museum of Natural History (Leiden, Netherlands).

Ecology | Evolutionary biology | Extinction | Environmental threats

Gourt Home::Gourt :: Extinction