Extinction

For other uses of "Extinction", see Extinction (disambiguation).

In biologyand ecology, extinction is the ceasing of existence of a speciesor group of taxa. The moment of extinction is generally considered to be the death of the last individual of that species. Extinction is usually a natural phenomenon; it is estimated that more than 99.9% of all species that have ever lived are now extinct^{[{{fullurl:Template:FULLPAGENAME}}#endnote_999Extinct]}. 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 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 (an ancient horse-like animal) is pseudoextinct, rather than extinct, because there are several extantspecies of horse, including zebraand donkeys. However, as fossil species typically leave no genetic material behind, it is 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, many environmental groups and 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 predatorsand 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 speciesare species that are in danger of becoming extinct; several organizations attempt to preserve recognized endangered species through a variety of conservationprograms.

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 statusextinct 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 zoosor other artificial environments. Some of these species are functionally extinct. When possible, modern zoological institutions attempt to maintain a viable populationfor species preservation and possible future reintroductionto 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 equilibriumhypothesis of Stephen Jay Gouldand Niles Eldredge.^{[{{fullurl:Template:FULLPAGENAME}}#endnote_TimeFrames]}

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 mammothand 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 ethicaland philosophicalquestions.

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 surviveor reproducein 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 populationswhich represent an entire species are much more vulnerable to these types of effects.

Natural selectionacts 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 poolgives a population a higher chance of surviving an adverse change in conditions. Effects that cause or reward a loss in genetic diversitycan increase the chances of extinction of a species. Population bottleneckscan dramatically reduce genetic diversity by severely limiting the number of reproducing individuals and make inbreedingmore frequent. The founder effectcan cause rapid, individual-based speciation and is the most dramatic example of a population bottleneck.

Habitat degradation

The degradation of a species' habitatmay alter the fitness landscapeto 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 contaminationor sterilizingthem. It can also occur over longer periods at lower toxicity levels by affecting life span, reproductive capacity, or competitiveness. DDTplayed such a role in killing off bald eaglesand other birds by thinning the eggshell walls of affected birds, thus lowering the survivability of offspring. Since this effect was discovered, DDT has been banned in many parts of the world and affected bird populations are recovering.

Habitat degradation can also take the form of a physical destruction of niche habitats. The widespread destruction of tropical rainforestsand replacement with open pastureland is widely sited as an example of this; elimination of the dense forest eliminated the infrastructure needed by many species to survive. For example, a fernthat 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 warminghas 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 waterand food can also be limited during habitat degradation, causing some species to become extinct.

Predation, competition, and disease

Coextinction

Main article: Coextinction

Effects

Mass extinctions

Main article: Mass extinction

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 extinction65 million years ago at the end of the Cretaceousperiod, is best known for having wiped out the non-aviandinosaurs, among many other species.

According to a 1998survey 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. Wilsonestimated^{[{{fullurl:Template:FULLPAGENAME}}#endnote_Wilson]} in 2002that 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

Main article: Human extinction

Human extinction refers to the possibility that the humanspecies 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

Extinction in species other than Homo sapiens

Because humans are a reasonably intelligent species capable of abstract thought, the views on the extinction of species other than Homo sapiens tend to be diverse, ranging from the preservative views of the most vehement eco-warriorto general apathy in the matter. However, it is seen that humans as a whole generally will act to protect against the extinction of species (primarily species that have a direct impact on human agriculture, such as cattle), if only to further preserve and perpetuate the human species. This is evidenced by organizations such as the United States Environmental Protection Agency(http://www.epa.gov), created by RepublicanPresident Richard Nixon.

Extinction in the species Homo sapiens

In a normal human being, a very strong instinctual urge to survive and, upon puberty and physical development, procreate (or at the very least have sexual contact, which normally leads to procreation) is prominent. Because of this, the general consensus of the human species can be presumed to be against the extinction of humankind. The natural urge to both survive as long as possible and mate can be and often is overcome by things such as personal, organizational, or religious beliefs, such as is the case with the Voluntary Human Extinction Movement (http://www.vhemt.org), by mental illness(possibly leading up to suicideand thus a zero percent chance of both living and continued mating), or sometimes by trained willpowerstrong enough to overcome the natural functions of the brain. Laws against genocidehave been ratified to help prevent human extinction as a direct result of unnatural death caused by another human being or group of human beings.

Planned extinction

Olivia Judsonis one of few modern scientists to have advocated the deliberate extinction of any species. Her controversial2003NY Timesarticle advocates "specicide" of thirty mosquitospecies through the introduction of recessive"knockout genes". Her defense of such an extreme measure rests on:

• Anophelesmosquitoes and Aedesmosquito represent only 30 species; eradicating these would save at least one million human lives per annum at a cost of reducing the genetic diversityof the familyCulicidaeby 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 programsoffer little realistic hope to the 300 million people in developing nationswho will be infected with acute illnesses this year; although trials are ongoing she writes that if they fail: "We should consider the ultimate swatting."^{[{{fullurl:Template:FULLPAGENAME}}#endnote_specicide]}

Scientists

Although scientists are generally opposed to future extinctions they have found historic extinctions very useful for research; in the early nineteenth centuryGeorges 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^{[{{fullurl:Template:FULLPAGENAME}}#endnote_Ideas16]} and lead to the first book publicizing the idea of evolution^{[{{fullurl:Template:FULLPAGENAME}}#endnote_Vestiges]}.

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 profitsthan corporate image, a move began (under campaign pressure) to account for corporate reputational riskfrom such environmental catastrophes.

The most recent example of the potential clash between industrial technology and species survival is in the wireless communicationrevolution. Although extensive studies have rejected the hypothesis that low-level exposure to microwave-frequency communication is harmful to human health (or is not statistically significant) few studies have been carried out on the effects of microwaves in other life forms. For instance, the dielectric heatingstudies are limited to their effects on human beings. Currently 1mW/cm² is taken to be the acceptable power density of such radiation. The most glaring omission is the absence of any study on other living species, and hence on the existing fragile eco-system.

The rapidly dwindling species of dragon-flies, butterfliesand fireflies from densely populated areas (where satellite foot-prints and mobile communication infrastructures are plentiful, Indiaor Chinafor instance) has been blamed by some on the rise of the mobile phone, although in the absence of studies, this is speculative.

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 most concerned about extinction risks.

Endangered species

Main article: Endangered species

See also

• Coextinction
• List of extinct animals
• Biodiversity
• Lazarus taxon
• Unified neutral theory of biodiversity
• Population genetics

External links

• A mathematical model for mass extinction
• Species disappearing at an alarming rate (MSNBC)
• Red List of Threatened Species
• The Wildlands Projecttakes action on wildlifeconservation to address the extinction crisisin the Americas.
• The Extinction Website
• Extinction forum
• Committee on recently extinct organisms
• ZMA - Threatened and Extinct Birdsin 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).

References

• Mass Extinction Underway | Biodiversity Crisis | Global Species Loss
• Univ. of Arizona lecture on extinction

Notes

^  A mass extinction summary lecturefrom the University of North Carolina at Chapel Hillestimates 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 2-4 familiesper million years. The American Museum of Natural Historysays that scientists estimate that "at least" 99.9% of all species of plantsand animals that have ever lived are now extinct. The Permian-Triassic extinctionalone killed off about 90 percent of marine species and 70 percent of the terrestrial vertebrate species then alive.

^  Biologist E.O. Wilson, Professor Emeritus and honorary curator of entomologyat Harvard University, in his 2002book The Future of Life (ISBN 0679768114). See also: The Sixth Extinction : Patterns of Life and the Future of Humankind by Richard Leakey ( ISBN 0385468091).

^  See: Niles Eldredge, Time Frames: Rethinking of Darwinian Evolution and the Theory of Punctuated Equilibria, 1986, Heinemann ISBN 0434226106

^  The pro-specicide article's full fair usetext is available on the animana pages (a socioecology blog).

^  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.

^  Robert Chambers, 1844, Vestiges of the Natural History of Creation, 1994reprint: University of Chicago Press ISBN 0226100731da:Uddød de:Aussterben es:Extinción fr:Extinction ja:絶滅 nl:Extinctie pt:Extinção sv:Utrotad zh:灭绝

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It uses material from the http://en.wikipedia.org/wiki/Extinction Wikipedia article Extinction.