Extinction

In biology and ecology, extinction is the ceasing of existence of a species or group of taxons. The moment of extinction is generally considered to be the death of the last individual of that species. Extinction is a natural phenomenon; it is estimated that 99.9% of all species that have ever lived are now extinctTemplate:Ref. In the theory of 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. Many of prehistoric extinct species have evolved into new species; for example the extinct Eohippus (an ancient horse-like animal) was the ancestor of several extant species including the horse, the zebra and the donkey. The Eohippus itself is no more, but its descendants live on. It is therefore said to be pseudoextinct.

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 predators, and other influences. According to the World Conservation Union (WCU, formerly the 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.

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.

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. DDT played such a role in killing off bald eagles and other birds by thinning the egg shell 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 rainforests and 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 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

Coextinction

Main article: Coextinction

Effects

Mass extinctions

Main article: Mass extinction

There have been at least five mass extinctions in the history of life, 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-feathered dinosaurs, called 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 estimatedTemplate:Ref 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.

Mass extinctions are a key part of the punctuated equilibrium hypothesis of Stephen Jay Gould and Niles Eldredge.Template:Ref

Human extinction

Main article: 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 of the 2,700 species on Earth, 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."Template:Ref

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 Template:Ref and lead to the first book publicizing the idea of evolutionTemplate:Ref.

Environmental movement

The environmental movement is strongly opposed to further extinction, and generally sees habitat preservation as preferable to prevention of extinction through captive breeding programs.

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.

The most recent example of the potential clash between industrial technology and species survival is in the wireless communication revolution. 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 heating studies 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, butterflies and fireflies from densely populated areas (where satellite foot-prints and mobile communication infrastructures are plentiful, India or China for 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

External links

A mathematical model for mass extinction (http://www.lassp.cornell.edu/newmme/science/extinction.html)
Species disappearing at an alarming rate (MSNBC) (http://www.msnbc.msn.com/id/6502368/)
Red List of Threatened Species (http://www.iucnredlist.org/)
The Wildlands Project (http://www.wild-earth.org/cms/page1090.cfm) takes action on wildlife conservation to address the extinction crisis in the Americas.

References

Mass Extinction Underway | Biodiversity Crisis | Global Species Loss (http://www.well.com/user/davidu/extinction.html)
Univ. of Arizona lecture on extinction (http://nitro.biosci.arizona.edu/courses/EEB105/lectures/extinction/extinction.html)

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