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Extinction

2007 Schools Wikipedia Selection. Related subjects: Environment; General
Biology

   The Dodo, shown here in illustration, is an often-cited example of
   extinction.
   Enlarge
   The Dodo, shown here in illustration, is an often-cited example of
   extinction.

   In biology and ecology, extinction is the cessation of existence of a
   species or group of taxa, reducing biodiversity. The moment of
   extinction is generally considered to be the death of the last
   individual of that species (although the capacity to breed and recover
   may have been lost before this point; see population bottleneck).
   Because a species' potential range may be very large, determining this
   moment is difficult, and is usually done retrospectively. This
   difficulty leads to phenomena such as Lazarus taxa, where a species
   presumed extinct abruptly "re-appears" (typically in the fossil record)
   after a period of apparent absence.

   Through evolution, new species arise through the process of speciation
   — where new varieties of 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, called
   living fossils, survive virtually unchanged for hundreds of millions of
   years. Only one in a thousand species that have existed remain today.

   Prior to the dispersion of humans across the earth, extinction was a
   purely natural phenomenon that generally occurred at a continuous low
   rate ( mass extinctions being relatively rare events). Starting
   approximately 100,000 years ago, and coinciding with an increase in the
   numbers and range of humans, species extinctions have increased to a
   rate unprecedented since the Cretaceous-Tertiary extinction event. This
   is known as the Holocene extinction event and is at least the sixth
   such extinction event. Some experts have estimated that up to half of
   presently existing species may become extinct by 2100.
     Conservation status
   the risk of extinction
          Extinction

   Extinct
   Extinct in the Wild
          Threatened

   Critically Endangered
   Endangered
   Vulnerable
          Lower risk

   Near Threatened
   Conservation Dependent
   Least Concern
           See also

   World Conservation Union
   IUCN Red List

Definition

   A species becomes extinct when the last existing member of that species
   dies. Extinction therefore becomes a certainty when there are no
   surviving individuals that 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
   poor health, age, sparse distribution over a large range, a lack of
   individuals of both sexes (in sexually reproducing species), or other
   reasons.
   Bark from the extinct Lepidodendron, which died out after the
   Carboniferous, likely due to competition from newer plant life.
   Enlarge
   Bark from the extinct Lepidodendron, which died out after the
   Carboniferous, likely due to competition from newer plant life.

   Pinpointing the extinction (or pseudoextinction) of a species requires
   a clear definition of that species. If it is to be declared extinct,
   the species in question must be uniquely identifiable from any ancestor
   or daughter species, or from other closely related 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.

   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. Local extinctions may be followed by a replacement of the
   species taken from other locations; Wolf reintroduction is an example
   of this. Species which are not extinct are termed extant. Those that
   are extant but threatened by extinction are referred to as endangered
   species.

   An important aspect of extinction at the present time are human
   attempts to preserve critically endangered species through the creation
   of the conservation status extinct in the wild. Species listed under
   this status by the World Conservation Union (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.

Pseudoextinction

   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 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.

Causes

   The passenger pigeon, gone since 1914, was hunted to extinction in a
   few decades.
   Enlarge
   The passenger pigeon, gone since 1914, was hunted to extinction in a
   few decades.

   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. Conservation biology uses the Extinction Vortex model to
   classify extinctions by cause. The question of whether more historical
   extinctions have been caused by evolution or by catastrophe is a
   subject of debate; Mark Newman, the author of Modeling Extinction
   argues for a mathematical model that falls between the two positions.
   When concerns about human extinction have been raised, for example in
   Sir Martin Rees' 2003 book Our Final Hour, concerns lie with climate
   change or technological disaster.

   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, habitat destruction, 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.

Genetic and demographic causes

   Population genetics and demographic phenomena affect the evolution, and
   therefore the risk of 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
   without forest to house it. Another example is the destruction of ocean
   floors by bottom trawling.

   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, leading to extinction.
   The Golden Toad was last seen on May 15, 1989. Decline in amphibian
   populations is ongoing worldwide.
   Enlarge
   The Golden Toad was last seen on May 15, 1989. Decline in amphibian
   populations is ongoing worldwide.

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. Human populations may themselves act as
   invasive predators. According to the "overkill hypothesis", the swift
   extinction of the megafauna in areas such as New Zealand, Australia,
   Madagascar and Hawaii resulted from the sudden introduction of human
   beings to environments full of animals that had never seen them before,
   and were therefore completely unadapted to their predation techniques.

Coextinction

   Coextinction refers to the loss of a species due to the extinction of
   another. An obvious example of coextinction is the extinction of
   parasitic insects following the loss of their hosts. Coextinction can
   also occur when a species loses its pollinator, or to predators in a
   food chain who lose their prey. According to Koh (2004), "Species
   coextinction is a manifestation of the interconnectedness of organisms
   in complex ecosystems ... While coextinction may not be the most
   important cause of species extinctions, it is certainly an insidious
   one".

Mass extinctions

   Apparent fraction of genera going extinct at any given time, as
   reconstructed from the fossil record (Graph not meant to include recent
   epoch of Holocene extinction event).
   Enlarge
   Apparent fraction of genera going extinct at any given time, as
   reconstructed from the fossil record (Graph not meant to include recent
   epoch of Holocene extinction event).

   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 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 attitudes on extinction

   Extinction is an important research topic in the field of zoology, and
   biology in general, and has also become an area of concern outside the
   scientific community. A number of organisations, such as the Worldwide
   Fund for Nature, have been created with the goal of preserving species
   from extinction. Governments have attempted, through enacting laws, to
   avoid human overharvesting or habitat destruction. While many
   human-caused extinctions have been accidental, humans have also engaged
   in the deliberate destruction of some species, such as dangerous
   viruses, and the extirpation of other problematic species has been
   suggested.

The scientific community

   The possibility of extinction was not widely accepted before the 1800s.
   When parts of the world had not been thoroughly examined and charted,
   scientists could not rule out that animals found only in the fossil
   record were not simply 'hiding' in unexplored regions of the Earth.
   Georges Cuvier is credited with establishing extinction as a fact in a
   1796 lecture to the French Institute.

   Although today the scientific community "stress[es] the importance" of
   maintaining biodiversity they have found historic extinctions useful
   for research; 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 and lead to the first book
   publicizing the idea of evolution. In the case of the Bay Checkerspot,
   scientists, including Paul R. Ehrlich, chose not to intervene in a
   local extinction, using it to study the danger to the world population
   Biologist Bruce Walsh of the University of Arizona states three reasons
   for scientific interest in the preservation of species; genetical
   resources, ecosystem stability, and ethics.

   Until recently, it had been universally accepted that the extinction of
   a species meant the complete 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,
   although the latter attempt has been abandoned. 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, primarily due to technological limitations, though
   bioethical and philosophical objections have also been raised. The
   concept of cloning extinct species was popularized in the successful
   novel and movie Jurassic Park.

Planned extinction

   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, and the polio
   virus is now confined to small parts of the world as a result of human
   efforts to cure the disease it causes.

   Olivia Judson is one of few modern scientists to have advocated the
   deliberate extinction of any species. Her September 25, 2003 New York
   Times article, "A Bug's Death", advocates "specicide" of thirty
   mosquito species through the introduction of recessive " knockout
   genes". Her arguments for doing so are:
     * Anopheles mosquitoes (which spread malaria) and Aedes mosquitoes
       (which spread dengue fever, yellow fever, elephantiasis, and other
       diseases) 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."

Other groups

   In the 1800s when extinction was first described, the idea of
   extinction was threatening to those who held a belief in the Great
   Chain of Being, a theological position that did not allow for "missing
   links".

   In modern times, commercial and industrial interests often have to
   contend with the effects of production on plant and animal life. When
   commercial technologies are tested the testing tends to concentrate on
   human effects. However, some technologies with no, or minimal, proven
   harmful effects on Homo sapiens can be devastating to wildlife (for
   example, 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 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.
   Nature preserves are created by governments as a means to provide
   continuing habitats to species crowded by human expansion. The 1992
   Convention on Biological Diversity has resulted in international
   Biodiversity Action Plan programmes, which attempt to provide
   comprehensive guidelines for government biodiversity conservation.

   People who live close to nature can be dependent on the survival of all
   the species in their environment and might be considered some of the
   people who should be most concerned about extinction risks. However
   with 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 often prioritize day-to-day survival
   over species conservation.

   Retrieved from " http://en.wikipedia.org/wiki/Extinction"
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