Extinction
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The Dodo, shown here in illustration, is an often-cited |
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 may be 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 are created by
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,
[Newman, Mark. "A Mathematical Model for Mass Extinction". Cornell University. May 20 1994. URL accessed July 30 2006.] although some species, called
living fossils, survive virtually unchanged for hundreds of millions of years. All but a tenth of one percent of species that have existed are extinct.
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
[Species disappearing at an alarming rate, report says. MSNBC. URL accessed July 26 2006.] 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.
[Wilson, E.O., The Future of Life (2002) (ISBN 0679768114). See also: Leakey, Richard. The Sixth Extinction : Patterns of Life and the Future of Humankind ( ISBN 0385468091 ).]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.
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. This distinction is discussed further in the article on
species definition. 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. 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.
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. 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.
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.
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 extinction of the
megafauna in areas such as
New Zealand,
Australia,
Madagascar and
Hawaii was the result of human migration into those areas at a time when
hunting techniques against which local animals were defenseless.
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"
[Koh, Lian Pih. Science, Vol 305, Issue 5690, 1632-1634, 10 September 2004.].
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.
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 or regrettable, humans have also engaged in the deliberate destruction of some forms of life, such as dangerous
viruses, and the extirpation of other problematic species has been suggested.
The scientific community
Although the scientific community "stress the importance" of maintainng biodiversity
[Committee on Recently Extinct Organisms. "Why Care About Species That Have Gone Extinct?". URL accessed July 30 2006.][Walsh, Bruce. Extinction. Bioscience at University of Arizona. URL accessed July 26 2006.] 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] 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".
[Discover Channel staff. Will mammoths walk again?. Discovery Channel. March 9 2001. URL accessed July 30 2006.] 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
[WHO FactsheetWHO meeting agenda Scientists certified it eradicated in December 1979, WHO formally ratified this on 8 May 1980 in resolution WHA33.3] - 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 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."
Other groups
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 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 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.
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
[Paul Ehrlich and Anne Ehrlich, Extinction, Random House, New York (1981) ISBN 0-394-51312-6].
*
Extinct organisms Category*
List of extinct animals*
List of extinct plants*
The current mass extinction with links to news reports about extinction
*
Red List of Threatened Species*
Naturalis - Extinct Birds: 3D images of extinct bird species in the collection of the National Museum of Natural History (Leiden, Netherlands).
*
The Extinction Website*
Scientific Research on Extinction From
ScienceDaily*
Committee on recently extinct organisms*
The Wildlands Project takes action on
wildlife conservation to address the extinction
crisis in the
Americas.
*
Alliance for Zero Extinctionszh-yue:絕種
