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. Extinction is usually a natural phenomenon; it is estimated that more than 99.9% of all species that have ever lived are now extinct.
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, 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.
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.
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.
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 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.
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.
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.
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The most often cited examples are that of the extinct passenger pigeon and its parasitic louse Columbicola extinctus and Campanulotes defectus. Recently, C. extinctus was rediscovered on band-tailed pigeon and C. defectus was found to be a likely case of misidentification, is the existing Campanulotes flavus. However, even though the passenger pigeon lice story has a happy ending (i.e. rediscovery), it is uncertain that other co-extinctions of other parasites, even on passenger pigeon, have not occurred.
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.
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Human extinction would be the extinction of the human species, Homo sapiens, whether on Earth (often as the result of a doomsday event) or from the entire universe, provided the species colonizes on other planets.
Attitudes to human extinction vary widely depending on beliefs concerning spiritual survival (souls, heaven, reincarnation, and so forth), the value of the human race, whether the human race evolves individually or collectively, and many other factors. Many religions prophesy an end time to the universe, so eventual human extinction is necessarily a part of the faith of many humans, to the extent that the end time means the absolute end of humanity (see eschatology), except for possibly their "souls".
Many people consider that the extinction of the entire species would be a much worse fate than the death of an individual. Although the mortality of the individual can be accepted as an inevitable part of the human condition, humans can nevertheless expect to attain some measurement of immortality through their progeny, or through contributions or advancement in culture or science. However, the extent to which this "immortality" can be achieved is subject to the continuation of the species as a whole, and human extinction would represent the termination of such expectations.
Fear of human extinction is said to be one of the motivating factors of the environmentalist movements of the 20th and 21st century.
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The Permian-Triassic (P-T or PT) extinction event, sometimes informally called the Great Dying, was an extinction event that occurred approximately 251.0 million years ago (mya), forming the boundary between the Permian and Triassic geologic periods. It was the Earth's most severe extinction event, with about 90 percent of all marine species and 70 percent of terrestrial vertebrate species going extinct.
At one time, this die-off was assumed to have been a gradual reduction over several million years. Now, however, it is commonly accepted that the event lasted less than a million years, from 252.3 to 251.4 MYA (both numbers ±300,000 years), a very brief period of time in geological terms. Organisms throughout the world, regardless of habitat, suffered similar rates of extinction, suggesting that the cause of the event was a global, not local, occurrence, and that it was a sudden event, not a gradual change.
New evidence from strata in Greenland shows evidence of a double extinction, with a separate, less dramatic extinction occurring 9 million years before the Permian-Triassic (P-T) boundary, at the end of the Guadalupian epoch. Confusion of these two events is likely to have influenced the early view that the extinction was extended.
For some time after the event, fungal species were the dominant form of terrestrial life. Though they only made up approximately 10% of remains found before and just after the extinction horizon, fungal species subsequently grew rapidly to make up nearly 100% of the available fossil record. However, some researchers argue that fungal species did not dominate terrestrial life, as their remains have only been found in shallow marine deposits. Alternatively, others argue that fungal hypha are simply better suited for preservation and survival in the environment, creating an inaccurate representation of certain species in the fossil record.
Explanatory theories
Many theories have been presented for the cause of the extinction, including plate tectonics, an impact event, a supernova, extreme volcanism, the release of frozen methane hydrate from the ocean beds to cause a greenhouse effect, or some combination of factors.
Plate tectonics
At the time of the Permian extinction, all the continents had recently joined to form the super-continent Pangaea and the super-ocean Panthalassa. This configuration radically decreased the extent and range of shallow aquatic environments and exposed formerly isolated organisms of the rich continental shelves to competition from invaders. As the planet's epicontinental systems coalesced; many marine ecosystems, especially ones that evolved in isolation, would not have survived those changes. Pangaea's formation would have altered both oceanic circulation and atmospheric weather patterns, creating seasonal monsoons. Pangaea seems to have formed millions of years before the great extinction, however, and very gradual changes like continental drift alone probably could not cause the sudden, simultaneous destruction of both terrestrial and oceanic life.
Impact event
When large bolides (asteroids or comets) impact Earth, the aftermath weakens or kills much of the life that thrived previously. Release of debris and carbon dioxide into the atmosphere reduces the productivity of life and causes both global warming and ozone depletion. Evidence of increased levels of atmospheric carbon dioxide exist in the fossil record. Material from the Earth's mantle released during volcanic eruption has also been shown to contain iridium, an element associated with meteorites. At present, there is only limited and disputed evidence of iridium and shocked quartz occurring with the Permian event, though such evidence has been very abundantly associated with an impact origin for the Cretaceous-Tertiary extinction event.
If an extraterrestrial impact triggered the Permian extinction event, scientists ask, where is the impact crater? Part of the answer may lie in the fact that there is no Permian-age oceanic crust remaining; all of it has been subducted, so plate tectonics during the last 252 million years have erased any possible P-T seafloor crater.
Adrian Jones, University College London, models the effects of impacts on the Earth's geological crust. After an impact, the crust rebounds to form a large shallow crater. Jones suggests that in a truly massive impact, the combined heat of the impact and rebound is enough to melt the crust. Lava floods through and the crater disappears beneath new crust. If Jones is right, the Permian meteorite crater can't be found because it doesn't exist.
But geologist John Gorter of Agip found evidence of a circular structure 200 kilometers in diameter called the Bedout, in currently submerged continental crust off the northwestern coast of Australia, and geologist Luann Becker, of the University of California, confirmed it, finding shocked quartz and brecciated mudstones.
The geology of the area of continental shelf dates to the end of the Permian. The Bedout impact crater is also associated in time with extreme volcanism and the break-up of Pangaea. "We think that mass extinctions may be defined by catastrophes like impact and volcanism occurring synchronously in time," Dr. Becker explains. "This is what happened 65 million years ago at Chicxulub but was largely dismissed by scientists as merely a coincidence. With the discovery of Bedout, I don't think we can call such catastrophes occurring together a coincidence anymore," Dr. Becker added in a news release.
It has also been proposed that such a collision might heat up ocean waters enough to produce "hypercanes," gigantic storms with winds possibly exceeding the speed of sound. Although not impossible, this theory has little supporting evidence.
Supernova
A supernova occurring within ten parsecs (1 parsec = 3.26 light years) of Earth would produce enough gamma radiation to destroy the ozone layer for several years. The resulting direct ultra-violet radiation from the sun would weaken or kill nearly all existing species. Only those deep in the oceans would be unaffected. Statistical frequency of supernovae suggests that one at the P-T boundary would not be unlikely. A gamma ray burst (the most energetic explosions in the universe; believed to be caused by a very massive supernova (hypernova) or two objects as dense as neutron stars colliding) that occurred within ~6000 light years would produce the same effect.
Volcanism
The P-T boundary was marked with many volcanic eruptions. In the Siberian Traps, now a sub-Arctic wilderness, over 200,000 square kilometers were covered in torrents of lava. The Siberian flood basalt eruption, the biggest volcanic effect on Earth, lasted for millions of years.
The acid rain, brief initial global cooling with each of the bursts of volcanism, followed by longer-term global warming from released volcanic gases, and other weather effects associated with enormous eruptions could have globally threatened life.
The theory is debated if volcanic activity, over such a long time, could alter the climate enough to kill off 95% of life on Earth. Volcanic activity affects the concentration of atmospheric gases directly, and, indirectly, the oceanic dissolved gases.
Increases in carbon dioxide enhance the greenhouse effect and cause global warming, which would reduce the temperature gradient between the equator and the poles. As a result, thermohaline circulation would slow and eventually stop.
The oceans would stagnate, and nutrients would fail to disperse themselves. Many marine ecosystems rely on upwelling and circulation of nutrients, oxygen included; without the regular circulation, organisms would starve or suffocate.
In addition, sulfur and particulates contribute to cooling, or volcanic winter, which usually lasts three to six months. Combinations of the two effects could produce a cooling cycle in which the climate alternatively warms then cools. Such temperature fluctuations could cause convective overturn of the oceans, bringing anoxic bottom waters to the surface; in an already oxygen-deprived environment, this would be fatal to many forms of life. Significant evidence supports this theory.
Fluctuations in air and water temperature are evident in the fossil record, and the uranium/thorium ratios of late Permian sediments indicate that the oceans were severely anoxic around the time of the extinction. Numerous indicators of volcanic activity at the P-T boundary are present, though they are similar to bolide impact indicators, including iridium deposits.
The volcanism theory has the advantage over the bolide theory, though, in that it is certain that an eruption of the Siberian Traps - the largest known eruption in the history of Earth - occurred at this time, while no direct evidence of bolide impact has been located.
Extinction Wikipedia
ELE - Extinction Level Event Wikipedia
Sea die-out blamed on volcanoes BBC - July 16, 2008
Undersea volcanic activity has been blamed for a mass extinction in the seas 93 million years ago.
Greatest Mysteries: What Causes Mass Extinctions? Live Science - August 8, 2007
Ancient Mass Extinctions Caused by Cosmic Radiation, Scientists Say National Geographic - April 20, 2007
Fossil research suggests 'mass dying' triggered teeming oceans Guardian - November 23, 2006
Dramatic shift from simple to complex marine ecosystems occurred 250M years ago at mass extinction PhysOrg - November 23, 2006
"Dinosaur Killer" Asteroid Only One Part of New Quadruple-Whammy Theory National Geographic - October 30, 2006
Lack of oxygen worsened 'Great Dying' MSNBC - April 2005
Great extinction came in phases BBC - April 2005
The Great Dying MSNBC - May 13, 2004
Hunt for Oil Leads to Crater Linked to 'Great Dying' Space.com - May 13, 2004
The biggest extinction event in Earth's history 250 million years ago The Great Dying, as it is known, is firmly established in the fossil record. It wiped out nearly 90 percent of species in the ocean and more than two-thirds of vertebrate species on land, or more by some accounts. The cause has not been pinned down. Scientists have speculated a
comet or asteroid may have been involved, and volcanism and climate change have also been blamed - Australia and Antarctica
Climate Change Plus Human Pressure Caused Large Mammal Extinctions In Late Pleistocene Science Daily - October 4, 2004
The Great Dying NASA - January 28, 2002
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