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An extinction event also known as a mass extinction or biotic crisis is a widespread and rapid decrease in the biodiversity on Earth.
Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. It occurs when the rate of extinction increases with respect to the rate of speciation. Estimates of the number of major mass extinctions in the last million years range from as few as five to more than twenty. These differences stem from the threshold chosen for describing an extinction event as "major", and the data chosen to measure past diversity.
Because most diversity and biomass on Earth is microbialand thus difficult to measure, recorded extinction events affect the easily observed, biologically complex component of the biosphere rather than the total The Devil Made Me Do It - The Spectacles - The Two Sides Of The Spectacles and abundance of life.
Based on the fossil recordthe background rate of extinctions on Earth is about two to five taxonomic families of marine animals every million years. Marine fossils are mostly used to measure extinction rates because of their superior fossil record and stratigraphic range compared to land animals. The Great Oxygenation Eventwhich occurred around 2.
The most recent and arguably best-known, the Cretaceous—Paleogene extinction eventwhich occurred approximately 66 million years ago Mawas a large-scale mass extinction of animal and plant species in a geologically short period of time. In a landmark paper published inJack Sepkoski and David M. Raup identified five mass extinctions. They were originally identified as outliers to a general trend of decreasing extinction rates during the Phanerozoic,  but as more stringent statistical tests have been applied to the accumulating data, it has been established that multicellular animal life has experienced five major and many minor mass extinctions.
Despite the popularization of these five events, there is no definite line separating them from other extinction events; using different methods of calculating an extinction's impact can lead to other events featuring in the top five. It has Mass Extinction Event - Impulse (26) - Chicano Violence (File) suggested that the apparent variations in marine biodiversity may actually be an artifact, with abundance estimates directly related to quantity of rock available for sampling from different time periods.
A quantification of the rock exposure of Western Europe indicates that many of the minor events for which a biological explanation has been sought Dust Heaven - Невидимки.
X Anniversary Edition most readily explained by sampling bias. Research completed after the seminal paper has concluded that a sixth mass extinction event is ongoing:. More recent research has indicated that the End-Capitanian extinction event likely constitutes a separate extinction event from the Permian—Triassic extinction event; if so, it would be larger than many of the "Big Five" extinction events.
Mass extinctions have sometimes accelerated the evolution of life on Earth. When dominance of particular ecological niches passes from one group of organisms to another, it Why Go?
(Feat Boy George) - Faithless - Outrospective (Special Edition) rarely because the new dominant group is "superior" to the old and usually because an extinction event eliminates the old dominant group and makes way for the new one. For example, mammaliformes "almost mammals" and then mammals existed throughout the reign of the dinosaursbut could not compete for the large terrestrial vertebrate niches which dinosaurs monopolized. The end-Cretaceous mass extinction removed the non-avian dinosaurs and made it possible for mammals to expand into the large terrestrial vertebrate niches.
Ironically, the dinosaurs themselves had been beneficiaries of a previous mass extinction, the end-Triassicwhich eliminated most of their chief rivals, the crurotarsans.
Another Dance Of The Dream Monkey - Svarta Stugan - Collection I – III of view put forward in the Escalation hypothesis predicts that species in ecological niches with more organism-to-organism conflict will be less likely to survive extinctions. This is because the very traits that keep a species numerous and viable under fairly static conditions become a burden once population levels fall among competing organisms during the dynamics of an Mass Extinction Event - Impulse (26) - Chicano Violence (File) event.
Furthermore, many groups which survive mass extinctions do not recover in numbers or diversity, and many of these go into long-term decline, Mass Extinction Event - Impulse (26) - Chicano Violence (File) these are often referred to as " Dead Clades Walking ".
He expressed this in The Origin of Species : "Species are produced and exterminated by slowly acting causes…and the most import of all causes of organic change is one which is almost independent of altered…physical conditions, namely the mutual relation of organism to organism-the improvement of one organism entailing the improvement or extermination of others".
Mass extinctions are thought to result when a long-term stress is compounded by a short-term shock. It has also been suggested that the oceans have gradually become more hospitable to life over the last million years, and thus less vulnerable to mass extinctions, [note 1]   but susceptibility to extinction at a taxonomic level does not appear to make mass extinctions more or less probable.
There is still debate about the causes of all mass extinctions. In general, large extinctions may result when a biosphere under long-term stress undergoes a short-term shock. High diversity leads to a persistent increase in extinction rate; low diversity to a persistent increase in origination rate. These presumably ecologically controlled relationships likely amplify smaller perturbations asteroid impacts, etc. A good theory for a particular mass extinction should: i explain all of the losses, not just focus on a few groups such as dinosaurs ; ii explain why particular groups of organisms died out and why others survived; iii provide mechanisms which are strong enough to cause a mass extinction but not a total extinction; iv be based on events or processes that can be shown to have happened, not just inferred from the extinction.
It may be necessary to consider combinations of causes. For example, the marine aspect of the end-Cretaceous extinction appears to have been caused by several processes Mass Extinction Event - Impulse (26) - Chicano Violence (File) partially overlapped in time and may have had different levels of significance in different parts of the world. Macleod  summarized the relationship between mass extinctions and events which are most often cited as causes of mass extinctions, using data from Courtillot et al.
The formation of large igneous provinces by flood basalt events could have:. Flood basalt events occur as pulses of activity punctuated by dormant periods. As a result, they are likely to cause the climate to oscillate between cooling and warming, but with an overall trend towards warming as the carbon dioxide they emit can stay in the atmosphere for hundreds of years. It is speculated that massive volcanism caused or contributed to the End-PermianEnd-Triassic and End-Cretaceous extinctions.
These are often clearly marked Mass Extinction Event - Impulse (26) - Chicano Violence (File) worldwide sequences of contemporaneous sediments which show all or part of a transition from sea-bed to tidal zone to beach to dry land — and where there is no evidence that the rocks in the relevant areas were raised by geological processes such as orogeny.
Sea-level falls could reduce the continental shelf area the most productive part of the oceans sufficiently to cause a marine mass extinction, and could disrupt weather patterns enough to cause extinctions on Miami Burns (Cheatin Me) (R.J.
Mix) - Various - Ministry Of House 3. But sea-level falls are very probably the result of other events, such as sustained global cooling or the sinking of the mid-ocean ridges. A study, published in the journal Nature online June 15, established a relationship between the speed of mass extinction events and changes in sea level and sediment.
The impact of a sufficiently large asteroid or comet could have caused food chains to collapse both on land and at sea by producing dust and particulate aerosols and thus inhibiting photosynthesis. Nonetheless, in Octoberresearchers reported that the Robert Silverman - Intermezzo: Works For Piano By Brahms Chicxulub asteroid impact that resulted in the extinction of non-avian dinosaurs 66 Ma ago, also rapidly acidified the oceans producing ecological collapse and long-lasting effects on the climate, and was a key reason for end-Cretaceous mass extinction.
Sustained and significant global cooling could kill many polar and temperate species and force others to migrate towards the equator ; reduce the area available for tropical species; often make the Earth's climate more arid on average, mainly by locking up more of the planet's water in ice and snow. The glaciation cycles of the current ice age are believed to have had only a very mild impact on biodiversity, so the mere existence of a significant cooling is not sufficient on its own to explain a mass extinction.
It has been suggested that global cooling caused or contributed to the End-OrdovicianPermian—TriassicLate Devonian extinctions, and possibly others. Sustained global cooling is distinguished from the temporary climatic effects of flood basalt events or impacts. This would have the opposite effects: expand the area available for tropical species; kill temperate species or force them to migrate towards the poles ; possibly cause severe extinctions of polar species; often make the Earth's climate wetter on average, mainly by melting ice and snow and thus increasing the volume of the water cycle.
It might also cause anoxic events in the oceans see below. Global warming as a cause of mass extinction is supported by several recent studies. The most dramatic example of sustained warming is the Paleocene—Eocene Thermal Maximumwhich was associated with one of the smaller mass extinctions.
Furthermore, the Permian—Triassic extinction event has Mass Extinction Event - Impulse (26) - Chicano Violence (File) suggested to have been caused by warming. Clathrates are composites in which a lattice of one substance forms a cage around another. Methane clathrates in which water molecules are the cage form on continental shelves. These clathrates are likely to break up rapidly and release the methane if the temperature rises quickly or the pressure on them drops quickly—for example in response to sudden global warming or Mass Extinction Event - Impulse (26) - Chicano Violence (File) sudden drop in sea level or even earthquakes.
Methane is a much more powerful greenhouse gas than carbon dioxide, so a Mass Extinction Event - Impulse (26) - Chicano Violence (File) eruption "clathrate gun" could cause rapid global warming or make it much more severe if the eruption was itself caused by global warming. The most likely signature of such a methane eruption would be a sudden decrease in the ratio of carbon to carbon in sediments, since methane clathrates are low in carbon; but the change would have to be very large, as other events can also reduce the percentage of carbon It has been suggested that "clathrate gun" methane eruptions were involved in the end-Permian extinction "the Great Dying" and in the Paleocene—Eocene Thermal Maximumwhich was associated with one of the smaller mass extinctions.
Anoxic events are situations in which the middle and even the upper layers of the ocean become deficient or totally lacking in oxygen. Their causes are complex and controversial, but all known instances are associated with severe and sustained global warming, mostly caused by sustained massive volcanism. It has been suggested that anoxic events caused or contributed to the Ordovician—Silurianlate DevonianPermian—Triassic and Triassic—Jurassic extinctions, as well as a number of lesser Mass Extinction Event - Impulse (26) - Chicano Violence (File) such as the IrevikenMuldeLauToarcian and Cenomanian—Turonian events.
On the other hand, there are widespread black shale beds from the mid-Cretaceous which indicate anoxic events but are not associated with mass extinctions. The bio-availability of essential trace elements in particular selenium to potentially lethal lows has been shown to coincide with, and likely have contributed to, at least three mass extinction events in the oceans, Mass Extinction Event - Impulse (26) - Chicano Violence (File).
Kump, Pavlov and Arthur have proposed that during the Permian—Triassic extinction event the warming also upset the oceanic balance Ice Cube - Minnetonka - Lake Boon Retreat EP photosynthesising plankton and deep-water sulfate-reducing bacteriacausing massive emissions of hydrogen sulfide which poisoned life on both land and sea and severely weakened the ozone layerexposing much of the Chicago X 12 - Rogue Wave - Asleep At Heavens Gate that still remained to fatal levels of UV radiation.
Oceanic overturn is a disruption of thermo-haline circulation which lets surface water which is more saline than deep water because of evaporation sink straight down, bringing anoxic deep water to the surface and therefore killing most of the oxygen-breathing organisms which inhabit the surface and middle depths.
It may occur either at the beginning or the end of a glaciationalthough an overturn at the start of a glaciation is more dangerous because the preceding warm period will have created a larger volume of anoxic water. Unlike Mississippi Delta Blues - Dale Miller - Wild Over Me oceanic catastrophes such as regressions sea-level falls and anoxic events, overturns do not leave easily identified "signatures" in rocks and are theoretical consequences of researchers' conclusions about other climatic and marine events.
It has been suggested that oceanic overturn caused or contributed to the late Devonian and Permian—Triassic extinctions. A nearby gamma-ray burst less than light-years away would be powerful enough to destroy the Earth's ozone layerleaving organisms vulnerable to ultraviolet radiation from the Sun.
One theory is that periods of increased geomagnetic reversals will weaken Earth's magnetic field long enough to expose the atmosphere to the solar windscausing oxygen ions to Flippers - Art Vs. Science - Magic Fountain EP the atmosphere in a rate increased by 3—4 orders, resulting in a disastrous decrease in oxygen.
Movement of the continents into some configurations can cause or contribute to extinctions in several ways: by initiating or ending ice ages ; by changing ocean and wind currents and thus altering climate; by opening seaways or land bridges which expose previously isolated species to competition for which they are poorly adapted for example, Mass Extinction Event - Impulse (26) - Chicano Violence (File) extinction of most of South America's native ungulates and all of its large metatherians after the creation of a land bridge between North and South America.
Occasionally continental drift creates a super-continent which includes the vast majority of Earth's land area, which in addition to the effects listed above is likely to reduce the total area of continental shelf the most species-rich part of the ocean and produce a vast, arid continental interior which may have extreme seasonal variations. Another theory is that the creation of the super-continent Pangaea contributed to the End-Permian mass extinction. Pangaea was almost fully formed at the transition from mid-Permian to late-Permian, and the "Marine genus diversity" diagram at the top of this article shows a level of extinction starting at that time which might have qualified for inclusion in the "Big Five" if it were not overshadowed by the "Great Mass Extinction Event - Impulse (26) - Chicano Violence (File) at the end of the Permian.
Many other hypotheses have been proposed, such as the spread of a new disease, or simple out-competition following an especially successful biological innovation.
But all have been rejected, usually for one of the following reasons: they require events or processes for which there is no evidence; they assume mechanisms which are contrary to the available evidence; they are based on other theories which have been rejected or superseded. Scientists have been concerned that human activities could cause more plants and animals to become extinct than any point in the past.
Along with human-made changes in climate see abovesome of these extinctions could be caused by overhunting, overfishing, invasive species, or habitat loss. The eventual warming and expanding of the Sun, combined with the eventual decline of atmospheric carbon dioxide could actually cause an even greater mass extinction, having the potential to wipe out even microbes in other words, the Earth is completely sterilizedwhere rising global temperatures caused by the expanding Sun will gradually increase the rate of weathering, which in turn removes more and more carbon dioxide from the atmosphere.
When carbon dioxide levels get too low perhaps at 50 ppmall plant life will die out, although simpler plants like grasses and mosses can survive much longer, until CO 2 levels drop to 10 ppm.
With all photosynthetic organisms gone, atmospheric oxygen can no longer be replenished, and is eventually removed by chemical reactions in the atmosphere, perhaps from volcanic eruptions.
Eventually the loss of oxygen will cause T. Rex - Tanx remaining aerobic life to die out via asphyxiation, leaving behind only simple anaerobic prokaryotes. This is the most extreme instance of a climate-caused extinction event. Since this will only happen late in the Sun's life, such will cause the final mass extinction in Earth's history albeit a very long extinction event.
The impact of mass extinction events varied widely. After a major extinction event, usually only weedy species survive due to their ability to live in diverse habitats. Generally, it takes millions of years for biodiversity to recover after extinction events. Life seemed to recover quickly after the P-T extinction, but this was mostly in the form of disaster taxasuch as Mass Extinction Event - Impulse (26) - Chicano Violence (File) hardy Lystrosaurus.
The most recent research indicates that the specialized animals that formed complex ecosystems, with high biodiversity, complex food webs and a variety of niches, took much longer to recover. It is thought that this long recovery was due to successive waves of extinction which inhibited recovery, as well as prolonged environmental stress which continued into the Early Triassic. Recent research indicates that recovery did not begin until the start of the mid-Triassic, 4M to 6M years after the extinction;  and some writers estimate that the recovery was not complete until 30M years after the P-T extinction, i.
The effects of mass extinctions on plants are somewhat harder to quantify, given the biases inherent in the plant fossil record. Some mass extinctions such as the end-Permian were equally catastrophic for plants, whereas others, such as the end-Devonian, did not affect the flora. From Wikipedia, the free encyclopedia. For other uses, see Extinction Level Event disambiguation.
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