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Extinction event

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For the Big Finish Productions audio play, see The Extinction Event.
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An extinction event (also extinction-level event, ELE) occurs when there is a sharp decrease in the number of species in a relatively short period of time. This decrease may be caused by one or both of:

Based on the fossil record, the background rate of extinctions on Earth is about two to five taxonomic families of marine invertebrates and vertebrates every million years.

Apparent extinction intensity, i.e. the 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)

Since life began on Earth, a number of major mass extinctions have greatly exceeded the background extinction rate present at other times. Though there were undoubtedly mass extinctions in the Archean and Proterozoic, it is only during the Phanerozoic Eon that the emergence of bones and shells in the evolutionary tree has provided a sufficient fossil record from which to make a systematic study of extinction patterns. The number of major mass extinctions attributed to this most recent 540 million years varies from source to source, with some authorities arguing for as few as 5 or more than 20. These differences stem primarily from the threshold chosen for describing an extinction event as "major", and what set of data one chooses to believe is the best measure of past diversity. These extinction events generally consist of participation by most major taxonomic classes and thus yield extinct birds, mammals, fishes, invertebrates and other simpler life forms.

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[edit] Major extinction events

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text:"Permian-~Triassic" at:251 shift:(-9,1)
text:"Triassic-~Jurassic" at:200 shift:(-9,1)
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text:"Holocene" at:0 shift:(-9,2)

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The classical "Big Five" mass extinctions identified by Raup and Sepkoski (1982) are widely agreed upon as some of the most significant: End Ordovician, Late Devonian, End Permian, End Triassic, and End Cretaceous.

These and a selection of other extinction events are highlighted below:

  1. 488 million years ago — a series of mass extinctions at the Cambrian-Ordovician transition (the Cambrian-Ordovician extinction events) eliminated many brachiopods and conodonts and severely reduced the number of trilobite species.
  2. 444 million years ago — at the Ordovician-Silurian transition two Ordovician-Silurian extinction events occurred, probably as the result of a period of glaciation. Marine habitats changed drastically as sea levels decreased, causing the first die-off, and then another occurred between 500 thousand to a million years later when sea levels rose rapidly. It has been suggested that a gamma ray burst may have triggered this extinction.<ref>Jha, Alok (April 11 2005). Sea life 'killed by exploding star'. The Guardian. Retrieved on 2006-03-17.</ref>
  3. 360 million years ago — near the Devonian-Carboniferous transition (the Late Devonian extinction) a prolonged series of extinctions led to the elimination of about 70% of all species. This was not a sudden event, with the period of decline lasting perhaps as long as 20 million years. However, there is evidence for a series of extinction pulses within this period.
  4. 251 million years ago — at the Permian-Triassic transition Earth's worst mass extinction (the P/T or Permian-Triassic extinction event) killed 53% of marine families, 84% of marine genera, about 96% of all marine species and an estimated 70% of land species (including plants, insects, and vertebrate animals.) It had enormous evolutionary significance because it ended the dominance of the mammal-like reptiles and created the opportunity for archosaurs and then dinosaurs to become the dominant land vertebrates. Because of the P/T extinction's size and significance there are several competing theories about what caused it. The main contenders appear to be: (A) The flood basalt event which created the Siberian Traps. This certainly happened, and it would have (i) caused food chains to collapse both on land and at sea by producing dust and particulate aerosols and thus inhibiting photosynthesis; (ii) emitted sulphur oxides which were precipitated as acid rain and poisoned many organisms, especially plants and planktonic organisms which relied on calcium carbonate shells - contributing further to the collapse of food chains. (B) A meteor strike, which would have had similar effects and perhaps exacerbated or even triggered the Siberian Traps flood basalt event. Researchers from Ohio State University and NASA have presented findings suggesting that a meteorite, some 50km (30 miles) in diameter, collided with Earth approximately 250 million years ago, south of Australia in what is now Wilkes Land, eastern Antarctica, called Wilkes Land crater. This conclusion is still controversial. (C) A severe anoxic event, a condition where the oceans became depleted of dissolved oxygen, and rich in highly toxic hydrogen sulfide. With the toxic gas bubbling up into the atmosphere, it poisoned much of the life on both land and sea, and then destroyed the atmosphere's protective ozone layer. <ref name="anoxicEvent">Ward, Peter D. Impact from the Deep, Scientific American, October 2006, p. 64-71. Retrieved on 2006-9-26.</ref> (D) The formation of Pangaea in the mid-Permian created a vast, arid continental interior and enormously decreased the continental shelf area which is the most prolific part of the seas. These effects would probably have caused a mass extinction on their own, but are probably not enough to explain the severity of the P/T extinction.
  5. 200 million years ago — at the Triassic-Jurassic transition (the Triassic-Jurassic extinction event) about 20% of all marine families as well as most non-dinosaurian archosaurs, most therapsids, and the last of the large amphibians were eliminated.
  6. 65 million years ago — at the Cretaceous-Paleogene transition (the K/T or Cretaceous-Tertiary extinction event) about 50% of all species became extinct, and this has great significance for humans because it ended the reign of the dinosaurs and opened the way for mammals to become the dominant land vertebrates. The K/T extinction was rather uneven, for example: all non-avian dinosaurs and all pterosaurs, plesiosaurs, mosasaurs and ammonites became extinct; marsupials, birds, plankton, teleosts, bivalves, snails, sponges, and sea urchins suffered heavy losses; but placental mammals, non-dinosaurian reptiles and amphibians appear to have got off relatively lightly. The main challenge for theories about this extinction is to explain why different groups fared so differently, and none of the most widely supported theories are good at explaining these differences. Many theories are totally unsatisfactory because if they were correct they would only explain the extinction of the dinosaurs. The main contenders are (A) The flood basalt event which created the Deccan Traps. This certainly happened, and it would have (i) caused food chains to collapse both on land and at sea by inhibiting photosynthesis; (ii) poisoned many plants and planktonic organisms with acid rain. Since the Deccan Traps basalt flows lasted from about 68 to 60 million years ago with a peak period of about 30,000 years 66 million years ago, this theory implies a fairly gradual extinction. (B) The asteroid or comet impact event which created the Chicxulub crater. In addition to inhibiting photosynthesis and thus collapsing food chains, this would have created a vast amount of acid rain because it landed in a bed of gypsum and would have splashed red-hot debris over a large area of North America because it was travelling northwards and landed at an angle of about 30° from horizontal. The debate between supporters of these two theories has centred on: (i) the speed of the extinction - the impact theory implies a very rapid extinction, while the flood basalt theory implies a more gradual extinction; (ii) which theory better explains geological abnormalities at the K/T boundary - an iridium-rich clay layer, tektites and shocked quartz granules.
  7. Present day — the Holocene extinction event. A 1998 survey by the American Museum of Natural History found that 70% of biologists view the present era as part of a mass extinction event, the fastest to have ever occurred. Some, such as E. O. Wilson of Harvard University, predict that man's destruction of the biosphere could cause the extinction of one-half of all species in the next 100 years. Research and conservation efforts, such as the IUCN's annual "Red List" of threatened species, all point to an ongoing period of enhanced extinction, though some offer much lower rates and hence longer time scales before the onset of catastrophic damage. The extinction of many megafauna near the end of the most recent ice age is also sometimes considered a part of the Holocene extinction event.<ref>Eldredge, Niles (June 2001). The Sixth Extinction. ActionBioscience.org. Retrieved on 2006-03-17.</ref>

[edit] Causes for mass extinction

With the exception of the Cretaceous-Tertiary mass extinction, which is widely attributed to an impact event, and modern day extinctions associated with the proliferation of human civilization, it is not well known what has caused other mass extinctions. Some of the hypotheses are discussed below.

  1. Impact events - The impact of a sufficiently large asteroid or comet could create Megatsunamis, global forest fires, and simulate nuclear winter from the dust it puts in the atmosphere. Taken together, it is not surprising that these and other related effects might be sufficiently severe as to disrupt the global ecosystem and cause extinctions. Only for the End Cretaceous extinctions is there strong evidence of such an impact. Circumstantial evidence of such events is also given for the End Permian, End Ordovician, End Jurassic and End Eocene extinctions.
  2. Climate change - Rapid transitions in climate may be capable of stressing the environment to the point of extinction. However, it is worth observing the recent cycles of ice ages are only believed to have had very mild impacts on biodiversity. Extinctions suggested to have this cause include: End Ordovician, End Permian, Late Devonian, and others.
  3. Anoxic events - Climate change, brought on by heavy volcanic activity, warmed the oceans, which lead to anoxic conditions, where the oceans were depleted of dissolved oxygen and were rich in hydrogen sulfide. That toxic gas poisoned life on both land and sea, and destroyed the ozone layer. Without the protective ozone, the sun's UV radiation killed off much of the life that still remained.<ref name="anoxicEvent"/>
  4. Volcanism - The formation of large igneous provinces, which can involve the outflow of millions of cubic kilometers of lava in a short duration, are suggested to poison the atmosphere and oceans in a way that may cause extinctions. This cause has been proposed for the End Cretaceous, End Permian, End Triassic, and End Jurassic extinctions.
  5. Astronomical event, such as a nearby nova, supernova or gamma ray burst - A nearby gamma ray burst (less than 6000 light years distance) could sufficiently irradiate the surface of the Earth to kill organisms living there and destroy the ozone layer in the process. From statistical arguments, approximately 1 gamma ray burst would be expected to occur in close proximity to Earth in the last 540 million years. This has been suggested as an explanation for the End Ordovician extinction event. However, a recent study by leading GRB researchers say that GRBs are not possible in metal rich galaxies like our own.<ref>Stanek, K. Z., O. Y. Gnedin, J. F. Beacom, A. P. Gould, J. A. Johnson, J. A. Kollmeier, M. Modjaz, M. H. Pinsonneault, R. Pogge, D. H. Weinberg (2006). "Protecting Life in the Milky Way: Metals Keep the GRBs Away". astro-ph/0604113.</ref>
  6. Plate tectonics - It has been suggested that the opening and closing of seaways and land bridges may play a role in extinction events as previously isolated populations are brought into contact and new dynamics are established in the ecosystem. This is most frequently discussed in relation to the End Permian mass extinction.
  7. Press/Pulse theory - A new theory proposed by Ian West and Nan Crystal Arens of Hobart & William Smith Colleges suggests that co-occurrence rate of long-term stress and catastrophic disturbance finally led big extinctions. This theory postulates a combination of deadly sudden catastrophes, called "pulses", with longer, steadier pressures on species called "presses". They postulate that this combination serve as trigger for mass extinction GSA Abstract.

Other hypotheses, such as the spread of a new disease or simple out-competition following an especially successful biological innovation are also considered; however, it is often thought that the major mass extinctions in Earth's history are too sudden and too extensive to have resulted solely from biological events.

[edit] Postulated extinction cycles

It has been suggested by several sources that biodiversity and/or extinction events may be influenced by cyclic processes. The best-known of these claims is the 26 to 30 million year viral cycle in extinctions proposed by Raup and Sepkoski (1986). More recently, Rohde and Muller (2005) have suggested that biodiversity fluctuates primarily on 62 ± 3 million year cycles.

It is difficult to evaluate the validity of these claims except through reduction to statistical arguments regarding how plausible or implausible it is for the observed data to exhibit a particular pattern, as the causes of most extinction events are still too uncertain to attribute to them any specific cause let alone a recurring one. Much early work in this area also suffered from poor knowledge of the geological time scale (errors > 10 million years at times), though the time scale now available (uncertainties all < 4 million years) should be adequate for studying these processes.

While the statistics alone have been judged as sufficiently compelling to warrant publication, it is important to consider processes that might be responsible for a cyclic pattern of extinctions and future work may focus on trying to find evidence of such processes.

One theory, for which no real evidence exists, suggests that the extinction cycle could be caused by the orbit of a hypothetical companion star dubbed Nemesis that periodically disturbs the Oort cloud, sending storms of large asteroids and comets towards the Solar System. Another similar theory suggests that the Solar System's oscillations through the plane of the galaxy results in periods of comet showers. Other theories suggest geological instabilities that might allow heat to periodically build up deep in the Earth, which is then released through mantle plumes, periods of major volcanism and active plate tectonics.

If any of these theories are correct, then it is worth noting that both Raup and Sepkoski and Rohde & Muller predict another naturally caused mass extinction event within the next 10 million years.

There is however no one single theory that can account for all of the specific extinctions. Although one theory may explain the mass extinctions on land it may not account for all of the extinctions in marine conditions. The only theory that accounts for most of the extictions is the gamma ray theory. This would explain the selective extinctions and also would explain mass speciation which follows mass extinction. The radiation causes manipulations in DNA and RNA which lead to the sudden development of new species and also the sudden disappearance of previous species.

[edit] Controversy

In 2005, Andrew Smith and Alistair McGowan of the Natural History Museum suggested that the apparent variations in marine biodiversity may actually be caused by changes in the quantity of rock available for sampling from different time periods.<ref>A. Smith & A. McGowan, 2005, "Cyclicity in the fossil record mirrors rock outcrop area", Biology Letters, Vol. 1, No. 4, pp. 443–445. DOI:10.1098/rsbl.2005.0345 </ref> The diversity of the marine life appears to be proportional to the amount of rock available for study. However, statistical analysis shows that only half of the apparent diversity modification can be attributed to this effect.

[edit] ELE in movies

[edit] See also

[edit] References

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[edit] Notes

[edit] External links

de:Massenaussterben es:Extinción masiva fr:Extinction massive it:Estinzione he:הכחדה המונית nl:Massaal uitsterven ja:大量絶滅 no:Masseutryddelse pl:Masowe wymieranie pt:Extinção em massa fi:Joukkosukupuutto sv:Massutdöende zh:生物集群灭绝

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