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Extinction event

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An extinction event (also known as: mass extinction; extinction-level event, ELE) occurs when there is a sharp decrease in the number of species in a relatively short period of time. Mass extinctions affect most major taxonomic classes present at the time — birds, mammals, reptiles, amphibians, fish, invertebrates and other simpler life forms. They 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.

Extinction Intensity

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.

There are differing estimates of the number of major mass extinctions in the last 540 million years, ranging from as few as five to more than twenty discrete extinctions. These differences stem primarily from: (a) the threshold chosen for describing an extinction event as "major"; and, what set of data one chooses as the best measure of past diversity.

Major extinction events

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 outlined below. The articles about individual mass extinctions describe their effects in more detail and discuss theories about their causes.

  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, and togther these are ranked by many scientists as the second largest of the five major extinctions in Earth's history in terms of percentage of genera that went extinct.
  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 — the period of decline lasted perhaps as long as 20 million years, and 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/Tr 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). The "Great Dying" had enormous evolutionary significance: on land it ended the dominance of the mammal-like reptiles and created the opportunity for archosaurs and then dinosaurs to become the dominant land vertebrates; in the seas the percentage of animals that were sessile dropped from 67% to 50%.
    The whole of the late Permian was a difficult time for at least marine life - even before the "Great Dying", the diagram shows a late-Permian level of extinction large enough to qualify for inclusion in the "Big Five".
  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. It 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; and in the seas it reduced the percentage of sessile animals again, to about 33%. The K/T extinction was rather uneven — some groups of organisms became extinct, some suffered heavy losses and some appear to have got off relatively lightly.
  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, possibly one of the fastest ever. 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.[1]

Evolutionary importance of mass extinctions

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 is 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 mammals existed throughout the reign of the dinosaurs, but 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.

Is the frequency of extinctions decreasing?

The diagram at the top of this page appears to show that:

  • The gaps between mass extinctions are becoming longer.
  • The average and background rates of extinction are decreasing.

Both of these phenomena could be explained by one or more of (Macleod, 2001): [1]

  • Reasonably complete fossils are very rare, most extinct organisms are represented only by partial fossils, and complete fossils are rarest in the oldest rocks. So paleontologists have mistakenly assigned parts of the same organism to different genera which were often defined solely to accommodate these finds (the story of Anomalocaris is a good example), and the risk of this mistake is higher for older fossils because these are often unlike parts of any living organism. Many of the "superfluous" genera are represented by fragments which are not found again and the "superfluous" genera appear to become extinct very quickly.
  • Martin (1994, 1996) has argued that the oceans have become more hospitable to life over the last 500M years and less vulnerable to mass extinctions: dissolved oxygen became more widespread and penetrated to greater depths; the development of life on land reduced the run-off of nutrients and hence the risk of eutrophication and anoxic events; and marine ecosystems became more diversified so that food chains were less likely to be disrupted.

It should also be noted that the relatively small number of mass extinctions makes any notions of "frequency decreasing" highly speculative. A sample of fewer than 15 extinction events is, in terms of probability theory, too small to be a reliable sign of any actual trend.

Causes of mass extinction

There is still debate about the causes of all mass extinctions before the Holocene.

Looking for the causes of particular mass extinctions

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 killing 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 which partially overlapped in time and may have had different levels of significance in different parts of the world.

Arens and West (2006) proposed a "press / pulse" model in which mass extinctions generally require 2 types of cause: long-term pressure on the eco-system ("press") and a sudden catastrophe ("pulse") towards the end of the period of pressure. Their statistical analysis of marine extinction rates throughout the Phanerozoic suggested that neither long-term pressure alone nor a catastrophe alone was sufficient to cause a significant increase in the extinction rate.

Most widely-supported explanations

Macleod (2001) summarised the relationship between mass extinctions and events which are most often cited as causes of mass extinctions, using data from Courtillot et al (1996), Hallam (1992) and Grieve et al (1996):

  • Flood basalt events: 11 occurrences, all associated with significant extinctions (see notes i & ii)
  • Sea-level falls: 12, of which 7 were associated with significant extinctions (see note ii).
  • Asteroid impacts producing craters over 100km wide: 1, associated with 1 mass extinction.
  • Asteroid impacts producing craters less than 100km wide: over 50, the great majority not associated with significant extinctions.

Notes:
(i) The earliest known flood basalt event is the one which produced the Siberian Traps and is associated with the end-Permian extinction.
(ii) Some of the extinctions associated with flood basalts and sea-level falls were significantly smaller than the "major" extinctions, but still much greater than the background extinction level.

Here are the most commonly-suggested causes of mass extinctions:

  1. Massive and sustained Volcanism
    The formation of large igneous provinces (flood basalt events), could have: produced dust and particulate aerosols which inhibited photosynthesis and thus caused food chains to collapse both on land and at sea; emitted sulfur oxides which were precipitated as acid rain and poisoned many organisms — contributing further to the collapse of food chains; emitted carbon dioxide and thus caused sustained global warming once the dust and particulate aerosols dissipated.
    Various scientists have suggested that massive volcanism caused or contributed to the End-Cretaceous, End-Permian, End Triassic and End Jurassic extinctions.
  2. Sea-level falls
    These 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 land.
    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.
    Sea-level falls are associated with most of the mass extinctions, including all of the "Big Five" — End-Ordovician, Late Devonian, End-Permian, End-Triassic, and End-Cretaceous.
  3. Impact events
    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. If it hit sulfur-rich rocks, it could also have emitted sulfur oxides which were precipitated as acid rain and poisoned many organisms — contributing further to the collapse of food chains. Some scientists have suggested that impacts could also have caused megatsunamis and / or global forest fires, but these ideas are now regarded as exaggerations.
    Only the end-Cretaceous extinction is associated with strong evidence of such an impact,
    but that impact is easily the largest for which there is strong evidence.
  4. Sustained global cooling
    This 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-Ordovician, Permian-Triassic, Late Devonian extinctions, and possibly others.
    Note: this item is labelled "sustained global cooling" to distinguish it from the temporary climatic effects of flood basalt events or impacts.
  5. Sustained global warming
    This would have the opposite effects: expand the area available for tropical species; kill temperate species or force them to migrate towards the Poles (or perish); 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).
    The most dramatic example of sustained warming is the Paleocene-Eocene Thermal Maximum, which was associated with one of the smaller mass extinctions.
  6. Anoxic events
    The causes of anoxic events are complex and controversial, but all known instances are associated with severe and sustained global warming, mostly caused by by massive sustained volcanism. Prolonged high temperatures directly reduce water's capacity for dissolved oxygen, which could cause a marine mass extinction. Kump, Pavlov and Arthur (2005) have proposed that the warming also upset the oceanic balance between photosynthesising plankton and deep-water sulfate-reducing bacteria, causing massive emissions of hydrogen sulfide which poisoned life on both land and sea and severely weakened the ozone layer, exposing much of the life that still remained to fatal levels of UV radiation.[2]
  7. 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. However, a recent study by Stanek et al (2006) says that GRBs are not possible in metal rich galaxies like our own. A proposal that a supernova or gamma ray burst had caused a mass extinction would also have to be backed up by astronomical evidence of such an explosion at the right place and time.
    It has been suggested that a supernova or gamma ray burst caused the End-Ordovician extinction.
  8. Continental drift
    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 the extinction of most American marsupials 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.
    It is widely thought that the creation of the super-continent Pangea 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 would have qualified for inclusion in the "Big Five" if it were not overshadowed by the "Great Dying" at the end of the Permian.
  9. Plate tectonics
    This is the mechanism which drives many of the possible causes of mass extinctions, especially volcanism and continental drift. So it is implicated in many extinctions, but in each case it is necessary to specify which manifestations of plate tectonics were involved.

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.

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.

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.[3] 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.

ELE in movies

  • Armageddon (film)
  • Deep Impact (film)
  • The Second Renaissance
  • The Day After Tomorrow
  • The Day After

See also

  • Doomsday event
  • Elvis taxon
  • Endangered species
  • Lazarus taxon
  • Outside Context Problem
  • Overpopulation
  • Rare species
  • Signor-Lipps Effect
  • Snowball Earth
  • The Revenge of Gaia

References

  1. ↑ Eldredge, Niles (June 2001). The Sixth Extinction. ActionBioscience.org. Retrieved on 2006-03-17.
  2. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named anoxicEvent
  3. ↑ 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
  • Arens, N.C. and West, I.D. (2006). "Press/Pulse: A General Theory of Mass Extinction?"" 'GSA Conference paper' Abstract
  • Berner, R.A., and Ward, P.D. (2004). "Positive Reinforcement, H2S, and the Permo-Triassic Extinction: Comment and Reply" describes possible positive feedback loops in the catastrophic release of hydrogen sulphide proposed by Kump, Pavlov and Arthur (2005).
  • Courtillot, V., Jaeger, J-J., Yang, Z., Féraud, G., Hofmann, C. (1996). "The influence of continental flood basalts on mass extinctions: where do we stand?" in Ryder, G., Fastovsky, D., and Gartner, S, eds. "The Cretaceous-Tertiary event and other catastrophes in earth history". The Geological Society of America, Special Paper 307, 513-525.
  • Cowen, R. (1999). "The History of Life". Blackwell Science. The chapter about extinctions is reproduced at http://www.ucmp.berkeley.edu/education/events/cowen1a.html
  • Grieve, R., Rupert, J., Smith, J., Therriault, A. (1996). "The record of terrestrial impact cratering". GSA Today 5: 193-195
  • Hallam, A. (1992). "Phanerozoic sea-level changes". New York; Columbia University Press.
  • MacLeod, N. (2001). "Extinction!"
  • Martin, R.E. (1995). "Cyclic and secular variation in microfossil biomineralization: clues to the biogeochemical evolution of Phanerozoic oceans". Global and Planetary Change Volume 11, Number 1
  • Martin, R.E. (1996). "Secular increase in nutrient levels through the Phanerozoic: Implications for productivity, biomass, and diversity of the marine biosphere". Palaios 11:209-219.
  • Kump, L.R., Pavlov, A., and Arthur, M.A. (2005). "Massive release of hydrogen sulfide to the surface ocean and atmosphere during intervals of oceanic anoxia". Geology v. 33, p.397–400. Abstract. Summarised by Ward (2006).
  • Stanek, K.Z., Gnedin, O. Y., Beacom, J. F., Gould, A. P., Johnson, J.A., Kollmeier, J.A., Modjaz, M., Pinsonneault, M.H., Pogge, R.D., Weinberg, H. (2006) "Protecting Life in the Milky Way: Metals Keep the GRBs Away"
  • Ward, P.D. (2006). "Impact from the Deep".

Notes

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