While extinction of population, genetic lineages or entire species are a common occurrence in the history of life, mass extinctions – brief times of crisis where both the amount and diversity of life sharply drop – are few events of huge importance that shape the history of a planet.
Despite the huge capacity for adaptation displayed by life, a rapid change in environmental conditions can bring the general extinction rate far above the speciation rate. Since the vast majority of biomass and biodiversity are found in bacteria, it's likely that not even the most catastrophic event can significantly dent Earth's biosphere, but it can still have grave consequences on its most evident part, macroscopic animals and plants.
Historical Mass Extinctions[edit | edit source]
In the Phanerozoic Eon, distinguished by the abundant presence of large-sized animals and plants, five major mass extinctions have been identified, with different causes and severity:
- The End Ordovician extinction (450–440 Ma) involved more than a hundred families of marine invertebrates, and the total victim count is estimated about 27% of all families, 57% of all genera and 60-70% of all species. A great reduction of early reef has been observed, along with a crisis of brachiopods, bryozoans, conodonts, trilobites and graptolytes. The most likely cause is the southwards movement of Gondwana, which triggered an ice age and thus caused the sea level to fall, destroyed habitats worldwide.
- The Late Devonian extinction (375–360 Ma) destroyed 19% of families, 50% of genera and 70% of species. It saw a great reduction of trilobites, graptolytes, brachiopods, placoderms; armoured jawless fish disappeared entirely; there was, again, a crisis of reefs, composed by organisms with a calcium-rich skeleton, because of accumulation of magnesium in seawater (they'll be replaced by aragonite-rich reefs). Another global cooling episode is suspected, perhaps triggered by a meteor impact; the magnesium raise, and a strong eutrophication, could have been produced by the formation of the first forests.
- The End Permian extinction (251 Ma), the largest one, destroyed 57% of families, 83% of genera and maybe 90-96% of marine species. Trilobites and most therapsids (then the dominant vertebrates) disappeared entirely, while brachiopods, mollusks, echinoderms, corals, fish, amphibians, reptiles, insects and plants are merely heavily damaged. The formation of Pangaea (which produced desertification and ecosystem mixing) is probably responsible, along with an ice age, probably the huge lava emission from the Siberian Traps and maybe an exhalation of methane from the oceanic clathrates (see below).
- The End Triassic extinction (205 Ma) destroyed 23% of families, 48% of genera and 70-75% of species. Biodiversity had not entirely recovered from the previous mass extinction; several groups of reptiles and amphibians disappear, opening the way to dinosaurs. Its cause is still unknown: volcanic events connected to Pangaea breakup are suspected.
- The End Cretaceous extinction (65 Ma) destroyed 17% of families, 50% of genera and 75% of species. Victims include most mollusks (such as ammonites) and reptiles (plesiosaurs, mosasaurs, pterosaurs and dinosaurs, except for birds); mammals, birds, crocodiles and small-sized reptiles are not heavily damaged. This extinction has been ascribed to the Chicxulub meteor impact in Mexico, and perhaps the volcanic activity of the Deccan Traps.
|End Ediacaran||542 Ma||Global anoxy? (sea current change?)||?|
|End Botomian||517 Ma||?||40%|
|Dresbachian Extinction||502 Ma||?||40%|
|End Cambrian||488 Ma||Glaciation? Hypoxia? Basaltic flood?||52%|
|End Ordovician||440 Ma||Glaciation, carbon dioxide drop; gamma ray burst?||57%|
|Ireviken Event||428 Ma||Deep-ocean anoxia? Climate change?||25%?|
|Late Devonian||375 Ma||Eutrophication from new forests; impact?||50%|
|Rainforest Collapse||318 Ma||Glaciation; carbon dioxide drop; impact?||30%?|
|Olson's Extinction||270 Ma||?||20%?|
|End Permian||251 Ma||Siberian Traps, likely impact, desertification||83%|
|Carnian Pluvial Event||232 Ma||Wrangellia basaltic flood? New monsoons?||?|
|End Triassic||205 Ma||Impact? Volcanism? Climate change?||48%|
|Toarcian Turnover||183 Ma||South Africa volcanoes, ocean anoxia||10%?|
|Aptian Extinction||117 Ma||Rajmahal Traps?||?|
|End Cretaceous||65 Ma||Chicxulub Impact, Deccan Traps?||50%|
|End Eocene||34 Ma||Global cooling (impact? Azolla event?)||20%|
|Middle Miocene||15 Ma||Global cooling?||?|
|Quaternary||0.05 Ma||Expansion of Homo sapiens; ice age end?||?|
Consequences of Mass Extinctions[edit | edit source]
After a mass extinction has taken its toll, bringing death on a great deal of species, it opens up spaces that were previously occupied and/or creates new niches that were non-existent because conditions were inhibited by previous organisms. Life then slowly but surely begins to recover, starting with pioneer organisms (fern spikes are a common marker of mass extinctions). Bouncing back, however, is difficult and it takes thousands, if not millions, of years to bring about an ecosystem of at least somewhat similar properties and liveability of its predecessor.
A biotic crisis clears the way for not only new species but also new sorts of habitats, affecting the Earth as a whole and causing the planet itself to change its variables. Climate is the most common factor altered by an extinction event, although effects on other circumstances are more unlikely but not unthinkable such as the change of the world's magnetic poles or the stimuli to a greater incidence of volcanic activity or tectonic geology.
Life usually gets the better out of a disaster, not only surviving but also innovating with the ever-increasing ingenuity, creativity and cleverness that belongs to nature, making animals and plants cross new milestones and advance in complexity, design and efficiency, toughening it even further to the next catastrophe and increasing the likeliness of withstanding another calamity, at times worse than anything that came before.
Possible Causes[edit | edit source]
The causes of mass extinctions can be many, and varied. The events listed below can be very common (eleven basaltic floods, twelve sea-level falls and at least one major asteroid impact have been recognised in the Phanerozoic), but usually they're not enough to induce a worldwide crisis of biodiversity; rather, these occur when several events overlap. Often, however, one of these events can trigger another, causing a catastrophic chain reaction of multiple disasters.
Biological Causes[edit | edit source]
Keystone species disappearance can be a cause for localized mass extinctions on a relatively small scale. Keystone species have a disproportionately large effect on their environment relatively to their biomass or productivity: for example, sea otters, that protect kelp forests by eating sea urchins; the australian plant Banksia prionotes, that for most of the year is the only food source for pollinators that, in turn, are vital for other plants; prairie dogs' tunnels aerate the ground and store rain water. The extinction of one of this species due to some small event (say, an aggressive parasite) can cause the collapse of an entire ecosystem.
Hydrogen sulphide eruptions can be produced by sulfate-reducing bacteria, whose metabolism, promoted by warm conditions and lack of oxygen, releases large amounts of hydrogen sulfide. This gas is extremely toxic for most oxygen-breathing organisms and weakens the ozone layer. The End Permian extinction probably saw this happening because of the strong anoxia.
Methane production can occur by metabolic processes such as the hydrogenation of carbon dioxide or from the fermentation of plant matter; vast populations of large-sized herbivores can release huge amounts of methane in the atmosphere. Methane is an extremely strong greenhouse gas, and can induce global warming. It's now thought that methane production from herbivore sauropods had a significant role in the Mesozoic warming of Earth.
Eutrophication is the effect of a large amount of nutrients, such as nitrates and phosphates, released in an environment, especially aquatic. Estuaries are by nature eutrophic, as they receive nutrients from rivers. The greater biomass can consume oxygen causing anoxia and death for oxygen-breathing organisms, which are replaced by algae and bacterial masses. It's usually a local event, but the formation of the first forests (and thus disgregation of the soil, and percolation of more nutrients in rivers) is suspected to have contributed to the Late Devonian extinction.
Climatological Causes[edit | edit source]
Geological Causes[edit | edit source]
Astronomical Causes[edit | edit source]
Impacts[edit | edit source]
The most common astronomical extinction cause is that of an asteroid or comet. Asteroids knocked out of orbit from the Asteroid Belt between Jupiter and Mars can hit the Earth, causing an extinction event. Comets affect the Earth similarly, but often come unexpectedly and at a faster speed. While the mainstream theory states that an asteroid caused the K–Pg extinction, it can also be said that a comet did. Apparently, comets may hit much more often than previously thought, however, and it is plausible that a comet hit Earth as recently as 12,000 years ago.
A combination of an impact and volcanism is theoretically needed to cause a major mass extinction, and in fact the Wilkes Land crater in Antarctica may have been an impact responsible for part of the Permian–Triassic extinction. A radical proposal is that impacts in major mass extinctions cause a long period of volcanism directly on the other side of the Earth, or on the impact crater's antipode, but this is supported by the Wilkes Land crater being opposite to the Siberian Traps during Pangaea and the Chicxulub crater being opposite to the Deccan Traps.
Gamma Ray Bursts[edit | edit source]
A disputed theory is that gamma ray bursts have caused past mass extinctions. It has been theorised that an exceptionally large supernova of a nearby star shot towards Earth, depleting the atmosphere, especially the ozone layer, and causing the End Ordovician mass extinction.
Examples in Speculative Biology[edit | edit source]
The Future Is Wild included a mass extinction between the second and third time periods of the series. The extinction was caused by a series of volcanic eruptions. During the mass extinction, vertebrates were heavily hit, with tetrapods going extinct and fish only represented by flish and sharks. After the extinction, the dominant group became cephalopods, such as squibbons.
Commonly in idea in speculative biology is a mass extinction in the near future caused by man. This had been speculated as a lesser extinction or true mass extinction, and is considered possible, thought the plausibility of this is debated. It is common to see carnivorans and artiodactyls hit very heavily by this extinction, such as in After Man: A Zoology of The Future and The Future Is Wild.
References[edit | edit source]
- Cambrian–Ordovician extinction event. Wikipedia. Retrieved 30 September 2014.
- Mass Extinctions. Florida International University. Retrieved 30 September 2014.
- Gamma–Ray Burst Caused Mass Extinction?. National Geographic News. Retrieved 30 September 2014.
- "Whale Killer". Walking with Beasts. BBC. First aired 22 November 2001.
- Jennifer Welsh, "Dinosaur Farts May Have Warmed Prehistoric Earth", LiveScience, 7 May 2012. <http://www.livescience.com/20125-dinosaur-farts-climate.html>
- "The Late Devonian", Palaeos, 2000. <http://palaeos.com/paleozoic/devonian/latedev.html>
- What Killed the Dinosaurs?. PBS. Retrieved 30 September 2014.
- Did a Comet Hit Earth 12,000 Years Ago?. Scientific American. Retrieved 30 September 2014.
- Wilkes Land crater. Wikipedia. Retrieved 30 September 2014.