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Extinction is the process in which groups of organisms die out. If over an extended period of time the birth rate of a species is less than the death rate, then extinction will eventually occur.
Extinction is a natural phenomenon predicted by Darwin in his theory of evolution. A species goes extinct if it is not able to adapt to changes in it's environment, or compete effectively with other organisms.
Over the history of the earth over 99% of all the species that have ever lived have gone extinct.
Mass extinctions are periodic rises in the extinction rate above the background level. They are events which are not caused by changes in habitat or competition but catastrophes.
Perhaps over 95% of all extinctions have occurred as background events, with the rest consisting of catastrophic events which:-
These are the mass extinctions, they cause a dramatic decrease in the earth's biodiversity, and punctuate the earth's history, and are used by Geologists to break up geological time. Recent thinking has however been to class events by their ecological impacts, rather than by the number of species lost. Click HERE for a brief summary on this topic.
Five main extinction events have been recognized, these are known as the BIG 5.
Of all the ways in which scientists have proposed that it is possible to cause a mass extinction here are a few of the front runners. Conceptually there are 4 main ways in which the earth has tried to wipe out life on our planet.
Now that's all very well and good but, how do these physical effects cause mass extinctions?
It is important to realise that numbers 3 and 4 are essentially ways which have been put forward to explain 1 and 2, though Snowball earth and Greenhouse earth are still theories in their own right, as they are to some extent self replicating, i.e. we rather get stuck in vicious circles as the more we warm the earth, the more greenhouse gasses we can potentially release, (at least in theory).
As we know today the addition of Carbon dioxide into the atmosphere, stops ultra violet rays that impact into the earth from leaving again, by trapping them in the same way as a greenhouse does. Carbon dioxide is not the only gas that stops the escape of UV radiation, Methane is much more efficient. So in the same way as the addition of these gasses to the atmosphere causes warming, so the removal of these gasses from the atmosphere would plunge the earth into 'snowball' conditions.
Carbon dioxide is removed from the atmosphere all the time by plants during the process of photosynthesis, but a far more efficient removal process is needed to plunge the earth into icehouse conditions.
Another common cause of cooling is ice ages, thought to be caused to be caused by astronomical forcing, related to the earths orbit.
A snowball earth type mechanism is currently blamed for the End Ordovician extinction.
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To create greenhouse conditions you need a rapid release of greenhouse gasses into the atmosphere so rapid that the feedback mechanisms ( i.e. plants and rocks) cannot compensate quick enough.
Methane clathrates (otherwise known as Methane Hydrates) build up in permafrost, and in seafloor sediments, through the action of Methanogen bacteria buried in organic sediment. Methane clathrates are metastable, (i.e. unstable in most conditions) and can easily be released by climatic change, a change in sea level or earthquakes. Any one of these events would release huge amounts of methane into the atmosphere, so potentially starting the cycle of global warming.
Carbon dioxide levels are controlled to some extent in the earth system by feedback mechanisms including photosynthesis and carbonate deposition. The largest reservoir of Carbon Dioxide is in the Oceans, the best way to generate a huge release of carbon dioxide would be to keep the two reservoirs of the atmosphere and the ocean separate, then stop ocean circulation. This would in effect create a loaded gun, which would be unloaded when circulation returned to the ocean.
Pnatholossa the giant world ocean may well have stopped circulating after the late Palaeozioic glaciations, the lack of ice at the poles would have meant there was no influx of cold water to drive the oceanic heat engine, and since there where no isolated basin seas there was no hypersaline water to drive a saline circulation belt. All that needed to occur then was overturn of water to release huge volumes of carbon dioxide, which may have triggered the End Permian extinction.
In many of the mass extinctions known on earth there is evidence of meteor impact, from some location in the world, in the case of the Cretaceous- Tertiary event, there is the Chicxulub crater of the coast of New Mexico, which is widely stated as at very least a contributing factor to the events.
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The theory of extinctions caused by extraterrestrial bodies was first popularised by Louis Alverez, in his work on the K-T (Cretaceous Tertiary event). The ideas have been expanded by many scientists. An example of this is the Nemesis Hypothesis, put forward by Raup and Sepkoski.
The Nemesis Hypothesis
The theory as based around the observations of Raup and Sepkoski on the incidence of marine extinctions through time. They proposed that the extinctions observed on the earth had a cyclicity of 26Ma. This paper was read by Dr Richard Miller of the University of California, who was the first to link the cyclicity to a astronomical event. The theory goes that a brown giant star circles our sun somewhere between Pluto and the nearest star Proxima Centauri. When this brown giant passes the sun it disturbs the Oort cloud, which lies beyond the reaches of Pluto, causing comet showers in the interior of the solar system. There are however a number of problems with this theory, not least that the Nemesis planet has not yet been discovered though supporters of the theory would be quick to point out that a brown giant star emits little or no light, so would be rather hard to find. Other problems are that in the vastness of space the chances of a comet disturbed from the Oort cloud consistently hitting the earth are rather slim, (probably somewhere less than your chances of winning the lottery) though if a large number of comets were pulled form the Oort cloud, the chances would increase.
It Must be stressed that the Nemesis Hypothesis, is just that a hypothesis, devised as a possible explanation for the cyclicity in extinctions discovered by Raup and Sepkoski. There is however doubt even in the cyclicity as many Palaentoloigists argue that the dates quoted in the study are flawed. The theory does however present an interesting point that mass extinctions may not be as random and unpredictable as it first seems.
In the End Triassic extinction event, at least 2 craters have been found of about the right age, the first is in Western Australia, where scientists have discovered a 75 mile (120km) wide crater. Another has been found in Quebec Canada, surrounding the Manicouagan Reservoir, with an age of 210 million years old. Which seems to add weight to the theory that mass extinctions may well also be in some way assisted by impacts.
One of the questions associated with meteor impacts is how could it cause a mass extinction. Obviously several hundred tons of rock, hurtling towards you faster than sound is bad news if you happen to be stood underneath it, but if the impact were large enough, if would be equally bad news if you were on the other side of the earth. Molten fragments of rock would rain down for hundreds of miles around, starting forest fires, which would belch carbon dioxide and smoke in to the atmosphere. One of the first effects of an impact globally would be earthquakes which would lead to huge tsunamis or tidal waves all around the globe, destroying coastlines. For those organisms which had survived the first few months of this an even worse fate would be instore, the ash and dust which had been ejected into the atmosphere by the impact and the fires, would eventually shroud the globe plunging the earth into perpetual night. This night would last for years and would cause a crash in the ecosystems on both land and sea. The first to suffer would be those reliant on sunlight like the plants, in turn the herbivores would suffer and eventually the carnivores. Until all levels of the food chain would be affected, even the scavengers and parasites (because after all even fleas need to live somewhere), causing the extinctions of many groups of animals, and plants.
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It doesn't take a lot of imagination to realize that being covered in molten rock, at temperatures in excess of 400oc would be rather bad news. However you need a bit more imagination to understand to terrors of huge volcanic eruptions. Over larger areas it is the gasses, and ash that do the killing, after all one eruption could not generate enough lava to cover the whole world and lavas only spread a short distance.
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Recent work around Yellowstone National park in the USA, discovered the preserved remains of a whole herd of Rhinos. When the remains were examined by forensic scientists an unusual cause of death was discovered. The rhinos had essentially drowned in their own bodily fluids, but how? It turns out that Yellowstone National Park is essentially the 'crater' of a huge volcano. In volcanic eruptions huge amounts of ash are erupted into the atmosphere much if this is tiny shards of volcanic glass. The Rhinos had breathed these shards of glass, which had lodged in their lungs, puncturing and cutting the skin and leading to internal bleeding.. Even today we are well aware of the devastating effects of ash, when it comes in the form of Pyroclastic flows, which sweep rapidly down mountains, like a boiling avalanche, yet even these effects of ash are local and could not cause a worldwide extinction event like the one we see at the end of the Triassic.
From studies of volcanic eruptions today we know that huge volumes of gas are erupted into the atmosphere. We know a large part of this is Carbon Dioxide, which as we have already seen is a greenhouse gas, other gasses include Sulphur Dioxide, which today is a major cause of acid rain, and it's associated effects. There is also a large amount of dust and ash ejected into the upper atmosphere, which helps to cause some of the beautiful sunsets we see, this however helps to scatter the light destined for the earths surface.
To create a mass extinction we need to eject huge volumes of gas into the atmosphere, which requires huge scale volcanic eruptions most often associated with flood basalts, such as the Siberian, and Deccan Traps, which occurr when plumes of hot magma rise from the core, and break the earths surface. A likely cause of End Triassic vulcanism is the opening of the Atlantic Ocean.
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The simplest answer to this is climate. Every species alive on earth today and so presumably in the past is adapted to a certain range of conditions. In the same way that if we were suddenly whisked off to the North Pole and expected to live there for a year with only a wooly jumper and a Mars bar, we would surely die. It is the same in an extinction event, climatic zones essentially shift around the globe (so us being placed on the north pole is not as far fetched as you first thought), or ecosystems are starved of light or nutrients. This results in plants and animals being out of equilibrium with their surroundings, this not only causes the death of individuals but whole species. With the death of a species so there is a gap in the food chain, and so even animals which have adapted to the new climate find themselves with no food so die out and so on (see box 1)
BOX 1. EXTINCTION PROBLEMS IN A STRANGE ECOSYSTEM
If we go back to our example of us sat at the North Pole, but this time with a whole box of Mars bars, assuming we did not freeze to death in the first day, we would slowly become more accustomed to the cold, maybe we would find shelter from the cold, but either way as soon as our prey (the Mars Bars) ran out, unless we found an alternative source of food we would starve. To create a more dynamic ecosystem let us assume we provide a food source for another organism such as a polar bear, now if we died what would the polar bear eat?
Unfortunately my example falls down here as no one is going to believe that without us eating Mars bars they're going to reproduce uncontrollably. However, in our North Pole ecosystem of Mars bars, us and Polar bears, let us assume we find extra clothes and shelter, so the cold no longer controls our numbers, and we manage to find the recipe and ingredients for mars bars (and by some freak coincidence they provide us with all the essential nutrients for life), in this case it would be the polar bears controlling our numbers since they are our direct predators. So if polar bears were wiped out, then our numbers would no longer be controlled, so our population would over many generations grow, until eventually we could no longer supply ourselves with mars bars, or we may have even eaten all the mars bars in the world. This would cause a huge population crash, or maybe even extinction of the human race (or at least those dependant on Mars Bars). This rather abstract example helps illustrate the point that all the trophic levels of a food chain (or web) need to be in place, otherwise there will be instability in all other populations.
It is really the change in the climate caused by the physical event, that causes the death of the organism (unless of course there were a very small population of a species and they all happened to be stood under a falling meteor). So the mechanisms we discussed on this page are really the means to an ends or the triggers if you like. These triggers don't have to be as dramatic for normal extinctions of course, maybe a plant species is replaced by a newer and better adapted plant, as happened when grasses took over from ferns. In this case maybe the herbivores which evolved to live on the previous plant species, couldn't live on the new species and so would go extinct. Leading to extinctions across the food web. This could possibly cause a mass extinction though we have no obvious examples in the record.
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