Mass Extinctions

Mass Extinctions

Mass Extinction
An extinction event is a sharp and sudden decrease in the diversity and abundance of macro and even microscopic life. Because the majority of diversity and biomass on earth is microbial in nature, an extinction event is often recorded on the most easily observed component of the biosphere.
Basic Facts:
Roughly over 98% of all species that have ever lived in the world are extinct; however extinction occurs at an uneven rate. It has been theorised that since life began on Earth, several major mass extinctions have already occurred beyond what science can measure. The most recent and famous is the Cretaceous-Tertiary Extinction event which saw the end of almost all dinosaur species; a species that had dominated the world for over 160 million years.
Causes:
Flood basalt events
The formation of large igneous provinces by flood basalt events could have produced massive amounts of dust and particulate aerosols inhibiting photosynthesis, sulphur oxides which precipitated acid rain and carbon dioxide that would have caused sustain global warming.
Impact events
One of the most popular causes, a impact by a sufficiently large comet or asteroid would have caused widespread destruction in addition to the collapse of food chains due to the tremendous amounts of dust thrown into the sky. This coupled with acid rain and mega-tsunamis and global forest fires would have been deadly.
Global Cooling
Sustained global cooling would kill many polar and temperate species and force others to migrate towards the equator. This is usually attributed to ice ages. Global Warming
This would have the opposite effect of the above. It would also cause the melting of polar ice and adding increasing volumes of water to the water cycle, causing flooding and anoxic events. C
ontinental 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. 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.
Effects
The impact of mass extinction events varied widely. The worst event, the Permian-Triassic extinction event, devastated life on earth and is estimated to have killed off over 90% of species. Life on Earth seemed to recover quickly after this extinction, but this was mostly in the form of disaster taxa, such as the 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 the successive waves of extinction which inhibited recovery, as well as to prolonged environmental stress to organisms 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-Tr extinction, i.e. in the late Triassic.

Jonah Wang 10A05