Extinctions Caused by Abiotic Factors
Fossil records show that over time, there have been five mass extinctions caused by natural physical (abiotic) phenomenon. A mass extinction is when a large proportion of the total number of species on Earth were wiped out. The Earth is currently undergoing a sixth mass extinction due to human activities (biotic factors). In a landmark paper published in 1982,
Jack Sepkoski and David M. Raup identified five mass extinctions.
1. Cretaceous –Tertiary extinction event (End Cretaceous or K-T extinction)
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65.5 million years ago
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Probably caused by impact of a several mile wide asteroid that created a huge crater now hidden beneath the Gulf of Mexico
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Other possible causes are gradual climate change and flood-like volcano eruptions near India.
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The extinction killed 16% of marine families, 47% of marine genera & 18% of land vertebrates, including dinosaurs.
2. Triassic –Jurassic extinction event (End Triassic)
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199-214 million years ago
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Probably caused by massive floods of lava from an opening in the Atlantic
Ocean
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23% of families, and 48% of all genera went extinct
3. Permian –Triassic extinction event (End Permian)
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251 million years ago
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Probably caused by a comet or asteroid impact – no direct evidence.
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Other possible cause was flood volcanism from Siberia, which destroyed algae and plants reducing oxygen levels in the sea.
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The largest extinction event in history
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Wiped out 95% of all species, 53% of marine families, 84% of marine genera and 70% of land species
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The "Great Dying" had enormous evolutionary significance: on land, it ended the primacy of mammal-like reptiles. In the seas, the percentage of animals that were sessile dropped from 67% to 50%.
4. Late Devonian extinction
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360 –375 million years ago.
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Cause unknown
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19% of all families, 50% of all genera went extinct
5. Ordovician –Silurian extinction event (End Ordovician or O-S)
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440 –450 million years
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Caused by a drop in sea levels as glaciers formed then by rising sea levels as glaciers melted.
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27% of all families and 57% of all genera became extinct

The average time between these mass extinction events is around 100 million years. The exception is the gap between events 2&3 above which were about 50 million years apart.
The total number of species on earth remains poorly understood. The NSF estimates there could be anywhere between 5-100 million species on the planet but science has only identified 1.8 million. It is impossible to get an accurate count as most undiscovered or undocumented species are small – insects, bacteria and other microbes.
“The Sixth Extinction”
Can be divided into two discrete stages:
 phase 1 began when the first modern humans began to disperse to different parts of the world about 100 000 years ago
 phase 2 began about 10 000 years ago when humans turned to agriculture
Current Situation:
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The development of agriculture and clearance of native ecosystems accelerated the pace of extinction.
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Over-population, invasive species and over-exploitation are fuelling the extinction.
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Pollution and the advent of global warming are also accelerating changes to the planet and increasing extinction rates in species that cannot adapt to the changing conditions or migrate to new areas.
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The background (natural) level of extinction (known from fossil records) is between 10 and 100 species per year.
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Human activities have increased this rate.
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Estimates from tropical areas suggest that the Earth is losing 27 000 species from those habitats alone.
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The rate of extinction varies for different groups of organisms
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Extinction sin the past took place over geological time (1000s of millions of years), allowing new species to evolve and replace the extinct species.
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Current changes to the planet are occurring much faster (over the period of human lifetimes)
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This does not allow time for new species to adapt, change and evolve
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Scientists have predicted that 50% of all species could be extinct by the end of the 21 st century.
Mammals as a case study:
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Mammal species have an average species lifespan (from origin to extinction) of about 1 million years.
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About 5000 known mammal species
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Extinction rate should be about 1 species every 200 years.
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Past 400 years have seen 89 mammal species extinctions
– about 45 times the previous rate.
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Another 169 mammal species are listed as critically endangered.