Background extinction rate

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Background extinction rate, also known as the normal extinction rate, refers to the standard rate of extinction in Earth's geological and biological history, excluding major extinction events, including the current human-induced Holocene extinction. There have been five mass extinction events throughout Earth's history.

Contents

Overview

Extinctions are a normal part of the evolutionary process, and the background extinction rate is a measurement of "how often" they naturally occur. Normal extinction rates are often used as a comparison to present day extinction rates, to illustrate the higher frequency of extinction today than in all periods of non-extinction events before it. [1]

Background extinction rates have not remained constant, although changes are measured over geological time, covering millions of years. [2] [3] [4]

Measurement

Background extinction rates are typically measured in order to give a specific classification to a species and this is obtained over a certain period of time. There are three different ways to calculate background extinction rate. [5] The first is simply the number of species that normally go extinct over a given period of time. For example, at the background rate one species of bird will go extinct every estimated 400 years. [6] Another way the extinction rate can be given is in million species years (MSY). For example, there is approximately one extinction estimated per million species years. [7] From a purely mathematical standpoint this means that if there are a million species on the planet earth, one would go extinct every year, while if there was only one species it would go extinct in one million years, etc. The third way is in giving species survival rates over time. For example, given normal extinction rates species typically exist for 5–10 million years before going extinct. [8]

Lifespan estimates

Some species lifespan estimates by taxonomy are given below (Lawton & May 1995). [9]

TaxonomySource of EstimateSpecies Average Lifespan (Millions of Years)
All Invertebrates Raup (1978)11
Marine Invertebrates Valentine (1970)5–10
Marine AnimalsRaup (1991)4
Marine AnimalsSepkoski (1992)5
All Fossil GroupsSimpson (1952)0.5–5
Mammals Martin (1993)1
Cenozoic MammalsRaup and Stanley (1978)1–2
Diatoms Van Valen8
Dinoflagellates Van Valen (1973)13
Planktonic Foraminifera Van Valen (1973)7
Cenozoic Bivalves Raup and Stanley (1978)10
Echinoderms Durham (1970)6
Silurian Graptolites Rickards (1977)2

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References

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Further reading