Background extinction rate

Background extinction rate, also known as the normal extinction rate, refers to the standard rate of extinction in earth's geological and biological history before humans became a primary contributor to extinctions. This is primarily the pre-human extinction rates during periods in between major extinction events.

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 three different ways. 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.[5] 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.[6] 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.[7]

Lifespan estimates

Some species lifespan estimates by taxonomy are given below (Lawton & May 1995).[8]

Taxonomy Source of Estimate Species Average Lifespan (Millions of Years)
All Invertebrates Raup (1978) 11
Marine Invertebrates Valentine (1970) 5–10
Marine Animals Raup (1991) 4
Marine Animals Sepkoski (1992) 5
All Fossil Groups Simpson (1952) 0.5–5
Mammals Martin (1993) 1
Cenozoic Mammals Raup and Stanley (1978) 1–2
Diatoms Van Valen 8
Dinoflagelates 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

Accuracy

The fact that the total number of species, in both the past and the present, is currently unknown makes it very difficult to accurately calculate the non-anthropogenically influenced extinction rates. As a rate, it is essential to know not just the number of extinctions, but also the number of non-extinctions. This fact, coupled with the fact that the rates do not remain constant, significantly reduces accuracy in estimates of the normal rate of extinctions.

References
  1. E.g. Julia Whitty (2007-04-30). "Animal Extinction - the greatest threat to mankind: By the end of the century half of all species will be extinct. Does that matter?". The Independent. Archived from the original on 2015-08-06. Retrieved 2010-05-20. By the most conservative measure - based on the last century's recorded extinctions - the current rate of extinction is 100 times the background rate.
  2. Thackeray, J. Francis (1990). "Rates of Extinction in Marine Invertebrates: Further Comparison Between Background and Mass Extinctions". Paleobiology. Paleontological Society. 16 (1): 22–4. doi:10.1017/s0094837300009702. ISSN 1938-5331. JSTOR 2400930.
  3. N.L. Gilinsky (1994). "Volatility and the Phanerozoic decline of background extinction intensity". Paleobiology. 20 (4): 445–458. doi:10.1017/S0094837300012926. JSTOR 2401228.
  4. Raymond, H, Ward, P: “Hypoxia, Global Warming, and Terrestrial Late Permian Extinctions” Page 389–401. Science 15, 2005. http://www.sciencemag.org/cgi/content/full/308/5720/398
  5. American Museum of Natural History, 1998. http://www.amnh.org/science/biodiversity/extinction/Intro/OngoingProcess.html Archived 2008-04-06 at the Wayback Machine
  6. Pimm, S.: “The Extinction Puzzle”, Project Syndicate, 2007. http://www.project-syndicate.org/commentary/pimm1
  7. May, R. Lawton, J. Stork, N: “Assessing Extinction Rates” Oxford University Press, 1995.
  8. Lawton, John H.; May, Robert McCredie (1995-01-01). Extinction Rates. Oxford University Press. ISBN 9780198548294.

Further reading
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