Background extinction rate
Background extinction rate, also known as ‘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.
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.
Background extinction rates are typically measured 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. 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. 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.
Lifespan estimates 
Some species lifespan estimates by taxonomy
|Taxonomy||Source of Estimate||Species Average Lifespan years (MYA)|
|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)||.5–5|
|Cenozoic Mammals||Raup and Stanley (1978)||1–2|
|Dinoflagelates||Van Valen (1973)||13|
|Planktonic Foraminifera||Van Valen (1973)||7|
|Cenozoic Bivalves||Raup and Stanley (1978)||10|
|Silurian Graptolites||Rickards (1977)||2|
Adapted from the book “extinction rates”, edited by Lawton, J, and May, R.
The fact that we do not currently know the total number of species, in the past nor the present, 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.
- 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. 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."
- Thackery, J: “Rates of extinction in marine invertebrates: Further comparison between background and mass extinctions” page 22–24. Paleobiology 16, 1990 http://www.jstor.org/view/00948373/di995943/99p0729h/0
- N.L. Gilinsky (1994). "Volatility and the Phanerozoic decline of background extinction intensity". Paleobiology 20 (4): 445–458. JSTOR 2401228.
- 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
- American Museum of Natural History, 1998. http://www.amnh.org/science/biodiversity/extinction/Intro/OngoingProcess.html
- Pimm, S.: “The Extinction Puzzle”, Project Syndicate, 2007. http://www.project-syndicate.org/commentary/pimm1
- May, R. Lawton, J. Stork, N: “Assessing Extinction Rates” Oxford University Press, 1995.
- Lawton j, May, R: “extinction rates” Oxford University Press, Science, 1995. http://books.google.com/books?id=jDZMXF4bCccC&pg=PA1&dq=Extinction+rate&sig=pjYsNJ60bsJbRUJBVUQXUwhRkjQ#PPA45,M1
Further reading 
- E.O. Wilson. 2005. The Future of Life. Alfred A. Knopf. New York, New York, USA
- C.Michael Hogan. 2010. Edenic Period. Encyclopedia of Earth. National Council for Science and Environment. ed. Galal Hassan, ed in chief Cutler Cleveland, Washington DC
- J.H.Lawton and R.M.May (2005) Extinction rates, Oxford University Press, Oxford.