Extinctions are normally occurring in nature, 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, in an attempt to illustrate the higher degree of extinction in present day than in all periods of non-extinction events before it. Though it may seem that they would be fairly constant, background extinction rates have not remained constant over time, although the changes are measured in geographic time, which can cover millions of years . Given that biodiversity has been increasing since at least 3.5 billion years ago, the background extinction rate must be smaller than the rate of speciation
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
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-anthropogenicly 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.