Background extinction rate
Encyclopedia
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 event
s.
, 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 have not remained constant, although changes are measured over geological time, covering millions of years.
Adapted from the book “extinction rates”, edited by Lawton, J, and May, R.
Extinction
In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...
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 event
Extinction event
An extinction event is a sharp decrease in the diversity and abundance of macroscopic life. They occur when the rate of extinction increases with respect to the rate of speciation...
s.
Overview
Extinctions are a normal part of the evolutionary processNatural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
, 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
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
of extinction today than in all periods of non-extinction events before it.
Background extinction rates have not remained constant, although changes are measured over geological time, covering millions of years.
Measurement
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 |
| 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 |
Adapted from the book “extinction rates”, edited by Lawton, J, and May, R.
Accuracy
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.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.