. . . an on-going process

Mass Extinction

The earth's geologic history has been punctuated by five massive extinctions. In scientific terms a "mass extinction" can be described as follows:

A mass extinction is any substantial increase in the amount of extinction (i.e., lineage termination) suffered by more than one geographically widespread higher taxon during a relatively short interval of geologic time, resulting in an at least temporary decline in their standing diversity [numbers of species].

- Sepkoski (in Donovan, 1989)

There have been five extinction episodes that are agreed upon generally to qualify as "mass extinctions."

Major phylogenetic groups affected at each extinction pulse:

1. Ordovician (440 mya) -- extinction of deep- shelf benthic faunas including trilobites, graptolites, and conodonts.

2. Devonian (365 mya) -- decimation of coral reefs, brachiopods, and calcareous foraminifera.

3. Permian (250 mya) -- estimated 96% extinctions at species level in the marine realm, and for the first time, drastic reduction in the number of terrestrial tetrapod families.

4. Triassic (210 mya) -- extinctions wiped out 23% of both marine and non-marine animal families, including sponges, gastropods, bivalves, cephalopods, brachiopods, insects, and vertebrates.

5. Cretaceous (65 mya) -- the extinction of all non-avian dinosaurs, plus substantial to complete losses among such diverse groups as ammonites, nannoplankton, rudists, and certain marine reptiles.

The end-Pleistocene extinction event does not qualify as a mass extinction. It is better classified as a taxon-specific event, affecting primarily the Class Mammalia (although birds and, to a lesser extent, reptiles were also affected). Nor was it global, although later in the Quaternary many other regions were affected by dramatic losses of a similar sort.

Background Extinction

Background extinctions are at the opposite end of the spectrum from mass extinctions. It is generally assumed that "natural" extinctions go on all the time, but at very low or "background" rates. Characteristic of background extinctions is that they occur in a random or uncorrelated manner. One high estimate for the recent background extinction rate for birds is one species extinction per 400 years. If only this natural rate of loss affected the number of bird species, no more than a couple of extinctions should have occurred in the past 800 years. Scientists estimate that the actual loss during this time period lies somewhere between 200 and 2,000! The fact that today's extinction rate vastly exceeds any estimation of the background extinction rate impels many scientists to conclude that we are now on the cusp of the so-called Sixth Extinction.