0.15 Extinction

Extinction    (the complete disappearance of a species from Earth) is an important part ofthe evolution of life on Earth. The current diversity of species is a product of the processes of extinction and speciationthroughout the previous 3.8 billion year history of life. Raup ( 1991 ) assumed that there might be 40 million species alive today, but between 5 and 50 billionspecies have lived at some time during the history of the Earth. Therefore, Raup estimated that 99.9% of all the life thathas existed on Earth is now extinct ); a species is assumed to be extinct when there is no reasonable doubt that the lastindividual has died ( IUCN, 2002 ). However, extinction has not occurred at a constant pace throughthe Earth’s history. There have been at least five periods when there has been a sudden increase in the rate ofextinction, such that the rate has at least doubled, and the extinctions have included representatives from many differenttaxonomic groups of plants and animals; these events are called mass extinctions . The timing of these mass extinctions is shown in .

Each of the first five mass extinctions shown in represents a significant loss of biodiversity - but recovery has been good on a geologic time scale. Massextinctions are apparently followed by a sudden burst of evolutionary diversification on the part of the remainingspecies, presumably because the surviving species started using habitats and resources that were previously "occupied" by morecompetitively successful species that went extinct. However, this does not mean that the recoveries from mass extinction havebeen rapid; they have usually required some tens of millions of years ( Jablonski, 1995 ).

It is hypothesized that we are currently on the brink of a "sixth mass extinction," but one that differs from previousevents. The five other mass extinctions predated humans and were probably the ultimate products of some physical process( e.g. climate change through meteor impacts), rather than the direct consequence of the action of some otherspecies. In contrast, the sixth mass extinction is the product of human activity over the last several hundred, or even severalthousand years. These mass extinctions, and their historic and modern consequences are discussed in more detail in the moduleson Historical perspectives on extinction and the current biodiversity crisis , and Ecological consequences of extinctions. .