Abstract
The oft-repeated
claim that Earth’s biota is entering a sixth “mass extinction” depends on
clearly demonstrating that current extinction rates are far above the
“background” rates prevailing between the five previous mass extinctions. Earlier
estimates of extinction rates have been criticized for using assumptions that
might overestimate the severity of the extinction crisis. We assess, using
extremely conservative assumptions, whether human activities are causing a mass
extinction. First, we use a recent estimate of a background rate of 2 mammal
extinctions per 10,000 species per 100 years (that is, 2 E/MSY), which is twice
as high as widely used previous estimates. We then compare this rate with the
current rate of mammal and vertebrate extinctions. The latter is conservatively
low because listing a species as extinct requires meeting stringent criteria. Even
under our assumptions, which would tend to minimize evidence of an incipient
mass extinction, the average rate of vertebrate species loss over the last
century is up to 100 times higher than the background rate. Under the 2 E/MSY
background rate, the number of species that have gone extinct in the last
century would have taken, depending on the vertebrate taxon, between 800 and
10,000 years to disappear. These estimates reveal an exceptionally rapid loss
of biodiversity over the last few centuries, indicating that a sixth mass
extinction is already under way. Averting a dramatic decay of biodiversity and
the subsequent loss of ecosystem services is still possible through intensified
conservation efforts, but that window of opportunity is rapidly closing.
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