Accelerated modern human–induced species losses: Entering the sixth mass extinction

TLDR

The claim that Earth is entering a sixth mass extinction hinges on demonstrating that current extinction rates far exceed the background rates that prevailed between previous mass extinctions, a point that has been challenged by earlier estimates that may overstate the crisis. The study aims to determine whether human activities are driving a mass extinction by applying extremely conservative assumptions. The authors compare a recent background extinction rate of 2 mammal extinctions per 10,000 species per 100 years with the current vertebrate extinction rate, using stringent criteria for declaring species extinct. They find that vertebrate extinction rates over the last century are up to 100 times higher than the background rate, implying that species that would have taken 800–10,000 years to disappear have vanished in a century, confirming an ongoing sixth mass extinction and highlighting a rapidly closing window for conservation.

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.