![]() ![]() Here, we analyze the modern rates of vertebrate species extinction and compare them with a recently computed background rate for mammals ( 7). Population-level extinction directly threatens ecosystem services and is the prelude to species-level extinction ( 18). Moreover, even in species that are not currently threatened, the extirpation of populations is frequent and widespread, with losses that far outstrip species-level extinctions ( 18, 25). Records of extinction for reptiles, amphibians, freshwater fishes, and other organisms have mainly been documented since the beginning of the 20th century ( 14, 17). More species extinction records date from the 19th century and include numerous species of mammals and birds. Written records of extinctions of large mammals, birds, and reptiles date back to the 1600s and include species such as the dodo ( Raphus cucullatus, extinguished in the 17th century), Steller’s sea cow ( Hydrodamalis gigas, extinguished in the 18th century), and the Rodrigues giant tortoise ( Cylindraspis peltastes, extinguished in the 19th century). For example, in the islands of tropical Oceania, up to 1800 bird species (most described in the last few decades from subfossil remains) are estimated to have gone extinct in the ~2000 years since human colonization ( 24). A growing body of evidence indicates that current species extinction rates are higher than the pre-human background rate ( 8– 15), with hundreds of anthropogenic vertebrate extinctions documented in prehistoric and historic times ( 16– 23). The loss of biodiversity is one of the most critical current environmental problems, threatening valuable ecosystem services and human well-being ( 1– 7). 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. These estimates reveal an exceptionally rapid loss of biodiversity over the last few centuries, indicating that a sixth mass extinction is already under way. 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. 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. The latter is conservatively low because listing a species as extinct requires meeting stringent criteria. We then compare this rate with the current rate of mammal and vertebrate extinctions. 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 assess, using extremely conservative assumptions, whether human activities are causing a mass extinction. Earlier estimates of extinction rates have been criticized for using assumptions that might overestimate the severity of the extinction crisis. 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. ![]()
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