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.
- Sixth mass extinction
- vertebrate extinctions
- rates of extinction
- background extinction
- modern vertebrate losses
The loss of biodiversity is one of the most critical current environmental problems, threatening valuable ecosystem services and human well-being (1–7). 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). 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). 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). More species extinction records date from the 19th century and include numerous species of mammals and birds. Records of extinction for reptiles, amphibians, freshwater fishes, and other organisms have mainly been documented since the beginning of the 20th century (14, 17). 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). Population-level extinction directly threatens ecosystem services and is the prelude to species-level extinction (18).
Here, we analyze the modern rates of vertebrate species extinction and compare them with a recently computed background rate for mammals (7). We specifically addressed the following questions: (i) Are modern rates of mammal and vertebrate extinctions higher than the highest empirically derived background rates? (ii) How have modern extinction rates in mammals and vertebrates changed through time? (iii) How many years would it have taken for species that went extinct in modern times to have been lost if the background rate had prevailed? These are important issues because the uncertainties about estimates of species loss have led skeptics to question the magnitude of anthropogenic extinctions (26) and because understanding the magnitude of the extinction crisis is relevant for conservation, maintenance of ecosystem services, and public policy.
Until recently, most studies of modern extinction rates have been based on indirect estimates derived, for example, on the rates of deforestation and on species-area relationships (11, 14). Problems related to estimating extinction since 1500 AD (that is, modern extinctions) have been widely discussed, and the literature reflects broad agreement among environmental scientists that biases lead to underestimating the number of species that have gone extinct in the past few centuries—the period during which Homo sapiens truly became a major force on the biosphere (1–4, 6–8, 14, 15). However, direct evaluation is complicated by uncertainties in estimating the incidence of extinction in historical time and by methodological difficulties in comparing contemporary extinctions with past ones.
Less discussed are assumptions underlying the estimation of background extinction rates. The lower these estimates, the more dramatic current extinction rates will appear by comparison. In nearly all comparisons of modern versus background extinction rates, the background rate has been assumed to be somewhere between 0.1 and 1 species extinction per 10,000 species per 100 years (equal to 0.1 to 1 species extinction per million species per year, a widely used metric known as E/MSY). Those estimates reflect the state of knowledge available from the fossil record in the 1990s (7, 9–13). In a recent analysis, which charted the stratigraphic ranges of thousands of mammal species, extinction rates were measured over intervals ranging from single years to millions of years, and the mean extinction rate and variance were computed for each span of time (7). In this way, the background extinction rate estimated for mammals was estimated at 1.8 E/MSY, here rounded upward conservatively to 2 E/MSY (that is, 2 extinctions per 100 years per 10,000 species). This is double the highest previous rough estimate.
Those previously estimated background rates were primarily derived from marine invertebrate fossils, which are likely to have greater species longevity than vertebrates (10, 15). Data deficiencies make it impossible to conduct empirical analyses (as was done for mammals) for non-mammal terrestrial vertebrates; therefore, we assume the background rates of other vertebrates to be similar to those of mammals. This supposition leads to a more conservative assessment of differences between current and past extinction rates for the vertebrates as a whole, compared with using the very low background extinction rate derived from marine invertebrates.
The analysis we present here avoids using assumptions such as loss of species predicted from species-area relationships, which can suggest very high extinction rates, and which have raised the possibility that scientists are “alarmists” seeking to exaggerate the impact of humans on the biosphere (26). Here, we ascertain whether even the lowest estimates of the difference between background and contemporary extinction rates still justify the conclusion that people are precipitating a global spasm of biodiversity loss.
Modern and background rates of vertebrate extinctions
Modern rates of vertebrate extinction were much higher than a background extinction rate of 2 E/MSY. Among the vertebrate taxa evaluated by the International Union of Conservation of Nature (IUCN), 338 extinctions have been documented since 1500 [“extinct” (EX), Table 1]. An additional 279 species have become either “extinct in the wild” (EW) or listed as “possibly extinct” (PE), totaling 617 vertebrate species summed over the three categories. Most extinctions have occurred in the last 114 years (that is, since 1900; Table 1). Our estimated “highly conservative” (that is, using data for EX species only) and “conservative” (that is, by including EX, EW, and PE) modern extinction rates for vertebrates varied from 8 to 100 times higher than the background rate (Table 2). This means, for example, that under the 2 E/MSY background rate, 9 vertebrate extinctions would have been expected since 1900; however, under the conservative rate, 468 more vertebrates have gone extinct than would have if the background rate had persisted across all vertebrates under that period. Specifically, these 468 species include 69 mammal species, 80 bird species, 24 reptiles, 146 amphibians, and 158 fish.
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Variation in modern extinction rates through time
Modern extinction rates have increased sharply over the past 200 years (corresponding to the rise of industrial society) and are considerably higher than background rates (Fig. 1). Rates of modern extinctions vary among vertebrate groups (Fig. 1). For example, amphibians, comprising of ~7300 species, show an accelerating rate of extinction: only 34 extinctions have been documented with a high level of certainty since 1500, yet >100 species have likely disappeared since 1980 (17, 23). This may not only reflect real trends but also a shortage of data for groups for which most species are not yet evaluated, such as reptiles and fish (21, 22).
Modern extinctions if background rate had prevailed
Our results indicate that modern vertebrate extinctions that occurred since 1500 and 1900 AD would have taken several millennia to occur if the background rate had prevailed. The total number of vertebrate species that went extinct in the last century would have taken about 800 to 10,000 years to disappear under the background rate of 2 E/MSY (Fig. 2). The particularly high losses in the last several decades accentuate the increasing severity of the modern extinction crisis.