Accelerated modern human–induced species losses: Entering the sixth mass extinction
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.
INTRODUCTION
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.
RESULTS
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.
| Vertebrate taxon | No. of species, IUCN 2014.3 | ||||
|---|---|---|---|---|---|
| Highly conservative rates (EX) | Conservative rates (EX + EW + PE) | No. of species evaluated by IUCN | |||
| Since 1500 | Since 1900 | Since 1500 | Since 1900 | ||
| Vertebrates | 338 | 198 | 617 | 477 | 59% (39,223) |
| Mammals | 77 | 35 | 111 | 69 | 100% (5,513) |
| Birds | 140 | 57 | 163 | 80 | 100% (10,425) |
| Reptiles | 21 | 8 | 37 | 24 | 44% (4,414) |
| Amphibians | 34 | 32 | 148 | 146 | 88% (6,414) |
| Fishes | 66 | 66 | 158 | 158 | 38% (12,457) |
Table 1 Numbers of species used in the Table 2 calculations of “highly conservative” and “conservative” modern extinction rates based on the IUCN Red List (17).
For the highly conservative rates, only species verified as “extinct” (EX) were included; for the conservative extinction rates, species in the categories “extinct in the wild” (EW) and “possibly extinct” (PE) were also included.
| Animal group | Elevation of modern rates with respect to expected rates | |||
|---|---|---|---|---|
| Highly conservative | Conservative | |||
| Since 1500 | Since 1900 | Since 1500 | Since 1900 | |
| Vertebrates | 8 | 22 | 15 | 53 |
| Mammals | 14 | 28 | 20 | 55 |
| Birds | 13 | 24 | 15 | 34 |
| Reptiles | 5 | 8 | 8 | 24 |
| Amphibians | 5 | 22 | 22 | 100 |
| Fishes | 5 | 23 | 12 | 56 |
Table 2 Elevation of “highly conservative” and “conservative” modern vertebrate extinction rates above background rate of 2 E/MSY (see table S2 for calculations).
For each assessment category, two periods are shown: extinction rates computed from 1500 to the present, and from 1900 to the present.
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).
Fig. 1 Cumulative vertebrate species recorded as extinct or extinct in the wild by the IUCN (2012).
Graphs show the percentage of the number of species evaluated among mammals (5513; 100% of those described), birds (10,425; 100%), reptiles (4414; 44%), amphibians (6414; 88%), fishes (12,457; 38%), and all vertebrates combined (39,223; 59%). Dashed black curve represents the number of extinctions expected under a constant standard background rate of 2 E/MSY. (A) Highly conservative estimate. (B) Conservative estimate.
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.
Fig. 2 Number of years that would have been required for the observed vertebrate species extinctions in the last 114 years to occur under a background rate of 2 E/MSY.
Red markers, highly conservative scenario; blue markers, conservative scenario. Note that for all vertebrates, the observed extinctions would have taken between 800 to 10,000 years to disappear, assuming 2 E/MSY. Different classes of vertebrates all show qualitatively similar trends.
DISCUSSION
Arguably the most serious aspect of the environmental crisis is the loss of biodiversity—the other living things with which we share Earth. This affects human well-being by interfering with crucial ecosystem services such as crop pollination and water purification and by destroying humanity’s beautiful, fascinating, and culturally important living companions (4, 5, 15, 27–30).
Our analysis shows that current extinction rates vastly exceed natural average background rates, even when (i) the background rate is considered to be double previous estimates and when (ii) data on modern vertebrate extinctions are treated in the most conservative plausible way. We emphasize that our calculations very likely underestimate the severity of the extinction crisis because our aim was to place a realistic “lower bound” on humanity’s impact on biodiversity. Therefore, although biologists cannot say precisely how many species there are, or exactly how many have gone extinct in any time interval, we can confidently conclude that modern extinction rates are exceptionally high, that they are increasing, and that they suggest a mass extinction under way—the sixth of its kind in Earth’s 4.5 billion years of history.
A final important point is that we focus exclusively on species, ignoring the extirpation of populations—the units relevant to ecological functioning and the delivery of ecosystem services (4, 5, 29). Population extinction cannot be reliably assessed from the fossil record, precluding any analysis along the lines of that presented here. Also, although it is clear that there are high rates of population extinction (18), existing data are much less reliable and far harder to obtain than those for species, which will remain true for the foreseeable future. Likewise, we have not considered animals other than vertebrates because of data deficiencies.
The evidence is incontrovertible that recent extinction rates are unprecedented in human history and highly unusual in Earth’s history. Our analysis emphasizes that our global society has started to destroy species of other organisms at an accelerating rate, initiating a mass extinction episode unparalleled for 65 million years. If the currently elevated extinction pace is allowed to continue, humans will soon (in as little as three human lifetimes) be deprived of many biodiversity benefits. On human time scales, this loss would be effectively permanent because in the aftermath of past mass extinctions, the living world took hundreds of thousands to millions of years to rediversify. Avoiding a true sixth mass extinction will require rapid, greatly intensified efforts to conserve already threatened species and to alleviate pressures on their populations—notably habitat loss, overexploitation for economic gain, and climate change (31–33). All of these are related to human population size and growth, which increases consumption (especially among the rich), and economic inequity (6). However, the window of opportunity is rapidly closing.
MATERIALS AND METHODS
To estimate modern extinction rates, we compiled data on the total number of described species and the number of extinct and possibly extinct vertebrate species from the 2014 IUCN Red List (17). In the IUCN’s list, extinct species can be viewed as the minimum number of actual extinctions during recent human history (that is, since 1500) because it lists species known to be extinct (EX), extinct in the wild (EW), and possibly extinct (PE, a subcategory within “critically endangered” reserved for species thought to be extinct, but not confirmed) (17) (table S1). We used the IUCN data to calculate modern extinction rates in two ways: (i) we estimate a “highly conservative modern extinction rate” by using the data exclusively on species listed as EX, and (ii) we calculate a “conservative extinction rate” by including also both EW and PE species (table S2). Including these latter two categories recognizes that there is only a slim chance that most of the species in those categories can reestablish viable populations in their native habitats. In terms of biological impact and the provision of ecosystem services, we consider EW and PE species to be functionally equivalent to EX species: even if some individuals still exist, their abundances are not sufficient to have a substantial influence on ecological function and processes.
The IUCN’s list is considered the authoritative, albeit likely conservative, assessment of the conservation status of plant and animal species. About 1.8 million species have been described since 1758 (when the current nomenclature system was developed), of which 1.3 million are animals (3, 17). Of these animal species, about 39,223 (of the currently counted 66,178) vertebrate species have been formally assessed and reported in the 2014 IUCN Red List (17). In the IUCN sample, mammals, birds, and amphibians have had between 88 and 100% of their known species evaluated, whereas only 44% of reptiles and 38% of fish species have been assessed (Table 1). We focus our comparisons on vertebrates because they are the group for which the most reliable data exist, both fossil and modern.
To produce conservative comparisons with modern extinctions, we assumed a background extinction rate of 2 E/MSY as the highest likely baseline average background extinction rate (7); that is, we should expect 2 extinctions per 10,000 vertebrate species per 100 years. That background extinction rate was empirically determined using the exceptionally good fossil records of mammals, combining extinction counts from paleontological databases and published literature on the fossil, subfossil, and historical records (7). Using the resulting high background extinction rate provides a stringent test for assessing whether current modern extinction rates indicate that a mass extinction event is under way. Previous estimates of background extinction rates for other taxa are invariably lower than the mammal-derived estimate of 2 E/MSY used here.
Acknowledgments
We would like to thank B. Young for helping us with the data on “possibly extinct species” published by IUCN. J. Soberon, C. Mendenhall, and J. Pacheco gave valuable suggestions on the manuscript. Funding: This work has been supported by the Programa de apoyo a proyectos de investigación e innovación tecnológica from UNAM. Competing interests: The authors declare that they have no competing interests.
Supplementary Material
Summary
Table S1. Definitions of IUCN categories (17) used to assess modern extinction rates.
Table S2. Estimation of modern extinction rates since 1500 and 1900.
Resources
File (1400253_sm.pdf)
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Information & Authors
Information
Published In
Science Advances
Volume 1 • Issue 5 • June 2015
History
Received: 23 December 2014
Accepted: 1 May 2015
Published online: 19 June 2015
Copyright
Copyright © 2015, The Authors.
This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.
RE: Figure 2 legend reversed?
Possibly it is my lack of understanding, but should the MORE conservative figure result in a smaller equivalent time to extinction?
Looking at Table S2 "1900 to 2014 Highly Conservative" F/E for Amphibians (for example) shows '22'
Looking at Table S2 "1900 to 2014 Conservative" F/E for Amphibians shows '100'.
This is reversed from Figure 2, which shows "Very Conservative" for Amphibians at 10,000 years and "Conservative" for Amphibians at 2,200 years.
RE: Accelerated modern human–induced species losses: Entering the sixth mass extinction
The article brings some interesting statistics showing the increase rate of extinctions among mammal and vertebrate. The over population of humans is obviously a great problem and has been for years. The population of humans in the early 1960s was around 3 billion and today over 7 billion and projected to be around 10 billion by 2020. No matter what mankind does that kind of overpopulation will put a stress on earth's ecosystems and animals and bio-systems. I remember scientists in the 1970s warning of overpopulation.. it is the fundamental basis for all the environmental problems. This article, if the stats are correct, bring this more to light.
However this article's very important point looses credibility with statements like: "All of these are related to human population size and growth, which increases consumption (especially among the rich), and economic inequity". Over population increases consumption - period. Consumption of resources is world wide and is effected by population increases world wide, not just the rich ( and I assume you meant rich countries). And population increases are happening more rapidly in the less "rich" countries. "Overexploitation for economic gain" is minor compared to just the sheer volume of people living on the earth that consume air and food and resources and create waste. The destruction of the rain forest in South America since the 1940s to make cheap farming land for the poor has probably done as much damage to species extinction as other factors. That rain forest is huge role in photosynthesis as well as maintaining millions of animal lives.
While this article is important in its statistics on animal extinctions, it barely touches on how the loss directly is caused by man or effects man or any solutions (which I understand that is not what this article was for). The article ended sounding of some sort of opinionated agenda which lessen its scientific credibility. Still very important.
Are we nearing "Dooms Day"?
Is human species who considered himself at the pedestal of superiority is inviting its own end? This is suggested in the present study by Ceballos et al. where they attempted to identify the next mass extinction. The pace at which species are getting extinct is alarming. Though there were reports earlier about the high rate of extinction of species in post industrialization era, yet lack of conclusive parameters and estimates always diluted the effect it should have on the policy makers and public. Comparison of "highly conservative modern extinction rate" and "background extinction rate" in this study provides a clear picture of rapid extinction of species. Man's consumerist approach has been responsible for bringing on the possibility of sixth mass extinction that could very well have been avoided by adopting small measures in light of knowledge of ecology and modern science.
Various anthropogenic factors like deforestation, aggressive exploitation of natural resources, mining and mindless dumping toxic wastes in natural environment may have plausible role for massive extinction. While we are trying to explore life on other planets, forgetting to concentrate to take measures to save our own planet. Man has lost the touch with nature and has become oblivious of his role of just a co-habitant on Earth not its master. Small steps like supporting the local animals and birds and separating solid wastes can go a long way in this direction. If we learn to appreciate the role of biodiversity for our well being, progress and future survival and found ways to nurture and conserve it in a sustainable manner, we can at least delay the mass extinction if not avoid it.
Consortium development and effective data collection and further analysis will lead to more awareness about the issue and conservation at global scale.
RE: Accelerated modern human-induced species losses: Enteringthe sixth mass extinction
Is a Sixth Global Mass Extinction Underway?
John C. Briggs*
In their recent article, published in Sciences Advances, Ceballos et al. (19 June 2015) endeavored to demonstrate that species extinctions by humans have caused the world to enter into its sixth mass extinction. As evidence, they compared extinction rates, among terrestrial vertebrates for the past few centuries, to the historical rate from the geological record. However, the historical rate was calculated only from data on mammal extinctions. The authors assumed that the historical mammal rate could be applied to all the other vertebrates (birds, reptiles, amphibians, and fishes). Using the mammal data, the authors determined that, without human influence, only nine vertebrate extinctions would have been expected since the year 1900.
According to the Red List (IUCN, 2014), 198 extinctions were recorded for all terrestrial vertebrates since the year 1900. Compared to the expected nine extinctions during that time (Caballos et al., 2015), an increase of 189 appears to indicate a disastrous human influence. Extending back to the year 1500, a total of 338 were documented by the Red List. In addition to these numbers, the authors recognized 279 more species under "extinct in the wild" and "possibly extinct" categories. These figures produced a grand total 617 vertebrate species lost or presumed lost since 1500. These impressive numbers persuaded the authors to declare that the evidence is "incontrovertible", that the recent extinction rates are unprecedented, and that the rate is highly unusual in Earth history. Therefore, they concluded that a mass extinction episode had been initiated.
The unfortunate aspect of this work is that no reference was made to other estimates of global extinctions using other faunal groups. Extinctions recorded in the marine environment were not considered, and no mention was made about concurrent gains in species diversity. About 90% of the terrestrial world's species are insects. A recent estimate by an entomologist (Dunn, 2005) indicates a species diversity of about 3.4 million. While many insect species are still undescribed, certain groups, (butterflies, tiger beetles, dragonflies, and damselflies) are well known and their extinctions during the past 500 years have been documented. Within those groups, which total 25,260 species, only two species have become extinct (Briggs, 2014). In the marine environment, which covers about 71% of the Earth's surface, the diversity of multicellular species is about 2.21 million (Mora et al., 2011); of this number, only 20 have been recorded to be extinct (Dulvy et al., 2009). Furthermore, a recent, uncited paleontological work on extinctions (MacLeod, 2013) provides valuable information on Pleistocene and modern extinction rates. Finally, there is good evidence that points to continual gains in diversity due to species invasions and rapid ecological speciation (Briggs, 2014).
Aside from the neglect of the published information cited above, the attempt to forecast a global extinction on the basis of only terrestrial vertebrate animals (Ceballos et al., 2015), is unconvincing when comparable information on other faunal groups is not utilized. Also, in dealing with the numbers of vertebrate extinctions, the authors failed to mention that most of them took place on oceanic islands. In regard to the mammals and birds, more than 95% of all extinctions during the past 500 years were island species (Loehle and Eschenbach, 2012). If we use the mammals and birds as surrogates for all the vertebrates, extinctions on the world's continents must have been very low. For example, only three of the 61 mammal extinctions took place on continents. The loss of isolated island species had little or no effect on continental ecology.
This is the latest in a series of books and papers, published over the past 40 years, devoted to the frightening vision of a human-caused mass extinction. Its use of colorful graphs, showing vertebrate species dying off by the hundreds, presents a doomsday scenario that many are likely to believe, unless they consider the global extinction evidence.
References
Briggs, J.C. (2014) Global biodiversity gain is concurrent with declining population sizes. Biodiversity Journal, 5, 447-452.
Ceballos, G., Ehrlich, P.R., Barnosky, A.D., García, A., Pringle, R.M. & Palmer, T.M. (2015) Accelerated modern human-induced species losses: entering the sixth mass extinction. Science Advances. 1:e 1400253.
Dulvy, N.K., Pinnegar, J.K., & Reynolds, J.D. (2009) Holocene extinctions in the sea. In: Turvey, S.T. (Ed.) Holocene extinctions. Oxford University Press, Oxford, pp. 129-150.
Dunn, R.R. (2005) Modern insect extinctions: the neglected majority. Conservation Biology, 19, 1030-1036.
IUCN (2014) www.iucnredlist.org (March 20, 2015).
Loehle, C. & Eschenbach, W. (2012) Historic bird and terrestrial mammal extinction: rates and causes. Diversity and Distribution, 18, 84-91.
MacLeod, N. (2013) The great extinctions. Firefly Books, Buffalo, NY.
Mora, C., Tittensor, D.P., Adl, S., Simpson, A.G.B. & Worm, B. (2011) How many species are there on Earth and in the ocean? PLoS Biol. 9, e21001127.
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*Department of Fisheries and Wildlife, Oregon State University, Corvallis, OR 97333, USA; e-mail: [email protected] Current address: 2320 Guerneville Rd., Santa Rosa, CA 95403,