Has the Earth’s sixth mass extinction already arrived?

Journal name:
Nature
Volume:
471,
Pages:
51–57
Date published:
DOI:
doi:10.1038/nature09678
Published online

Palaeontologists characterize mass extinctions as times when the Earth loses more than three-quarters of its species in a geologically short interval, as has happened only five times in the past 540million years or so. Biologists now suggest that a sixth mass extinction may be under way, given the known species losses over the past few centuries and millennia. Here we review how differences between fossil and modern data and the addition of recently available palaeontological information influence our understanding of the current extinction crisis. Our results confirm that current extinction rates are higher than would be expected from the fossil record, highlighting the need for effective conservation measures.

At a glance

Figures

  1. Relationship between extinction rates and the time interval over which the rates were calculated, for mammals.
    Figure 1: Relationship between extinction rates and the time interval over which the rates were calculated, for mammals.

    Each small grey datum point represents the E/MSY (extinction per million species-years) calculated from taxon durations recorded in the Paleobiology Database30 (million-year-or-more time bins) or from lists of extant, recently extinct, and Pleistocene species compiled from the literature (100,000-year-and-less time bins)6, 32, 33, 89, 90, 91, 92, 93, 94, 95, 96, 97. More than 4,600 data points are plotted and cluster on top of each other. Yellow shading encompasses the ‘normal’ (non-anthropogenic) range of variance in extinction rate that would be expected given different measurement intervals; for more than 100,000years, it is the same as the 95% confidence interval, but the fading to the right indicates that the upper boundary of ‘normal’ variance becomes uncertain at short time intervals. The short horizontal lines indicate the empirically determined mean E/MSY for each time bin. Large coloured dots represent the calculated extinction rates since 2010. Red, the end-Pleistocene extinction event. Orange, documented historical extinctions averaged (from right to left) over the last 1, 30, 50, 70, 100, 500, 1,000 and 5,000 years. Blue, attempts to enhance comparability of modern with fossil data by adjusting for extinctions of species with very low fossilization potential (such as those with very small geographic ranges and bats). For these calculations, ‘extinct’ and ‘extinct in the wild’ species that had geographic ranges less than 500km2 as recorded by the IUCN6, all species restricted to islands of less than 105km2, and bats were excluded from the counts (under-representation of bats as fossils is indicated by their composing only about 2.5% of the fossil species count, versus around 20% of the modern species count30). Brown triangles represent the projections of rates that would result if ‘threatened’ mammals go extinct within 100, 500 or 1,000years. The lowest triangle (of each vertical set) indicates the rate if only ‘critically endangered’ species were to go extinct (CR), the middle triangle indicates the rate if ‘critically endangered’ + ‘endangered’ species were to go extinct (EN), and the highest triangle indicates the rate if ‘critically endangered’ + ‘endangered’ + ‘vulnerable’ species were to go extinct (VU). To produce Fig. 1 we first determined the last-occurrence records of Cenozoic mammals from the Paleobiology Database30, and the last occurrences of Pleistocene and Holocene mammals from refs 6, 32, 33 and 89–97. We then used R-scripts (written by N.M.) to compute total diversity, number of extinctions, proportional extinction, and E/MSY (and its mean) for time-bins of varying duration. Cenozoic time bins ranged from 25 million to a million years. Pleistocene time bins ranged from 100,000 to 5,000 years, and Holocene time bins from 5,000years to a year. For Cenozoic data, the mean E/MSY was computed using the average within-bin standing diversity, which was calculated by counting all taxa that cross each 100,000-year boundary within a million-year bin, then averaging those boundary-crossing counts to compute standing diversity for the entire million-year-and-over bin. For modern data, the mean was computed using the total standing diversity in each bin (extinct plus surviving taxa). This method may overestimate the fossil mean extinction rate and underestimate the modern means, so it is a conservative comparison in terms of assessing whether modern means are higher. The Cenozoic data are for North America and the Pleistocene and Holocene data are for global extinction; adequate global Cenozoic data are unavailable. There is no apparent reason to suspect that the North American average would differ from the global average at the million-year timescale.

  2. Extinction magnitudes of IUCN-assessed taxa  in comparison to the 75% mass-extinction benchmark.
    Figure 2: Extinction magnitudes of IUCN-assessed taxa 6 in comparison to the 75% mass-extinction benchmark.

    Numbers next to each icon indicate percentage of species. White icons indicate species ‘extinct’ and ‘extinct in the wild’ over the past 500years. Black icons add currently ‘threatened’ species to those already ‘extinct’ or ‘extinct in the wild’; the amphibian percentage may be as high as 43% (ref. 19). Yellow icons indicate the Big Five species losses: Cretaceous + Devonian, Triassic, Ordovician and Permian (from left to right). Asterisks indicate taxa for which very few species (less than 3% for gastropods and bivalves) have been assessed; white arrows show where extinction percentages are probably inflated (because species perceived to be in peril are often assessed first). The number of species known or assessed for each of the groups listed is: Mammalia 5,490/5,490; Aves (birds) 10,027/10,027; Reptilia 8,855/1,677; Amphibia 6,285/6,285, Actinopterygii 24,000/5,826, Scleractinia (corals) 837/837; Gastropoda 85,000/2,319; Bivalvia 30,000/310, Cycadopsida 307/307; Coniferopsida 618/618; Chondrichthyes 1,044/1,044; and Decapoda 1,867/1,867.

  3. Extinction rate versus extinction magnitude.
    Figure 3: Extinction rate versus extinction magnitude.

    Vertical lines on the right illustrate the range of mass extinction rates (E/MSY) that would produce the Big Five extinction magnitudes, as bracketed by the best available data from the geological record. The correspondingly coloured dots indicate what the extinction rate would have been if the extinctions had happened (hypothetically) over only 500years. On the left, dots connected by lines indicate the rate as computed for the past 500years for vertebrates: light yellow, species already extinct; dark yellow, hypothetical extinction of ‘critically endangered’ species; orange, hypothetical extinction of all ‘threatened’ species. TH: if all ‘threatened’ species became extinct in 100years, and that rate of extinction remained constant, the time to 75% species loss—that is, the sixth mass extinction—would be ~240 to 540years for those vertebrates shown here that have been fully assessed (all but reptiles). CR: similarly, if all ‘critically endangered’ species became extinct in 100years, the time to 75% species loss would be ~890 to 2,270years for these fully assessed terrestrial vertebrates.

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Author information

Affiliations

  1. Department of Integrative Biology, University of California, Berkeley, California 94720, USA

    • Anthony D. Barnosky,
    • Nicholas Matzke,
    • Susumu Tomiya,
    • Guinevere O. U. Wogan,
    • Brian Swartz,
    • Tiago B. Quental,
    • Charles Marshall,
    • Jenny L. McGuire,
    • Emily L. Lindsey,
    • Kaitlin C. Maguire,
    • Ben Mersey &
    • Elizabeth A. Ferrer
  2. University of California Museum of Paleontology, California, USA

    • Anthony D. Barnosky,
    • Susumu Tomiya,
    • Brian Swartz,
    • Tiago B. Quental,
    • Charles Marshall,
    • Jenny L. McGuire,
    • Emily L. Lindsey,
    • Kaitlin C. Maguire &
    • Elizabeth A. Ferrer
  3. University of California Museum of Vertebrate Zoology, California, USA

    • Anthony D. Barnosky,
    • Susumu Tomiya,
    • Guinevere O. U. Wogan &
    • Jenny L. McGuire
  4. Human Evolution Research Center, California, USA

    • Ben Mersey
  5. Present address: Departamento de Ecologia, Universidade de São Paulo (USP), São Paulo, Brazil

    • Tiago B. Quental
  6. Present address: National Evolutionary Synthesis Center, 2024 W. Main Street, Suite A200, Durham, North Carolina 27705, USA.

    • Jenny L. McGuire

Contributions

All authors participated in literature review and contributed to discussions that resulted in this paper. A.D.B. planned the project, analysed and interpreted data, and wrote the paper. N.M. and S.T. performed key data analyses and interpretation relating to rate comparisons. G.O.U.W., B.S. and E.L.L. assembled critical data. T.B.Q. and C.M. provided data, analyses and ideas relating to diversity dynamics and rate-magnitude comparisons. J.L.M. helped produce figures and with N.M., S.T., G.O.U.W., B.S., T.B.Q., C.M., K.C.M., B.M. and E.A.F. contributed to finalizing the text.

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Comments

  1. Report this comment #19649

    Mark Thompson said:

    I agree with your point Ryskin and this is an error that creeps up in conservation literature and the analysis on rates of extinction. However, the notion of individuality in the hierarchical view of life may help in this regard (i.e., treating species as individuals – Ghislen, Hull and Gould wrote extensively on this topic). I have several concern about using species or even genera to examine rates of extinction over time.

    Foremost, as you point out it is not the right statistic, but for reasons that pertain to ecosystem services. If we are interested in biodiversity loss conservation biologists need to start thinking beyond the species category and look at the functional diversity as Peter Kareiva has written about for biodiversity 'cold spots'. I live in Prince George BC – we have very few species of amphibians, one salamander species only. However, this single salamander species holds a great amount of biomass and functional utility in our forests. Not only does it contribute to the knowledge of what salamanders are about for conservation efforts that can reach people in a very direct way, they are more important because we lack ecological exchangeability through redundant eco-type that could take its place if lost. Gerardo Ceballos & Paul Ehlrich wrote about population losses in mammals (http://www.sciencemag.org/content/296/5569/904) and correctly state that the population levels of biodiversity as the more sensitive indicator of natural capital – this is where the bulk of the functional biomass is concentrated. Saving species hotspots is not necessarily the most effective way to for conservation biologists to realize their goals to conserve biodiversity.

    Moreover, Peter Ward has written about the nature of biodiversity over time in his paper 'The Father of All Mass Extinctions'. "It has long been argued that endemic centers are among the most important places to save. But the point is that endemic centers exist because they have not produced large numbers of successful species. Endemic centers are often living museums of very ancient species that do not have much potential for future evolution." Hence, we have a new emergent context to deal with in the Anthropocene and looking at species diversity in isolation to set conservation targets can put us well off the mark. If you look at population losses, estimated to be 20X the rate of species loss, I think the situation looks even more dire than Barnosky indicates in this article.

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