COMMENTARY
COMMENTARY
Extinction tsunami can be avoided Thomas E. Lovejoya,1
In many senses, the recent publication in PNAS by Ceballos et al. (1) on population losses and declines in vertebrates can be traced back to efforts early in the 20th century led by the American Committee for International Wildlife Protection to document the extinction phenomenon (2–4). The focus at the time was very much on individual species, not on extinction rates, and certainly not on precursor population declines. It is interesting to reflect on one of the most famous extinctions caused by humans, namely, the Passenger Pigeon. The original populations were staggeringly large and so impressive that in the 1870s, the Passenger Pigeon was chosen to adorn one of the gateposts of the British Museum of Natural History in South Kensington to represent the Nearctic region. Its extinction would have seemed inconceivable, given the enormous populations and the unpredictability of nesting sites. With the advent of a predator aided by telegraph to advertise the otherwise unpredictable nesting sites and by rail to transport the harvest to markets, the populations declined and winked out one by one until all that remained was the single female Martha, which died at the Cincinnati Zoo in 1916. Basically, there was no easy way to keep track, whereas today, in contrast, not only is there an army of birdwatchers but also, in the digital age, the ability to monitor population trends by, in this case, an electronic database, e-bird. By the time of the United Nations Conference on the Environment in Stockholm in 1972, it was clear that environmental change, including extinction, was pervasive and accelerating. That fact could have been derived from examining the Red List of Endangered Species kept by the International Union for the Conservation of Nature and Natural Resources or other sources like the list of endangered plant species produced by the Smithsonian Institution at the request of the US Congress in 1975 (5). The number of red-listed species has been increasing dramatically over time. The first projection of species extinctions globally was published for the Global 2000 Report for the President in 1980 (6). Often referred to as a prediction, it was, in fact, a projection, the point of which is to stimulate awareness and preventative action so that
the “results” do not convert into reality (and make it a prediction).
Estimating Extinction Rates Subsequent efforts to estimate extinction rates have been far more sophisticated and generally fall into two clusters: One compares current extinction rates against a “normal” background rate, and the other focuses on habitat reduction and has approached it through analyses of reduction of natural areas and species/area relationships. Both approaches focus pretty much on vertebrates because they are generally better known (i.e., vertebrate species have been mostly, but not completely, described) than other life forms. Extinction has been part of life on Earth pretty much since its origin. Some people think of it in terms of the five previous great extinction events in the history of life (7), but most hardly think of it at all, let alone realize that even without human pressures on the environment, there is a background extinction rate. One approach endeavors to determine to what extent human action has increased that “background rate,” and that approach, in turn, largely restricts the analyses to vertebrate species. Birds are by far the best known group of organisms. A detailed analysis of birds indicates a natural background rate of one extinction per million species per year (1 E/MSY), concludes that current extinctions are 100-fold that rate (100 E/MSY), and projects a 21st century rate of 1,000 E/MSY. That study concludes that extinction rates are likely to be higher for other groups of organisms (8). The theory of island biogeography (9), although initially focused on understanding the number of species on different islands, soon included consideration of species number in habitat “islands” in otherwise converted landscapes. Before that time, habitat fragmentation was not considered a serious conservation issue because the loss of species from isolated fragments occurs over time and is only obvious in retrospect. We now know that the impoverishment of isolated habitat fragments is quite dramatic and ubiquitous. It is a major conservation issue, with 70% of remaining forest within 1 km of the forest’s edge (10). The most
a Department of Environmental Science and Policy, George Mason University, Fairfax, VA 22030 Author contributions: T.E.L. wrote the paper. The author declares no conflict of interest. See companion article 10.1073/pnas.1704949114. 1 Email: [email protected].
www.pnas.org/cgi/doi/10.1073/pnas.1711074114
PNAS Early Edition | 1 of 2
detailed study is a 38-y project (and still running) in the Central Amazon that continues to reveal consequences from the initial fragmentation (11). In that study, a 100-ha forest fragment lost half of its forest interior bird species in less than 15 y (12). A critical aspect of habitat fragmentation is the time lag identified as “extinction debt” (13). Derived from first principles, it is borne out in many real-world examples as isolated fragments lose biodiversity and simplify over time. From a conservation perspective, a positive aspect is that if the fragment can be connected to another portion of natural habitat, then the process of species loss can be halted, and even reversed if the missing species occur in the habitat to which the fragment has been rejoined.
Addressing the Extinction Tsunami Put in this context, the new study by Ceballos et al. (1) on widespread population losses and declines in vertebrates suggests that the continuing focus on extinctions and their prevention leads conservation and conservation science to exert more of their total conservation effort at the end of the extinction process (rather than earlier) than might otherwise be wise. Although the sheer finality of extinction certainly justifies serious concern and effort, in a world of global change where the negative trends are essentially ubiquitous and accelerating, conservation and conservation science would do well to think and act in a more proactive way. Can we deflect the extinction tsunami implied in the major population declines? A wider lens for conservation and conservation science would also identify critical processes that could otherwise undercut locally focused efforts. A prime example would be the hydrological cycle of the Amazon by which half of Amazon rainfall is generated internally in the basin (by rainfall evaporating off the complex surfaces of the forest and through transpiration) (14). Inherent in the Amazon hydrological cycle is the question of how much deforestation could cause that cycle to degrade. Although that threshold is not entirely obvious (my personal view is that it is at around 20% deforestation), it is quite clear that although today’s Amazon is in excess of 50% under some form of protection, that impressive amount of conservation could be seriously threatened if the Amazon were to tip into Amazon dieback toward a more savannah-like climate and vegetation.
Similarly, those individuals concerned with biodiversity conservation and avoiding extinction would also be wise to be cognizant of change on a planetary scale. Of course, biodiversity loss is only one part of this change, but the planetary boundaries (15, 16) are quite instructive. Of the planetary conditions for which there are data, a small number (e.g., carbon cycle, nitrogen cycle) have exceeded their dimensions in preindustrial times. Biodiversity loss, as might be expected by extinction levels beyond the background rate, constitutes by far the greatest excursion beyond preindustrial conditions. The planetary boundaries work has received some criticism. Some object to the use of the word “boundaries,” but the real point is about going beyond the conditions that nourished the rise of human civilization. Not surprisingly, the carbon cycle and climate change exceed the historical boundary. Similarly, the distortion of the nitrogen cycle has produced dead zones in coastal waters and estuaries, doubling in number every decade for the past four decades. Biodiversity loss (extinction) is by far the greatest excursion “beyond safe operating conditions,” because biodiversity loss and extinction are caused by direct impacts like harvesting or habitat destruction, as well as by indirect impacts of other environmental change. This work traces back to early work of Ruth Patrick on freshwater systems in the mid-Atlantic, which demonstrated quite clearly that the number and kind of species in a stream or river reflect not only natural conditions but also the combined effects of human activity in the watershed (17). In other words biodiversity integrates the impacts of all kinds of environmental change. Combined with the large population declines documented in mammal and other vertebrate species, these regional and global forces put enormous pressure on biological diversity. It would be wise to address them in conservation science, thinking and planning well in advance of extinction thresholds. The Half-Earth Project (18) is just such an attempt to increase conservation efforts to the scale required. This project, along with the similar concept of “Nature Needs Half“ (19), calls for something on the order of half the planet to be dedicated primarily to nature. A conservation vision of that scale may seem impossibly dreamy, but the massive and ubiquitous vertebrate population declines argue for a new approach in which human ambition is embedded in nature.
1 Ceballos G, Ehrlich PR, Dirzo P (2017) Biological annihilation via the ongoing sixth mass extinction signaled by vertebrate population losses and declines. Proc Natl Acad Sci USA, 10.1073/pnas.1704949114. 2 Harper F (1945) Extinct and Vanishing Mammals of the Old World, Special Publication 12 (American Committee for International Wildlife Protection, New York). 3 Allen GM (1942) Extinct and Vanishing Mammals of the Western Hemisphere, Special Publication No. 11 (American Committee for International Wildlife Protection, New York). 4 Greenway JC, Jr (1958) Extinct and Vanishing Birds of the World, Special Publication No. 13 (American Committee for International Wildlife Protection, New York). 5 Ripley SD (1975) Report on Endangered and Threatened Plant Species of the United States (US Government Printing Office, Washington, DC). Available at https:// archive.org/details/reportonendanger00unse. Accessed July 14, 2017. 6 Lovejoy TE (1980) Projection of species extinctions, Global 2000 Report to the President (Council on Environmental Quality, Washington, DC), Vol 2, pp 327–331. 7 Raup DM, Sepkoski JJ, Jr (1982) Mass extinctions in the marine fossil record. Science 215:1501–1503. 8 Pimm S, Raven P, Peterson A, Sekercioglu CH, Ehrlich PR (2006) Human impacts on the rates of recent, present, and future bird extinctions. Proc Natl Acad Sci USA 103:10941–10946. 9 MacArthur RH, Wilson EO (1963) An equilibrium theory of insular zoogeography. Evolution 17:373–387. 10 Haddad NM, et al. (2015) Habitat fragmentation and its lasting impact on Earth’s ecosystems. Sci Adv 1:e1500052. 11 Laurance WF, et al. (May 30, 2017) An Amazonian rainforest and its fragments as a laboratory of global change. Biol Rev Camb Philos Soc, in press. 12 Ferraz G, et al. (2003) Rates of species loss from Amazonian forest fragments. Proc Natl Acad Sci USA 100:14069–14073. 13 Tilman DR, et al. (1994) Habitat destruction and the extinction debt. Nature 371:65–66. 14 Salati E, et al. (1979) Recycling of water in the Amazon basin: An isotopic study. Water Resour Res 15:1250–1258. 15 Rockström J, et al. (2009) A safe operating space for humanity. Nature 461:472–475. 16 Steffen W, et al. (2015) Sustainability. Planetary boundaries: Guiding human development on a changing planet. Science 347:1259855. 17 Patrick R (1949) A proposed biological measure of stream conditions based on a survey of Conestoga Basin, Lancaster County, Pennsylvania. Proc Acad Nat Sci Philadelphia 101:277–341. 18 Wilson EO (2016) Half-Earth: Our Planet’s Fight for Life (WW Norton, New York). 19 Locke H (2013) Nature Needs Half: A necessary and hopeful new agenda for protected areas. Parks 19:1–18.
2 of 2 | www.pnas.org/cgi/doi/10.1073/pnas.1711074114
Lovejoy
COMMENTARY
Extinction tsunami can be avoided Thomas E. Lovejoya,1
In many senses, the recent publication in PNAS by Ceballos et al. (1) on population losses and declines in vertebrates can be traced back to efforts early in the 20th century led by the American Committee for International Wildlife Protection to document the extinction phenomenon (2–4). The focus at the time was very much on individual species, not on extinction rates, and certainly not on precursor population declines. It is interesting to reflect on one of the most famous extinctions caused by humans, namely, the Passenger Pigeon. The original populations were staggeringly large and so impressive that in the 1870s, the Passenger Pigeon was chosen to adorn one of the gateposts of the British Museum of Natural History in South Kensington to represent the Nearctic region. Its extinction would have seemed inconceivable, given the enormous populations and the unpredictability of nesting sites. With the advent of a predator aided by telegraph to advertise the otherwise unpredictable nesting sites and by rail to transport the harvest to markets, the populations declined and winked out one by one until all that remained was the single female Martha, which died at the Cincinnati Zoo in 1916. Basically, there was no easy way to keep track, whereas today, in contrast, not only is there an army of birdwatchers but also, in the digital age, the ability to monitor population trends by, in this case, an electronic database, e-bird. By the time of the United Nations Conference on the Environment in Stockholm in 1972, it was clear that environmental change, including extinction, was pervasive and accelerating. That fact could have been derived from examining the Red List of Endangered Species kept by the International Union for the Conservation of Nature and Natural Resources or other sources like the list of endangered plant species produced by the Smithsonian Institution at the request of the US Congress in 1975 (5). The number of red-listed species has been increasing dramatically over time. The first projection of species extinctions globally was published for the Global 2000 Report for the President in 1980 (6). Often referred to as a prediction, it was, in fact, a projection, the point of which is to stimulate awareness and preventative action so that
the “results” do not convert into reality (and make it a prediction).
Estimating Extinction Rates Subsequent efforts to estimate extinction rates have been far more sophisticated and generally fall into two clusters: One compares current extinction rates against a “normal” background rate, and the other focuses on habitat reduction and has approached it through analyses of reduction of natural areas and species/area relationships. Both approaches focus pretty much on vertebrates because they are generally better known (i.e., vertebrate species have been mostly, but not completely, described) than other life forms. Extinction has been part of life on Earth pretty much since its origin. Some people think of it in terms of the five previous great extinction events in the history of life (7), but most hardly think of it at all, let alone realize that even without human pressures on the environment, there is a background extinction rate. One approach endeavors to determine to what extent human action has increased that “background rate,” and that approach, in turn, largely restricts the analyses to vertebrate species. Birds are by far the best known group of organisms. A detailed analysis of birds indicates a natural background rate of one extinction per million species per year (1 E/MSY), concludes that current extinctions are 100-fold that rate (100 E/MSY), and projects a 21st century rate of 1,000 E/MSY. That study concludes that extinction rates are likely to be higher for other groups of organisms (8). The theory of island biogeography (9), although initially focused on understanding the number of species on different islands, soon included consideration of species number in habitat “islands” in otherwise converted landscapes. Before that time, habitat fragmentation was not considered a serious conservation issue because the loss of species from isolated fragments occurs over time and is only obvious in retrospect. We now know that the impoverishment of isolated habitat fragments is quite dramatic and ubiquitous. It is a major conservation issue, with 70% of remaining forest within 1 km of the forest’s edge (10). The most
a Department of Environmental Science and Policy, George Mason University, Fairfax, VA 22030 Author contributions: T.E.L. wrote the paper. The author declares no conflict of interest. See companion article 10.1073/pnas.1704949114. 1 Email: [email protected].
www.pnas.org/cgi/doi/10.1073/pnas.1711074114
PNAS Early Edition | 1 of 2
detailed study is a 38-y project (and still running) in the Central Amazon that continues to reveal consequences from the initial fragmentation (11). In that study, a 100-ha forest fragment lost half of its forest interior bird species in less than 15 y (12). A critical aspect of habitat fragmentation is the time lag identified as “extinction debt” (13). Derived from first principles, it is borne out in many real-world examples as isolated fragments lose biodiversity and simplify over time. From a conservation perspective, a positive aspect is that if the fragment can be connected to another portion of natural habitat, then the process of species loss can be halted, and even reversed if the missing species occur in the habitat to which the fragment has been rejoined.
Addressing the Extinction Tsunami Put in this context, the new study by Ceballos et al. (1) on widespread population losses and declines in vertebrates suggests that the continuing focus on extinctions and their prevention leads conservation and conservation science to exert more of their total conservation effort at the end of the extinction process (rather than earlier) than might otherwise be wise. Although the sheer finality of extinction certainly justifies serious concern and effort, in a world of global change where the negative trends are essentially ubiquitous and accelerating, conservation and conservation science would do well to think and act in a more proactive way. Can we deflect the extinction tsunami implied in the major population declines? A wider lens for conservation and conservation science would also identify critical processes that could otherwise undercut locally focused efforts. A prime example would be the hydrological cycle of the Amazon by which half of Amazon rainfall is generated internally in the basin (by rainfall evaporating off the complex surfaces of the forest and through transpiration) (14). Inherent in the Amazon hydrological cycle is the question of how much deforestation could cause that cycle to degrade. Although that threshold is not entirely obvious (my personal view is that it is at around 20% deforestation), it is quite clear that although today’s Amazon is in excess of 50% under some form of protection, that impressive amount of conservation could be seriously threatened if the Amazon were to tip into Amazon dieback toward a more savannah-like climate and vegetation.
Similarly, those individuals concerned with biodiversity conservation and avoiding extinction would also be wise to be cognizant of change on a planetary scale. Of course, biodiversity loss is only one part of this change, but the planetary boundaries (15, 16) are quite instructive. Of the planetary conditions for which there are data, a small number (e.g., carbon cycle, nitrogen cycle) have exceeded their dimensions in preindustrial times. Biodiversity loss, as might be expected by extinction levels beyond the background rate, constitutes by far the greatest excursion beyond preindustrial conditions. The planetary boundaries work has received some criticism. Some object to the use of the word “boundaries,” but the real point is about going beyond the conditions that nourished the rise of human civilization. Not surprisingly, the carbon cycle and climate change exceed the historical boundary. Similarly, the distortion of the nitrogen cycle has produced dead zones in coastal waters and estuaries, doubling in number every decade for the past four decades. Biodiversity loss (extinction) is by far the greatest excursion “beyond safe operating conditions,” because biodiversity loss and extinction are caused by direct impacts like harvesting or habitat destruction, as well as by indirect impacts of other environmental change. This work traces back to early work of Ruth Patrick on freshwater systems in the mid-Atlantic, which demonstrated quite clearly that the number and kind of species in a stream or river reflect not only natural conditions but also the combined effects of human activity in the watershed (17). In other words biodiversity integrates the impacts of all kinds of environmental change. Combined with the large population declines documented in mammal and other vertebrate species, these regional and global forces put enormous pressure on biological diversity. It would be wise to address them in conservation science, thinking and planning well in advance of extinction thresholds. The Half-Earth Project (18) is just such an attempt to increase conservation efforts to the scale required. This project, along with the similar concept of “Nature Needs Half“ (19), calls for something on the order of half the planet to be dedicated primarily to nature. A conservation vision of that scale may seem impossibly dreamy, but the massive and ubiquitous vertebrate population declines argue for a new approach in which human ambition is embedded in nature.
1 Ceballos G, Ehrlich PR, Dirzo P (2017) Biological annihilation via the ongoing sixth mass extinction signaled by vertebrate population losses and declines. Proc Natl Acad Sci USA, 10.1073/pnas.1704949114. 2 Harper F (1945) Extinct and Vanishing Mammals of the Old World, Special Publication 12 (American Committee for International Wildlife Protection, New York). 3 Allen GM (1942) Extinct and Vanishing Mammals of the Western Hemisphere, Special Publication No. 11 (American Committee for International Wildlife Protection, New York). 4 Greenway JC, Jr (1958) Extinct and Vanishing Birds of the World, Special Publication No. 13 (American Committee for International Wildlife Protection, New York). 5 Ripley SD (1975) Report on Endangered and Threatened Plant Species of the United States (US Government Printing Office, Washington, DC). Available at https:// archive.org/details/reportonendanger00unse. Accessed July 14, 2017. 6 Lovejoy TE (1980) Projection of species extinctions, Global 2000 Report to the President (Council on Environmental Quality, Washington, DC), Vol 2, pp 327–331. 7 Raup DM, Sepkoski JJ, Jr (1982) Mass extinctions in the marine fossil record. Science 215:1501–1503. 8 Pimm S, Raven P, Peterson A, Sekercioglu CH, Ehrlich PR (2006) Human impacts on the rates of recent, present, and future bird extinctions. Proc Natl Acad Sci USA 103:10941–10946. 9 MacArthur RH, Wilson EO (1963) An equilibrium theory of insular zoogeography. Evolution 17:373–387. 10 Haddad NM, et al. (2015) Habitat fragmentation and its lasting impact on Earth’s ecosystems. Sci Adv 1:e1500052. 11 Laurance WF, et al. (May 30, 2017) An Amazonian rainforest and its fragments as a laboratory of global change. Biol Rev Camb Philos Soc, in press. 12 Ferraz G, et al. (2003) Rates of species loss from Amazonian forest fragments. Proc Natl Acad Sci USA 100:14069–14073. 13 Tilman DR, et al. (1994) Habitat destruction and the extinction debt. Nature 371:65–66. 14 Salati E, et al. (1979) Recycling of water in the Amazon basin: An isotopic study. Water Resour Res 15:1250–1258. 15 Rockström J, et al. (2009) A safe operating space for humanity. Nature 461:472–475. 16 Steffen W, et al. (2015) Sustainability. Planetary boundaries: Guiding human development on a changing planet. Science 347:1259855. 17 Patrick R (1949) A proposed biological measure of stream conditions based on a survey of Conestoga Basin, Lancaster County, Pennsylvania. Proc Acad Nat Sci Philadelphia 101:277–341. 18 Wilson EO (2016) Half-Earth: Our Planet’s Fight for Life (WW Norton, New York). 19 Locke H (2013) Nature Needs Half: A necessary and hopeful new agenda for protected areas. Parks 19:1–18.
2 of 2 | www.pnas.org/cgi/doi/10.1073/pnas.1711074114
Lovejoy
