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date: 11 December 2019


Summary and Keywords

The prospect of extinction, the complete loss of a species or other group of organisms, has long provoked strong responses. Until the turn of the 18th century, deeply held and widely shared beliefs about the order of nature led to a firm rejection of the possibility that species could entirely vanish. During the 19th century, however, resistance to the idea of extinction gave way to widespread acceptance following the discovery of the fossil remains of numerous previously unknown forms and direct experience with contemporary human-driven decline and the destruction of several species. In an effort to stem continued loss, at the turn of the 19th century, naturalists, conservationists, and sportsmen developed arguments for preventing extinction, created wildlife conservation organizations, lobbied for early protective laws and treaties, pushed for the first government-sponsored parks and refuges, and experimented with captive breeding. In the first half of the 20th century, scientists began systematically gathering more data about the problem through global inventories of endangered species and the first life-history and ecological studies of those species.

The second half of the 20th and the beginning of the 21st centuries have been characterized both by accelerating threats to the world’s biota and greater attention to the problem of extinction. Powerful new laws, like the U.S. Endangered Species Act of 1973, have been enacted and numerous international agreements negotiated in an attempt to address the issue. Despite considerable effort, scientists remain fearful that the current rate of species loss is similar to that experienced during the five great mass extinction events identified in the fossil record, leading to declarations that the world is facing a biodiversity crisis. Responding to this crisis, often referred to as the sixth extinction, scientists have launched a new interdisciplinary, mission-oriented discipline, conservation biology, that seeks not just to understand but also to reverse biota loss. Scientists and conservationists have also developed controversial new approaches to the growing problem of extinction: rewilding, which involves establishing expansive core reserves that are connected with migratory corridors and that include populations of apex predators, and de-extinction, which uses genetic engineering techniques in a bid to resurrect lost species. Even with the development of new knowledge and new tools that seek to reverse large-scale species decline, a new and particularly imposing danger, climate change, looms on the horizon, threatening to undermine those efforts.

Keywords: conservation biology, de-extinction, Endangered Species Act, captive breeding, rewilding, biodiversity, island biogeography, SLOSS debate, sixth extinction, mass extinction


Biological extinction is the complete loss of a species or other group of organisms, either from natural causes or by human hands. While Western naturalists initially resisted the notion that the disappearance of species was possible, since the early 19th century, extinction has not only been viewed as real but also as key to understanding the earth’s biological diversity. During that century, naturalists discovered a growing number of past extinctions in the fossil record and documented the decline and loss of numerous contemporary species, first on islands and then on continents. An initial indifference to the prospect of human-caused extinction gave way to a growing concern by the end of the century, concern that resulted in the development of arguments for species preservation, the founding of early wildlife protection organizations, the passage of pioneering conservation laws, successful negotiation of the first international wildlife treaties, and preliminary experiments with refuges and captive breeding programs.

During the 20th century and continuing to the present, scientists have sought to get a better handle on the scope, scale, and causes of contemporary extinction by pursuing a series of global inventories of lost and vanishing species and by carrying out ecological and life-history studies of those that remained. The rise of the modern environmental movement in the post-World-War-II period heightened political and financial support for the study and protection of threatened wildlife and led to the passage of the U.S. Endangered Species Act of 1973, a bold, sweeping, and controversial law that sought both to prevent extinction and to restore depleted plant and animal populations to sustainable levels. Despite this and numerous other national and international conservation initiatives, by the end of the 20th century, scientists warned that the world was facing a biodiversity crisis, an accelerating rate of species loss that was on par with or even exceeded the five mass extinction events that paleontologists first identified during this period.

Concerned scientists not only sought to publicize the biodiversity crisis and lobby for protective laws and treaties, they also established a new mission-oriented, interdisciplinary field—conservation biology—that focused explicitly on responding to and countering the problem of extinction. Recently they have also proposed controversial new approaches to the crisis beyond the regulations, laws, conventions, captive breeding programs, and protective refuges that have predominated thus far. Advocates of rewilding propose that the structure and healthy functioning of ecosystems might be restored and biodiversity loss reduced by establishing large-scale core reserves, creating connections between those reserves to facilitate species migration, and safeguarding or reintroducing the apex predators that play a key role in shaping ecosystems. At the same time, proponents of de-extinction believe that recent developments in genomics and reproductive technologies offer the hope of resurrecting lost species using the DNA found in fossils and other museum specimens. Whether or not these approaches ultimately prove fruitful, scientists warn that accelerating climate change represents a grave new threat to the survival of much of the world’s biota.

Discovering Extinction

The Order of Nature

Until the end of the 18th and the beginning of the 19th centuries, the prevailing view in the West was that extinction was impossible. In the process of trying to construct inventories of the world’s biota, naturalists had noted the local disappearance of some species, but they almost universally denied that the loss of any creature across its entire range could occur because it violated deeply held beliefs about the balance of nature and the great chain of being (Barrow, 2009, pp. 19–23).

Both ideas about the order of nature trace their roots back to antiquity, and indeed, historian of ecology Frank Egerton (1973) has argued “a balance-of-nature concept is part of most cosmologies” (p. 324). By the end of the 17th century, the theologian Thomas Burnet (1697) was using a specific term, the “oeconomy of nature,” to describe the “well ordering of the great Family of living Creatures” (pp. 203–204). The Swedish taxonomist Carolus Linnaeus fleshed out that expression in an influential essay that helped lay the foundations for the science of ecology following its publication in 1791. There Linnaeus noted that each animal not only feeds on its own prey but is also eaten by other species, thereby providing an early example of what ecologists would later refer to as a “food chain.” At the same time, he denied that even particularly voracious predators could “destroy a whole species,” because Providence had established an order that ensured a “just proportion” among all creatures, an order in which the total loss of any creature seemed out of the question (Linnaeus, 1791, p. 119).

The great chain of being, or scala naturae, provided a second, related basis for rejecting the possibility of extinction (Lovejoy, 1966). According to this widely embraced notion, the diversity of the natural world could be imagined as a long chain encompassing every possible kind of organism in a single continuous series. For those under the sway of the great chain of being, the loss of even a single species was unacceptable because it represented a threat to the entire edifice.

The Problem of Fossils

Fossils, gathered in the cabinets of curiosity that proliferated across Europe during the Renaissance, provided physical evidence that might have overturned the widespread belief that extinction violated notions about the order of nature. But for many years, naturalists failed to differentiate between the inorganic and organic origins of the objects they labelled as fossils, which literally means “things dug up.” As historian of science Martin Rudwick (1985) has argued, even those fossilized remains that seemed to have come from once-living beings presented thorny interpretive challenges: some of them, like the ammonites, had no known analogs among existing species, for example. But rather than relinquishing the assumption about a divinely ordered nature where extinction was unthinkable, naturalists clung to the hope that the organic remains of creatures that differed from those of any known living animal might still be found in deep oceans or other unexplored regions of the globe.

This is precisely the strategy that Thomas Jefferson, an avid fossil collector and founder of North American paleontology, pursued at the end of the 18th century. In Notes on the State of Virginia, Jefferson (1787/1955) defended his decision to include the mammoth among a list of native mammals he compiled by asserting the impossibility of extinction: “Such is the oeconomy of nature, that no instance can be produced of her having permitted any one race of her animals to become extinct; of her having formed any link in her great work so weak as to be broken” (pp. 53–54). Confident in this belief, Jefferson (1803/1978, p. 63) instructed Lewis and Clark to keep an eye out for “rare and extinct” creatures during their federally sponsored western exploring expedition in the early 19th century.

The French naturalist Georges Cuvier finally broke the grip that the deeply held belief in an ordered nature presented to the acceptance of extinction. In groundbreaking papers published in 1796 and 1806, Cuvier pointed out the improbability that any unknown creature as large as an elephant might still be discovered roaming the earth. Using principles of comparative anatomy that he had developed, he also differentiated between the two living species of elephant and the fossil elephants that had been unearthed thus far, including the mastodon, which he named and described for the first time. Cuvier later characterized a raft of other lost species, asserting they had been destroyed in the series of geological catastrophes that had periodically swept the globe (Rudwick, 2014, pp. 104–114).

Implicating Humans in Island Extinctions

Following Cuvier’s proof, naturalists not only widely accepted the reality of extinction but also began routinely implicating humans in species loss. In the 1820s, the Scottish zoologist and physician James Fleming argued that human predation was responsible not only for recently documented declines in wildlife populations but also the prehistoric loss of large mammals whose fossil remains were being unearthed throughout Great Britain (Rehbock, 1985). A decade later the British geologist Charles Lyell (1832) argued that far-reaching changes in landscapes across geological time, the subsequent migration of species beyond their previous ranges, and human colonization had repeatedly destroyed species, and that far from being a rare event, extinction was a “regular and constant part of Nature” (p. 141).

The destructive potential of humans soon received confirmation with the published accounts of the loss of three island birds in the mid-18th century: the dodo, the moa, and the great auk. The dodo (Raphus cucullatus) became the first widely circulated symbol of wildlife extinction, especially after it was featured in Lewis Carroll’s Alice’s Adventures in Wonderland (1865; Fuller, 2002; Pinto-Correia, 2003). This swan-sized, flightless bird once inhabited Mauritius, an Indian Ocean island roughly 500 miles east of Madagascar. Within less than a century after the arrival of the Portuguese in the early 16th century, the species fell victim to overhunting and predation from the animals that Europeans had introduced to the island. The destruction proved so complete that by the end of the 18th century, naturalists began to doubt whether the dodo had ever actually existed. Using evidence from a small number of paintings, several historical accounts, and a single surviving partial specimen, Hugh Strickland and Alexander Melville’s study, The Dodo and Its Kindred (1848) concluded that the bird and its cousins had not only lived but that they provided the “first clearly attested instances of extinction of organic species through human agency” (p. 5).

A few years earlier the British paleontologist and comparative anatomist Richard Owen began reconstructing the story of another lost island bird, the moa, based on a specimen that had come from a trader on the east coast of North Island, New Zealand. Owen published the first account of the species in 1838, and over the next 40 years would go on to name and describe nearly two dozen different moa species. As early as 1844 he thought it “not altogether improbable” that not long after the “Polynesian colony first set foot on the island,” the flightless moa “fell prey to the progenitors of the current Maoris” (as quoted in Worthy & Holdaway, 2002, p. 56).

The great auk (Pinguinus impennis) provided a more recent example of the destruction of an island bird through human agency (Fuller, 1999). The species had once inhabited the boreal and low-Arctic waters of the North Atlantic, where it was hunted for its meat, eggs, feathers, and oil and widely used as fishing bait. As the great auk became rare, egg and skin collectors also decimated the species. The last well-documented encounter occurred in 1844, on Eldey, a volcanic island 10 miles from Iceland, when a natural history dealer in Reykjavík encouraged a local fisherman to capture what turned out to be the final two specimens of the prized bird. The British ornithologist Alfred Newton collaborated with John Wolley to gather material for a monograph on the species, which was never completed, and following Wolley’s death in 1859, Newton published a series of articles highlighting the fate of the great auk (Cowles, 2013).

As these human-caused island extinctions were coming to light, the British naturalist Charles Darwin was slowly developing the theory of evolution by natural selection. Although apparently not directly influenced by the loss of the dodo, the moa, and the great auk, his first-hand observations about insular organisms played a critical role in his ideas about the creation of new species and the demise of old ones, as did his reading of Lyell’s geological works. In On the Origin of Species, one of the most influential books of the 19th century, Darwin (1859) declared that “We need not marvel at extinction” (p. 322) for it was part of the regular order of nature, a product of natural selection that was constantly acting to shape all living creatures. Following Lyell, Darwin believed that the level of biological diversity had remained roughly constant throughout the history of life, with the rate of extinction and the rate of species replacement staying about equal. He thus recast the old notion of the economy of nature as a dynamic equilibrium.

Documenting Continental Extinctions

With the exception of Alfred Newton, the naturalists who wrote about extinction up to this point seemed largely indifferent about its occurrence. The well-publicized demise or near-demise of several species in the second half of the 19th century shattered that apathy and led to the first systematic efforts to save declining wildlife.

In North America, the nearly simultaneous decline of the passenger pigeon (Ectopistes migratorius) and the American bison (Bison bison), two once-superabundant species, prompted widespread concern about the issue of wildlife extinction. Many eyewitness reports detail sightings of passenger pigeon flocks that blackened the skies for hours, and the total population of the species at the time of European contact may have reached as high as four billion birds (Schorger, 1955). Similarly, 20–30 million bison, the largest land mammal on the continent, once resided on the Plains (Isenberg, 2000). While numerous factors diminished both species, commercial hunting on a massive scale, facilitated by the expansion of telegraph and railroad networks that linked rural areas to urban markets, was the primary cause of their dramatic population reductions. The final large nestings of the passenger pigeon took place in western Wisconsin and northern Pennsylvania in 1882, and the last confirmed sighting of the bird in the wild was in 1902 (Greenberg, 2014). Martha, a captive bird in the Cincinnati Zoo who drew her last breath in 1914, was likely the final living passenger pigeon.

The American bison nearly met a similar fate, reaching a population in the wild of less than a thousand before being rescued from the jaws of extinction. Driven by nationalistic pride and nostalgic longing for a rapidly fading past, the American taxidermist turned conservationist and zoo director William T. Hornaday played a leading role in establishing two new institutions, the National Zoological Garden in Washington, DC (1889) and the New York Zoological Garden (1899), that he hoped would provide a safe haven for numerous endangered native animals. He also helped found and led a new organization, the American Bison Society, that sought to preserve its namesake. After several failed attempts, Hornaday managed to begin building a bison breeding herd in the South Bronx, and in 1907 he shipped 15 of the animals back west, to the newly established Wichita National Game Reserve in Oklahoma Territory, the first of several reintroduction projects involving the species (Barrow, 2009, pp. 108–124).

In Europe, the extinction of the aurochs (Bos primigenius), the ancestor of domestic cattle, in 1627 seems not to have garnered much attention or concern (Vuure, 2005). However, the decline of the continent’s largest surviving land mammal, the European bison or wisent (Bison bonasus), probably had more of an impact. The species once ranged throughout western, central, and south-eastern Europe, where it helped shape broadleaf forests and forested steppe ecosystems. Deforestation and overhunting dramatically reduced its range and overall population, and by the end of the 19th century, there were only two populations remaining in the wild: in the Białowieża Forest that straddles the border between current-day Poland and Belarus and in the West-Caucasus Mountains in Southern Russia (Pucek, 2004).

The extinction of the quagga (Equus quagga quagga) provided another example of the destructive potential of humans. Impressive herds of this equid species, whose front half resembled a zebra while its back half looked more like a horse, once ranged across the great plains of southern Africa. Dutch settlers fenced off much of the quagga’s range and hunted them for sport, to reduce competition for grazing land, and to provide meat for their workers. By 1860 only a small remnant population survived near the Vaal River, and the last wild quaggas were captured around 1870. The final quagga, held in the Artis Zoo in Amsterdam, died in 1889 (De Vos, 2014).

Early Responses to the Extinction Threat

As the story of the bison reveals, romantic nostalgia and ardent nationalism were two key arguments in early campaigns to rescue endangered species. Assertions about the economic value of organisms—as sources of food, drugs, leather, fiber, oils, and other commodities—also loomed large in many pioneering conservation initiatives. The longstanding belief that every organism plays a vital role in maintaining the stability of the world—an idea initially suggested by the notion of economy of nature and later reinforced with the emergence of the science of ecology—provided a fourth justification for species preservation. The evolutionary argument that, as the product of eons of time, species were living monuments every bit as worthy of protection as ancient cultural monuments, was also occasionally voiced. At the same time, naturalists maintained that plants, animals, and biotic communities should be preserved to allow for the possibility of the continued scientific study of them. Ethical arguments, either that organisms had a fundamental right to continued existence or that protecting declining plant and animal populations was an important obligation to future human generations, also began to emerge during the first half of the 20th century. Finally, many of those supporting conservation efforts felt a strong sense of emotional attachment to threatened species and landscapes that they sought to protect.

Moved by these considerations and a growing sense that wildlife populations were under threat, naturalists, sportsmen, and concerned citizens began mobilizing by the end of the 19th century. In England, the earliest evidence of interest in protecting declining non-game species came when a committee within the British Association for the Advancement of Science joined forces with the (Yorkshire) Association for the Protection of Sea Birds in 1868 to lobby for the Sea Birds Preservation Act of 1869 (Cowles, 2013). More than two decades later the Plumage League and the Fur and Feather League merged to create the Society for the Protection of Birds, an organization that received a royal charter in 1904 (Allen, 1994, pp. 176–185; Doughty, 1975, pp. 92–97).

Numerous other wildlife conservation organizations sprouted up in Europe and the United States during this period. The first local and state Audubon Societies were founded in the mid-1880s, while the National Association of Audubon Societies, the predecessor to the National Audubon Society, was launched in 1905 (Barrow, 1998; Doughty, 1975). The Boone and Crockett Club, an organization of well-to-do American sportsmen, was created in 1887 (Reiger, 2000), while in 1903 a group of influential naturalists, sportsmen, and colonial officials concerned about the plight of wildlife in Britain’s colonial possessions in Africa and Asia established the Society for the Preservation of Wild Flora and Fauna of the Empire (now Flora and Fauna International; Adams, 2004; Mackenzie, 1988). In 1922, T. Gilbert Pearson, the president of the National Association of Audubon Societies, joined forces with several European colleagues to form the International Committee for the Protection of Birds (now BirdLife International; Barrow, 2009). Following prodding from the Dutch naturalist Peter van Tienhoven, in 1928 the International Union of Biological Sciences established a conservation organization that would soon be known as the International Office for the Protection of Nature (Boardman, 1981).

Conservationists also secured groundbreaking legislation and treaties aimed at protecting threatened wildlife. In 1900, European colonial powers signed the Convention for the Preservation of Wild Animals, Birds[,] and Fish in Africa (the London Convention of 1900), an international agreement that never entered force because most signatories failed to ratify it. That agreement was superseded by the Convention Relative to the Preservation of Fauna and Flora in the Natural State (the London Convention of 1933), which established a two-class classification schema for threatened wildlife in Africa: schedule A for species requiring special protection and schedule B for those requiring less protection (Cioc, 2009; Mackenzie, 1988). In 1902, 12 European nations signed the International Convention on the Protection of Useful Birds (Adams, 2004, p. 44). Nine years later, officials from the United States, Canada, Russia, and Japan negotiated an agreement that environmental historian Kirkpatrick Dorsey (1998) has argued “snatched the fur seal from the jaws of extinction” (p. 163). In 1916, the U.S. and British officials acting on behalf of Canada negotiated the Migratory Bird Treaty, an agreement that was extended to Mexico in 1936 (Cioc, 2009; Dorsey, 1998).

Documenting Extinction

Inventories of Extinction

The widespread decline of wildlife also prompted efforts to get a more precise handle on the scope and scale of the problem. Museum-based naturalists compiled the earliest catalogs of endangered and extinct species (Barrow, 2009). The wealthy British banker and naturalist Walter Rothschild, who amassed a massive natural history museum at his family estate at Tring Park, published the first extensive inventory of birds believed to be lost in 1907. His lavishly illustrated volume, Extinct Birds, included 166 species, most of which were known from fossils. In the 1920s, curators at the Museum of Comparative Zoology (MCZ) at Harvard began a card catalog of rare and vanishing bird species as part of their campaign to fill out their collection with representatives from every known avian genus. Based on this catalog, John C. Phillips (1929) published a list of nearly 150 “extinct and vanishing birds of the Western Hemisphere,” a third of which he considered “certainly” or “probably” extinct.

Philips joined with MCZ colleague and primatologist Harold J. Coolidge to organize the American Committee for Wild Life Protection in 1930. An initial priority of the new organization was the creation of a comprehensive inventory of the world’s vanishing mammals. After attending the 1933 London Convention meetings as an official observer, Phillips returned with a renewed conviction that an authoritative inventory of the world’s lost and threatened mammals was needed. After numerous delays, MCZ curator Glover M. Allen’s Extinct and Vanishing Mammals of the Western Hemisphere, finally appeared in 1942, while Francis Harper’s Extinct and Vanishing Mammals of the Old World was published three years later. Not until 1954 did the American Committee publish James C. Greenway’s companion volume for birds (Barrow, 2009).

In the immediate aftermath of the war, the British biologist and conservationist Julian Huxley led the effort to create the International Union for the Preservation of Nature (soon renamed the International Union for the Conversation of Nature [IUCN]), an outgrowth of the newly established United Nations Educational, Scientific[,] and Cultural Organization (UNESCO; Holdgate, 1999; McCormick, 1989). The new organization, which included representatives from national governments and NGOs, sought to use the power of science to shield the natural world from the onslaught of civilization, but from the beginning rescuing endangered species was among its highest priorities. At its first conference in 1949, delegates approved a resolution highlighting 13 birds and 14 mammals of particular concern, and, at the prodding of Harold J. Coolidge, established an International Survival Office, which would later be renamed the Survival Service Commission. In 1961 the British ornithologist Max Nicholson helped establish the World Wildlife Fund to provide financial support for the IUCN and other conservation organizations, and two years later, the British wildlife artist, aviculturalist, and IUCN board member Peter Scott created the first Red Data Books, to systematically document the status of endangered species (Barrow, 2009).

The First Field Studies of Species Threatened with Extinction

While museum-based naturalists compiled inventories of the world’s endangered and extinct species, their field-based colleagues pursued an alternative strategy: ecological and life history studies. In the late 1920s, the Harvard-trained ornithologist Alfred O. Gross completed the first detailed, long-term study of an endangered species, the heath hen (Tympanuchus cupido cupido), chronicling its final years on Martha’s Vineyard, an island off the coast of Massachusetts (Gross, 1928). Following Gross’s study, the pioneering game manager and conservationist Aldo Leopold (1936) called for a “Conservation Inventory of Threatened Species,” including more studies of declining wildlife, a better understanding of the tools needed to rescue those species, and improved coordination between conservation organizations and relevant governmental agencies.

Soon after Leopold’s call, the National Association of Audubon Societies established a fellowship program to support the field work of several graduate students researching endangered species. Among those who benefited from this new funding source was the Cornell doctoral student James T. Tanner, who spent several seasons studying the elusive ivory-billed woodpecker (Campephilus principalis) at a site in northeast Louisiana that was being actively logged. In the years leading up to and following the publication of Tanner’s report in 1942, the newly re-named National Audubon Society fought unsuccessfully to obtain protection for the Singer Tract, and the bird was last spotted there in 1944. Audubon also sponsored the research of Carl Koford, who spent more than 400 days in the remote mountain regions of central and southern California observing the rare California condor (Gymnogyps californianus), the largest soaring land bird in North America, before publishing his report in 1953.

Legislating Endangered Species Protection in the Environmental Age

Technological developments in the post-World War II era had the paradoxical effect of exacerbating the threats to wildlife across the globe and, through advances in and popularization of the science of ecology, rendering those threats more visible to a broader public (Worster, 1994). Particularly notable in its impact was the bomb, an unprecedented destructive force that released dangerous radioactive fallout that was soon being detected in biotic communities, food supplies, and human bodies across the globe (Winkler, 1993). In an influential bestselling book, Silent Spring (1962), Rachel Carson used growing concern about fallout from routine atmospheric testing of nuclear weapons to strengthen her case about the dangers posed by the new synthetic pesticides that began to be widely used in agricultural production and public health campaigns in the second half of the 20th century. A series of highly publicized environmental disasters—oil spills, smog events, fish kills, algal blooms, and others—not only further raised public consciousness about contemporary environmental threats but also led to calls for stronger laws and regulations to protect human and land health (McCormick, 1989).

Among the many innovative protective laws that emerged from the modern environmental movement were a series of pioneering Endangered Species Acts passed by the U.S. government. In 1964, the Fish and Wildlife Service created a new committee modeled on the IUCN Survival Service Commission, the Committee on Rare and Endangered Wildlife Species (CREWS), which began compiling its own list of native vertebrate species threatened with extinction using a format similar to that found in the IUCN Red Book. The first federal endangered species list, released that same year, contained entries on about 60 vertebrate species, the majority of which were birds. That list, in turn became the impetus for the Endangered Species Preservation Act of 1966, which required the Department of the Interior to continue compiling an official list of wildlife threatened with extinction but only protected vertebrate species on federal refuges. The Endangered Species Conservation Act of 1969 expanded the Lacey Act’s ban on interstate commerce in unlawfully obtained wildlife to include reptiles, amphibians, mollusks, and crustaceans in addition to birds and mammals and extended protection to animals threatened with extinction around the world rather than species native to the United States. It also called for passage of an international convention on trade in endangered species (Petersen, 2002).

Concern about the plight of marine mammals provided additional pressure for strengthening federal endangered species protections. The growing popularity of oceanariums, whale watching tours, and motion pictures featuring cetaceans came at a time when marine mammals faced increasing pressure from overharvesting and, in the case of dolphins, incidental taking from tuna fishing. As early as 1931, 26 nations had signed a League of Nations-sponsored convention that established rules for harvesting whales, although the initial agreement was only partially observed. Following World War II, 16 nations negotiated the International Convention for the Regulation of Whaling (1946), an agreement that established the International Whaling Commission and granted it the authority to set catch limits (Dorsey, 2013). When that body failed to stem the decline of whale populations, in 1970 U.S. officials listed eight of the most threatened whales on its official endangered species list. Two years later Congress passed the Marine Mammal Protection Act, which sought not just to prevent extinction of species facing overexploitation but to achieve “optimum sustainable populations” for each of them (Barrow, 2009).

In March 1973, 80 nations signed the Convention of Trade in Endangered Species (CITES), an IUCN-led initiative that had been in the works for over a decade. That agreement created a global system of import and export certificates for species threatened with extinction, with a three-tiered classification that offered increased protection for those species facing the gravest danger (Boardman, 1981).

Concerned that existing laws in the United States did not go far enough, scientists, environmentalists, and policymakers also gained passage of the Endangered Species Act of 1973. That strong, sweeping legislation sought to protect not only organisms immediately at risk of extinction (“endangered”) but also those where the risk was less imminent (“threatened”). It also extended coverage to all animals and plants (except pests, bacteria, and viruses), broadly defined the activities that would be prohibited for listed species, and barred federal agencies from taking actions jeopardizing those species. Passed at the height of the modern environmental movement, the Endangered Species Act of 1973 gained unanimous approval in the Senate and near unanimous approval in the House. Congress strongly supported legislation it believed was intended to save iconic species facing extinction, like the bald eagle (Haliaeetus leucocephalus), the American alligator (Alligator mississippiensis), and the whooping crane (Grus americana; Petersen, 2002).

The new legislation soon became embroiled in a controversy that revealed its much broader protective scope. In 1975, the Tennessee Valley Authority had nearly completed construction of the Tellico Dam on the Little Tennessee River, when opponents of the controversial project successfully petitioned to have a small, newly discovered minnow, the snail darter (Percina tanasi), listed under the terms of the Endangered Species Act. When the TVA refused to stop construction of the dam, critics sought an injunction to force them to do so. The ensuing case, TVA v. Hill, made it all the way to the U.S. Supreme Court, which ruled in 1978 that the clear intent of the Endangered Species Act was to ensure that federal agencies did not jeopardize the continued existence of listed species or their habitats. Congress responded by amending the ESA to create a new committee, the so-called God Committee, that had the authority to exempt federal agencies from the provision of the law prohibiting them from potentially harming endangered species. When the God Committee ruled in favor of the protecting the snail darter, Congressional delegates from Tennessee inserted a rider into a popular appropriation bill that directed the TVA to complete the dam. Before its gates were closed, snail darters were transplanted into several neighboring rivers and the species was also later found at additional sites, leading its status to be upgraded from “endangered” to “threatened” (Plater, 2013).

The spotted owl controversy raged a decade later. As early as the 1970s, ornithologists had warned about the decline of the northern spotted owl (Strix occidentalis caurina), a subspecies dependent on old-growth conifer forests in the Pacific Northwest to survive. Since each breeding pair required an estimated 400 to 1,000 hectares of habitat, the Fish and Wildlife Service feared the economic and political consequences of listing the bird in a region that remained highly reliant on the lumber industry. The Seattle Audubon Society filed a lawsuit that forced the federal government to act, and it declared the species as threatened in 1990. With the logging industry claiming as many as 30,000 jobs were at stake, a series of conservation plans for the bird were drawn up and then promptly abandoned before the Pacific Northwest Forest Management Plan, a federal, landscape-level plan, was finally adopted in 1994. A federal court upheld that plan, which reduced logging to less than 25% of the levels harvested in the 1980s and protected two-thirds of the remaining old-growth forests in the region (Petersen, 2002; Yaffee, 1993). Although the controversy eventually died down, the northern spotted owl has continued to experience alarming population declines not only from habitat loss but also from incursions by the barred owl, which outcompetes the northern spotted owl for food and habitat.

The Endangered Species Act granted authority for extraordinary interventions to rescue species on the brink of extinction. One striking example was the California condor, which appeared on the first U.S. endangered species list in 1967. At the time Carl Koford published his pioneering study of the species in 1953, he estimated that only 60 of the birds remained. Koford and his sponsor, the National Association of Audubon Societies, strongly opposed proposals to begin captive breeding experiments with the species and instead stressed the need for habitat preservation. One-and-a-half million acres of prime condor habitat were eventually set aside in an effort to save the bird, a strategy that ultimately proved ineffective. By 1983, when only 21 California condors remained in the wild, the Condor Research Center received permission to obtain a limited number of condor eggs to begin a captive breeding program. Only two years later, when the total condor population had plummeted to just nine birds, the center received authorization to trap the all the remaining wild condors. While that move proved controversial, researchers enjoyed much success breeding the species. They struggled, however, to find ways to keep the captive-bred condors safe once they began releasing them back to the wild in 1992 (Snyder & Snyder, 2000). In the cases of at least two other endangered species during this period, the red wolf (Canis rufus) and the black-footed ferret (Mustela nigripes), ESA-mandated recovery programs removed all known individuals from the wild in last-ditch efforts to save them through captive breeding and re-introduction programs (Beeland, 2013; Clark, 1997; Jachowski, 2014).

The SLOSS Debate

New policies aimed at preventing extinction were increasingly informed by developments in science, particularly the science of ecology, which studies the relationship between organisms and their abiotic and biotic environments. Although the field of ecology has deep roots, not until the first half of the 20th century did scientists begin to lay down its foundations in an explicit, self-conscious fashion. During this period, Henry Cowles and Frederic Clements developed notions of ecological succession, Victor Shelford and Charles Elton explored the structure of animal communities, Arthur Tansley coined the term “ecosystem,” and Vladimir Vernadsky fleshed out the idea of the biosphere (Hagen, 1992; Worster, 1994). From the 1930s to the 1970s, G. Evelyn Hutchinson, the “father of modern ecology,” proved particularly influential on the emerging field through his own contributions and by mentoring numerous students who went on to become outstanding ecologists in their own right (Slack, 2010).

One of Hutchinson’s protégés was Robert MacArthur, who in the 1960s teamed up with the Harvard biologist E. O. Wilson, to push ecology in more theoretically and mathematically robust directions (Quammen, 1996). In “An Equilibrium Theory of Insular Zoogeography” (1963) and The Theory of Island Biogeography (1967), MacArthur and Wilson argued that the number of species on a given island was the result of a dynamic equilibrium between the rate at which new species colonized it and the rate that resident species went extinct. Species richness thus depended on an island’s size—the area effect, with larger islands containing more species than smaller islands—and its location—the distance effect, with islands closer to continents or large islands supporting more species than those more distant.

Scientists and conservationists soon recognized that the theory of island biogeography provided potential insight into the impact of habitat fragmentation and the design of nature reserves. In a landmark article in the mid-1970s, Jared Diamond launched a controversy that raged in conservation circles for more than a decade: the Single Large or Several Small (SLOSS) debate. Diamond (1975) argued that to support the maximum number of species in a given area, a large land reserve was better than several smaller reserves, closely grouped reserves were better than those spaced farther apart, linked reserves were better than those that were isolated, and round were better than elongated reserves. A year later, Daniel Simberloff and Lawrence Abele (1976) published an article in Science challenging Diamond’s claims and arguing that careful case-by-case reviews of how particular nature reserves actually functioned on the ground were preferable to sweeping generalizations like those that Diamond had offered (Franco, 2013).

The initial exchange provoked a long series of replies and numerous empirical studies into the impact of habitat fragmentation on species diversity (Franco, 2013; Shafer, 1990). The most important of these was the Biological Dynamics of Forest Fragments Project (BDFFP) that Thomas Lovejoy, a student of G. Evelyn Hutchinson, launched in 1979 with the help of Richard O. Bierregaard, Jr. Lovejoy and his collaborators convinced Brazilian farmers bringing new land into production in the central Amazon rainforest and to leave fragments of different sizes and with different distances between them so they could study the ensuing changes in habitat and species composition. Those studies confirmed the importance of edge effect—changes in community or population structure that occur at the boundaries of habitat—as a key driver of fragment dynamics, affecting microclimate, tree mortality, carbon storage, fauna, and other characteristics. As of 2010, researchers associated with BDFFP had produced 562 publications and 143 graduate theses presenting their findings (Laurence et al., 2011, p. 57).

The Emergence of Conservation Biology

Deepening concern about the threat of extinction also led to the founding of a new mission-oriented biological discipline aimed not just at producing new knowledge but also stemming the loss. While the ultimate roots of the field that became known as “conservation biology” are many, the immediate origins can be traced back to the tireless advocacy of Michael Soulé, who earned his doctorate studying the genetic fitness of wild plant and animal populations under Paul Ehrlich (Franco, 2013; Meine, Soulé, & Noss, 2006). In 1978, Soulé organized the first International Conference on Conservation Biology, where he issued an impassioned plea for a bolder, more courageous response to the growing extinction crisis: “The world was on the first of the worst biological extinction in 61 million years,” he noted, “and it was high time academics and conservationists overcame the barriers between their field to work together and save plants and animals” (as quoted in Gibbons, 1992).

One product of the gathering was an influential volume, Conservation Biology: An Evolutionary-Ecological Perspective, that Soulé and his student Bruce Wilcox edited. There they proclaimed conservation biology was an emerging “mission-oriented discipline comprising both pure and applied science” that focused on “the knowledge and tools of all biological disciplines, from molecular biology to population, on one issue—nature conservation” (Soulé & Wilcox, 1980, p. 1). A series of meetings and publications followed, leading to the Second International Conference on Conservation Biology in 1985. On the final day, participants voted to formally organize the Society for Conservation Biology. After the meeting, Soulé offered an expanded definition of the field he had been working to create. The goal of conservation biology, he wrote, “is to provide principles and tools for preserving biological diversity.” Given the increasingly desperate plight of the world’s biota, it was necessarily a “crisis discipline” that required its practitioners to act through a “synthetic, eclectic, and interdisciplinary structure” and “before knowing all the facts.” Moreover, conservation biology was normative rather than merely descriptive, since its practitioners viewed the “untimely extinction of populations and species as bad” and sought to use the power of science to prevent that clearly undesirable outcome whenever possible (Soulé, 1985). A year later the society initiated a new journal, Conservation Biology.

After facing limited opposition, the new field of conservation biology prospered. Its creators encountered initial resistance from some wildlife managers and ecologists, who dismissed the new enterprise as a passing fad that offered little beyond what they had been doing for many years. By 1992, however, more than 5,000 scientists had joined the Society for Conservation Biology, at least 16 graduate programs offered advanced training in the field, and federal agencies and private foundations established significant funding opportunities for biodiversity research (Barrow, 2009, p. 357).

The Biodiversity Crisis

The launching of conservation biology played out against a backdrop of growing public and scientific interest in the issue of wildlife extinction and an emerging sense that the planet was facing a biodiversity crisis (Takacs, 1996; Franco, 2013; Farnham, 2007). In 1979, ecologist Norman Myers published The Sinking Ark, a book warning that the current extinction rate of up to one species per day could skyrocket to one species per hour within a decade. Two years later, Paul and Anne Ehrlich authored Extinction: The Causes and Consequences of the Disappearance of Species (1981), which cautioned that destroying species was like removing rivets from an airplane: just as at some point the craft would no longer remain airworthy, accelerating species loss threatened the functioning of Spaceship Earth. As one measure of the increasing attention to the issue, during the 1950s only 13 English-language books included the word “extinction” in the title. That figure rose to 79 by the 1970s, 104 by the 1980s, and 237 by the 1990s (Barrow, 2009, p. 353).

Not long after the Ehrlichs published their alarming book, officials from the National Academy of Sciences and the Smithsonian Institution began planning a national forum on the threat of wild species loss. Seeking an eye-catching name for the gathering, Walter G. Rosen coined the term “biodiversity,” a contraction of the words “biological diversity” that Raymond Dasmann had first used as early as 1968 as a way to talk about species richness. The National Forum of BioDiversity, held in Washington, DC, convened to much fanfare in September 1986. Organizers downlinked the final evening to an estimated audience of 10,000 viewers, while the historic gathering also received wide coverage in the media. Dubbing themselves the Club of Earth, one group of preeminent biologists in attendance (including Paul Ehrlich, E. O. Wilson, Peter Raven, Thomas Eisner, Ernst Mayr, and others) called a press conference to publicize the grave risk that biodiversity loss posed, declaring that “The species extinction crisis is a threat to civilization second only to the threat of thermonuclear war” (as quoted in Takacs, 1996, p. 38). Two years later, Wilson, a leading voice in the growing chorus of scientists warning about the dangers of species loss, published an edited volume with the title Biodiversity, and the term quickly caught on in scientific circles and among the broader public.

One challenge facing those worried about the extinction crisis was obtaining a more complete inventory of the world’s species. At the time that Biodiversity was published, E. O. Wilson (1988, p. 305) estimated that about 1.4 million species of living organisms had been described out of a total that he estimated to exceed five million. But Wilson also acknowledged that a 1982 study by the entomologist Terry Erwin potentially pushed up previous global estimates of species numbers by an order of magnitude. Responding to the uncertainty of the task before them, declining federal funding for non-defense related research, the biodiversity crisis, and concerns about the diminishing status of their field, in 1994 biological systematists issued a report calling for an ambitious research initiative they dubbed Systematics Agenda 2000. There they called for a 25-year-long international program to “discover, describe[,] and inventory global species diversity,” to synthesize the resulting inventory into a “predictive classification system that reflects the history of life,” and to develop the information systems necessary to make the results easily retrievable so they could benefit both science and society (Claridge, 1994). The initiative was updated in 2012 as Systematics Agenda 2020 (Daly, Herendeen, Guralnick, Westneat, & McDade, 2012).

Biologists and conservationists also pursued new international agreements to provide stronger protections for threatened species. As early as 1982, the IUCN began calling for a new treaty that would strengthen the existing patchwork of more than 300 international environmental agreements that were then in force, thereby providing the tools to “conserve biodiversity at the genetic, species, and ecosystem levels” (Holdgate, 1999, pp. 513–516; Stevens, 1992). Five years later, the Governing Council of the United Nations Environment Program also expressed interest in the idea. Developing nations, however, voiced fears that a new global biodiversity treaty would promote a “Northern agenda,” thwarting full utilization of their own natural resources while forcing them to shoulder the costs of implementing the agreement. They also worried about the practice of “bioprospecting,” the appropriation of biological compounds for commercial development without adequate compensation. Negotiators managed to allay those fears by the time of the Earth Summit, a United Nations Conference on Environment and Development held in Rio de Janiero in 1992, where the Convention of Biological Diversity was formally opened for signature. The final version, which went into the force in 1993, articulated three main goals: the conservation of, sustainable use of, and equitable sharing of benefits derived from biodiversity (Pitt, 1994). When the United States failed to sign the agreement at Rio de Janiero and the Senate refused to ratify the treaty once it was signed, the nation abandoned its longstanding role as a world leader in endangered species protection.

The “New Catastrophism” and Mass Extinction

A renewed interested in mass extinctions at the end of the 20th century supported the appreciation for the importance of biological diversity (Sepkoski, 2012, 2015). The geological catastrophism that had taken root in the late 18th and early 19th centuries gave way to a new way of thinking about biological extinction that began in Darwin’s day and continued for more than a hundred years. That consensus viewed natural species loss as slow and gradual, as balanced by the appearance of new species, and as generally fostering progressive change, since over time competition weeded out the “unfit” while facilitating the survival of those best adapted to their environment. In the 1970s and 1980s, what would come to be known as a “new catastrophism” in paleontology, and evolutionary biology began to challenge this Darwinian consensus. Proponents of this new view argued that extinction could sometimes be the result of geologically sudden events that had the power to destroy a broad swath of life forms. They also maintained that selection was “nonconstructive,” that is successful adaptation to the pre-extinction selective regime provided no guarantee of survival during mass extinction events, when previously non-adaptive features might prove advantageous.

The new catastrophism was grounded in the careful study of patterns in the fossil record, but as historian of science David Sepkoski (2015, p. 71) has argued, it was also shaped by (and contributed to) the broader cultural anxiety surrounding the threat of nuclear annihilation that reigned during the Cold War. What paleontologists during this period began to see in the fossil record was not the steady state of species richness that Darwinians had long asserted, but rather a pattern of steep rises and falls in the levels of biodiversity over the history of life. In an influential study, David Raup and Jack Sepkoski (1984) documented five particularly devastating mass extinction events over the past 500 million years, each of which destroyed anywhere from 50% to 95% of all species. They also found a remarkable periodicity to those mass extinctions, and others that were significant though less destructive: they seemed to occur at roughly 26 million year intervals. Sepkoski and Raup argued that the Darwinian model of gradual, constant adaptation through natural selection failed to account for these extinction events, which seemed to bend the trajectory of evolution, wiping out long-enduring taxa, like the dinosaurs, and ushering into prominence new ones, like the mammals. While the normal or “background” extinction rate might follow Darwinian expectations—that is, it tended to be slow and constant—mass extinction events did not seem to correspond with Darwinian notions in terms of their speed or selectivity.

Sepkoski and Raup’s extinction studies coincided with other discoveries that heightened their scientific impact and broader cultural significance. In 1980, a team of researchers led by the physicist Luis Alvarez, his son, the geologist Walter Alvarez, and several colleagues announced that at numerous sites around the world they had found concentrations of iridium hundreds of times larger than normal in sedimentary layers at the Cretaceous-Tertiary boundary (Alvarez, Alvarez, Asaro, & Michel, 1980). They calculated that this iridium anomaly resulted from a comet or asteroid roughly 10 kilometers in diameter that had spread debris around the globe after striking the Earth with an energy more than two million times greater than the largest thermonuclear device ever detonated. Although the Alvarez team failed to locate the impact site, researchers subsequently identified the Chicxulub Crater in the Yucatán Peninsula in Mexico as the spot where the massive bolide had collided with the Earth more than 65 million years ago, setting in motion a chain of events that led to the fall of the dinosaurs.

Fears about global cooling that might follow even a modest exchange of multiple nuclear weapons also resonated with and reinforced the new catastrophism in paleontology (Badash, 2009). In 1982, as the Cold War heated up following Ronald Reagan’s election to the U.S. presidency, the Dutch atmospheric chemist Paul J. Crutzen and his American colleague, John W. Birks, estimated that firestorms ignited by a nuclear war would carry soot high into the atmosphere, blocking up to 99% of the sun reaching the Earth. The result would not only be darkness for many weeks but also large-scale changes in surface temperatures and wind systems around the globe (Crutzen & Birks, 1982). One year later, the team of Richard P. Turco, Owen Toon, Thomas P. Ackerman, James B. Pollack, and Carl Sagan, all of whom had previously studied Martian dust storms or asteroid impact events, published “Nuclear Winter: Global Consequences of Multiple Nuclear Explosions,” a much more detailed account of this bleak scenario, which relied on computer modeling (Turco, Toon, Ackerman, Pollack, & Sagan, 1983). The public had first encountered the vivid, memorable term “nuclear winter” two months earlier when Sagan, a planetary astronomer and well-known science popularizer, published an account in Parade, a magazine supplement that appeared in Sunday newspapers across the United States. There he delivered the ominous warning that the dust, smoke, and soot injected into the atmosphere from the detonation of numerous nuclear weapons would most likely plunge the planet into a prolonged state of cold and darkness, killing off many plant and animal species, destroying civilization, and perhaps even rendering humans extinct (Sagan, 1983; Davis, 2001).

Thus, by the 1980s and 1990s, thinking about extinction in popular and scientific circles had expanded from concern about the loss of individual species to concern about mass extinction events, like the kind paleontologists had been documenting in the fossil record. As early as 1979, Norman Myers made an explicit analogy between the accelerating rate of extinction, which could lead to the loss of more than a million species by the end of the century and many millions soon thereafter, and the rapid disappearance of the dinosaurs at the end of the Cretaceous period (Myers, 1979, p. ix). More than a decade later, E. O. Wilson (1992, p. 32) decried the “sixth great extinction spasm,” reinforcing the comparison between mass extinction events in the geological record and the contemporary biodiversity crisis. Soon afterward, paleoanthropologist and wildlife conservationist Richard Leakey and science writer Roger Lewin titled their book The Sixth Extinction (1995). There they warned that humans were on a trajectory not only destroy as many as a half of the world’s species over the next century but also themselves: “Homo sapiens is in the throes of causing a major biological crisis, a mass extinction, the sixth such event to have occurred in the past half billion years. And we, Homo sapiens, may also be among the living dead.” (p. 245). By the time the science writer Elizabeth Colbert published her Pulitzer Prize-winning book, The Sixth Extinction: An Unnatural History (2014), the comparison between the current human-driven extinction crisis and the five mass extinction events documented in the fossil record had become a common means of stressing the scale and urgency of global biodiversity loss.

Amphibian Decline

The decline of amphibians garnered particular attention and concern during this period. While reports about the decrease or loss of this class of animals had periodically surfaced, not until scientists gathered for the First World Congress of Herpetology in 1989 did they discover how widespread the problem was. At numerous sites across the globe, even those with pristine environments, amphibian populations seemed to be experiencing population crashes. Several species, like the gastric brooding frogs in Australia (Rheobatrachus), the golden toad of the Monteverde Cloud Forest in Costa Rica (Incilius periglenes), and the web-footed coqui in Puerto Rico (Eleutherodactylus karlschmidti), had even met with extinction (Stuart, 2012).

Hoping to clarify the scope and causes of the problem, in 2001 the IUCN and World Conservation Union launched the Global Amphibian Assessment (GAA). Three years later the GAA report concluded that the level of threat to amphibians was greater than that facing birds or mammals, with 43.2% of species showing a decline and 32.5% (1,856 species) being globally threatened (i.e., listed in the IUCN Red List Categories of Vulnerable, Endangered, or Critically Endangered) vs. 12% of birds and 23% of mammals (Stuart et al., 2004). Some of the declines were associated with habitat loss or overharvesting, but pollution, climate change, and chytrid fungus infections also seemed to play an important role. In 2005, 100 amphibian scientists and conservationists gathered for the Global Amphibian Summit, which produced the Amphibian Conservation Action Plan, published in 2007 (Stuart, 2012). That plan called for “unified global strategy incorporating survival assurance colonies, disease research, and habitat protection,” while simultaneously acting on other threats, including climate change, over-harvesting, and environmental toxins (Gascon et al., 2007, p. 5).


Fear that the rate of extinction continues to accelerate, despite protective efforts and improved knowledge about the causes and consequences of species loss, has recently led to bold new proposals for addressing the biodiversity crisis. One innovative approach is rewilding, a term that the wilderness activist and Earth First! founder Dave Foreman coined sometime before its first appearance in print in 1990 (Fraser, 2009, p. 365). Rewilding is a conservation approach that seeks to restore the structure and healthy functioning of large-scale ecosystems while reducing biodiversity loss. It pursues these goals by protecting extensive core reserve areas, establishing connectivity between them, and safeguarding or reintroducing so-called keystone species, particularly large apex predators, that play a disproportionate role in shaping ecosystems. Rewilding proponents hope to bring back the levels of biodiversity and ecosystem function that existed in a given area prior to human intervention and to maintain those levels with a minimum of ongoing management. The development of the theory of island biogeography, the discussion of optimal reserve design during the SLOSS debates, the discovery that even relatively large protected areas continued to experience species loss, and a series of ecological studies demonstrating the profound influence of large predators on landscapes all played a role in the emergence of rewilding as a conservation strategy.

In 1991, conservation biologists Michael Soulé and Reed Noss joined forces with Dave Foreman, Doug Tompkins (founder of the Esprit and North Face clothing companies), and David Johns (an attorney and environmentalist) to launch an initiative called the Wildlands Project (and later the Wildlands Network; Fraser, 2009, p. 32). The group began developing a vision for rewilding on a continental scale through the protection and restoration of four Continental MegaLinkages (later called Wildways) that spanned wide swaths of North America—the Pacific, the Spine of the Continent, the Atlantic, and the Arctic Boreal. The hope was to provide the extensive territory and connectivity that apex predators need to thrive in each of these areas. In an influential paper, “Rewilding and Biodiversity: Complementary Goals for Continental Conservation,” Soulé and Noss (1998) presented a spirited defense of rewilding, which they argued could be characterized in its most simple form as providing the “cores, corridors, and carnivores” needed to accomplish conservation on a continental scale (p. 22). Six years later Foreman authored a book-length account of the need for and scientific foundations of the Wildlands Project with the title Rewilding North America: A Vision for Conservation in the 21st Century (2004). Soon conservationists were initiating rewilding projects around the globe, including the Yellowstone to Yukon Conservation Initiative in North America, the European Greenbelt along the former Iron Curtain, Rewilding Europe, and the Peace Parks Foundation in Southern Africa.

The most audacious and controversial form of this new approach to conservation is Pleistocene rewilding, which uses the end of the last Ice Age as the baseline for restoration efforts. Naturalists had long been aware that numerous large animals, particularly mammals, had gone extinct in the late Pleistocene period, and many of them initially blamed prehistoric hunters for their disappearance. Beginning in the mid-19th century and continuing for more than 100 years, however, they tended to regard climate change as the main culprit in this species loss. In the mid-20th century, armed with data from radiocarbon dating, pollen analysis, and stratigraphic studies, the American geoscientist Paul S. Martin argued again for human culpability in the late Pleistocene extinctions. Martin, who may have also been influenced by similar ideas published at about the same time by the Russian climatologist Mikhail I. Budyko, focused on the North American continent, where over 100 species of large mammals vanished in a period of roughly 1,000 years. He maintained, however, that a similar process had occurred at different times in different locations around the world: the extinction of large mammals followed soon after the arrival of humans in a process he called “Pleistocene overkill” (Martin, 1973; Frances, 2002).

Martin’s theory not only generated much discussion and criticism over the next several decades, but it also inspired some conservationists to begin think about the possibility of Pleistocene rewilding. Following a visit to Kruger National Park in South Africa, the Brazilian ecologist Mauro Galetti (2004) suggested that Pleistocene Parks might be created in parts of the Cerrado and the Pantanal in Brazil by re-introducing mega-herbivores, like horses, impalas, elephants, and guanacos, that could fill the ecological niches of species that were now extinct in the region. Martin (2005), also an early proponent Pleistocene rewilding, called for efforts to expand the ranges of numerous North American species whose geographic distributions had experienced contraction since the end of the Pleistocene and for the reintroduction of contemporary analogues of many of the large mammals that vanished during that period. African and Asian elephants might fill the ecological niche of extinct mastodons and mammoths, African lions and cheetahs might replace their lost American relatives, and numerous other herbivores and predators could be reintroduced to help restore ecological functioning in areas that Martin referred to as Quaternary Parks. As Josh Donlan, Martin, Foreman, Soulé, and several other colleagues argued in a commentary published in Nature that same year and a longer article published in 2006, Pleistocene rewilding would not only help protect Asian and African species that were at risk in their native lands but also “change the underlying premise of conservation biology from managing extinction to actively restoring natural processes” (Dolan et al., 2005, p. 913, 2006).

Not surprisingly, the idea of Pleistocene rewilding provoked strong criticism on a variety of grounds. Some feared that introducing Asian and African species into North America would divert scarce resources from more proven conservation methods. Others pointed out that the consequences of rewilding were unpredictable since North America’s flora and fauna had evolved considerably since the Pleistocene and exotic species often wreak havoc outside their native ranges. Another objection was the pervasive intolerance of predators in much of the United States. How would residents who routinely balked at the presence of mountain lions, wolves, and coyotes respond to a plan to introduce African lions and cheetahs in their regions? (Fraser, 2009, pp. 297–298; Caro, 2007). Despite these criticisms, plans have moved forward with reintroducing the Bolson tortoise (Gopherus flavomarginatus), equids, and camelids at several locations, but not more controversial exotics, like lions, cheetahs, and elephants.

An earlier example of Pleistocene rewilding, in practice if not in name, dates back to 1988, when the Soviet geophysicist Sergey Zimov established an unusual nature reserve in remote northeastern Siberia. There, on the Kolyma River, in the Sakha Republic of Russia, Zimov began reintroducing the megafauna that shaped this subarctic environment before facing extinction some 10,000 years ago (Andersen, 2017; Lovgren, 2005). He did so to test the theory that humans had caused the late-Pleistocene loss of the region’s large herbivores, to begin restoring the once vast grasslands-steppe ecosystem that predominated in Siberia during the last Ice Age, and to slow the thawing of Arctic permafrost from global climate change. Zimov began his experiment with a modest 50 ha (125 acre) site to which he re-introduced the Yakutian horse (Equus ferus caballus). But soon the reserve that he began calling Pleistocene Park—a name probably inspired by Michael Crichton’s bestselling novel, Jurassic Park (1990), and the blockbuster movie that followed (1993)—would expand to include reindeer, elk, and muskox in a 50-square-mile area. Just as Josh Donlan, Paul Martin, and their colleagues were promoting the idea of Pleistocene rewilding in North America, Zimov published an account of Pleistocene Park in Science (Zimov, 2005). If successful, Zimov hoped his grassland restoration experiment could be expanded across much of Siberia, thereby slowing the melting of the region’s permafrost, which sequesters more total carbon content than all of the planet’s rainforests. He was also optimistic that efforts to resurrect the mammoth through de-extinction would allow him to reintroduce the largest of the herbivores that once dominated the region.


At the turn of the 20th century, molecular biologists developed powerful new technologies—like whole organism cloning and DNA extraction, sequencing, and assembly—that might be used to revive once lost species through a process known as de-extinction. The Human Genome Project, launched in 1990 with the goal of determining the complete sequence of human DNA, and the birth of Dolly the Sheep, the first mammal cloned from an adult somatic cell in 1997, raised hopes that innovative techniques like these might be successfully deployed not only to help rescue endangered species but also resurrect extinct ones.

One early de-extinction project was an attempt to bring back the thylacine (Thylacinus cynocephalus), also known as the Tasmanian tiger. This carnivorous marsupial, named for dark brown vertical stripes running down its back, once ranged throughout mainland Australia, Tasmania, and as far north as New Guinea. It was probably extinct or near extinct in Australia by the time Europeans began colonizing the region in the late 18th century, a victim of competition with the dingoes that humans had introduced earlier. The thylacine continued to survive in Tasmania, though it faced relentless persecution from bounty hunters and farmers, who feared it preyed on their sheep. As a result, the species was extremely rare in the wild by the 1920s, when limited efforts to protect it finally began. The last well-documented thylacine died at the Hobart Zoo in 1936 (Paddle, 2000). Although periodic claims of sightings continued after that date, the IUCN declared the species officially extinct in 1982 and the Tasmanian government did the same in 1986, one year before the successful cloning of Dolly. By that point, the species had become a cultural icon in Australia, as well as a source of both nationalistic pride and deep regret.

In an attempt to resurrect the lost species, the Australian Museum in Sydney launched the Thylacine Cloning Project in 1999. According to Mike Archer, the director of the Australian Museum and founder of the project, it did so at least partly to try to atone for humanity’s role in wiping out the species (Vangelova, 2003). Staff scientists managed to extract DNA fragments from a thylacine pup preserved in alcohol and two dried specimens before the museum abandoned the project in 2005, when it turned out the DNA was too degraded to be assembled into a library, the next step in the cloning process (Kerr & Smith, 2005).

Meanwhile, several scientific teams enjoyed limited success in cross-species cloning with endangered or very recently extinct animals (Vangelova, 2003, Shapiro, 2015, pp. 142–144). In early 2001, Advanced Cell Technology (ACT) of Worcester, Massachusetts, announced that it had succeeded in cloning a rare wild ox called a guar (Bos gaurus), native to South and Southeast Asia, using a domesticated cow as the surrogate mother. The cloned guar survived only two days, however, before falling victim to dysentery. Two years later, staff from ACT, Trans Ova Genetics, and the San Diego Zoo cloned a banteng (Bos javanicus), an endangered wild bovine from Java. Soon thereafter, a group of Spanish and French scientists cloned a bucardo (Capra pyrenaica pyrenaica), a subspecies of the Spanish ibex, a wild goat native to the northern Iberian peninsula. The team extracted somatic cells from tissue samples taken from the last living bucardo and then fused them with oocytes from a domesticated goat with its nuclei removed. The resulting cells were then placed into a surrogate domesticated goat mother, but by then the final bucardo had died, rendering the subspecies extinct. While the experiment resulted in the birth of a cloned bucardo, it died moments after birth from a lung deformity.

De-extinction projects aimed at resurrecting long-lost species, like the woolly mammoth (Mammuthus primigenius), have proven even more challenging since they involve obtaining viable nuclei from animals that have been dead for thousands of years or more. Emboldened by the recent discovery of a frozen mammoth thigh bone in the Sakha Republic that was so well preserved it contained greasy marrow, in 2011, the Japanese scientist Akira Iritani announced that his team would clone a mammoth within five years (Shapiro, 2015, pp. 86–93). At the time of his brash claim, Iritani, a professor emeritus at Kyoto University, and his colleague Kazufumi Goto had been pursuing what they dubbed the Mammoth Creation Project for more than a decade. A year later, in 2012, Hwang Woo-Suk, of the Sooam Biotech Research Foundation in South Korea, declared that he too was working closely with Russian scientists in a competing effort to clone a mammoth (Shapiro, 2015, p. 93). Woo-Suk, who had been the first to clone a domesticated dog using somatic cell nuclear transfer, later enjoyed success cloning coyotes and grey wolves. Despite both team’s previous success with cloning, neither has secured the intact mammoth nuclei needed to revive the mammoth.

Genome engineering offers an alternative de-extinction strategy. Since the late 2000s, the Harvard geneticist and molecular engineer George Church, a pioneer in numerous areas of genomics, has been developing plans to create a mammoth-like creature by inserting snippets of mammoth DNA into an Asian elephant (Elephas maximus). Church’s approach, which became technically feasible following discovery of the CRISPR gene editing tool in 2012, involves extracting enough DNA from well-preserved specimens to sequence the mammoth genome, comparing it to the Asian elephant genome, figuring out which sites on an Asian elephant genome need to be altered to make it more closely resemble a mammoth genome, cutting and pasting synthesized strands of mammoth DNA into the nucleus of an elephant cell using the CRISPR tool, and then cloning that engineered mammoth cell using nuclear transfer (Shapiro, 2015, pp. 116–118; Wade, 2008). A 2015 study comparing the genomes of the two species identified differences in more than 1,642 protein-coding genes between the mammoth and the elephant, differences that seem mostly related to adaptations that allowed the mammoth to survive in an Arctic environment. Although Church’s team managed to make only 45 relevant alterations to the elephant genome by early 2017, he believes they are only two years away from creating a mammoth-elephant hybrid embryo, which they hope can then bring to term using an artificial womb (Pilcher, 2017).

Encouraged by discussions about the possibility of resurrecting the mammoth, in 2011 American environmentalist and technoenthusiast Stewart Brand became convinced that genomic techniques might also be used to restore the passenger pigeon to life (Rich, 2014). Brand, who is best known for his countercultural Whole Earth Catalog, hoped that enough DNA could be extracted from the 100s of passenger pigeon specimens in museums around the world that it could be sequenced. With the resulting DNA code as a blueprint, researchers might then use genetic engineering techniques to modify parts of the genome of a closely related species, the band-tailed pigeon, until it resembled the living passenger pigeon. The resulting cells could then be introduced into a band-tailed pigeon’s embryo, which, if everything worked as planned, would hatch into a band-tailed pigeon that possessed either passenger pigeon sperm or eggs.

With financial backing from Brand and his wife, Ryan Phelan, in 2012 the Long Now Foundation launched Revive & Restore, which seeks to coordinate, promote, and support efforts to bring back the passenger pigeon and numerous other lost species (Rich, 2014; Revive & Restore, n.d.). Critics worry that de-extinction programs divert money and attention from more tried-and-true conservation efforts, fail to address the problem of habitat loss, and promote a hubristic belief in the power of techno-fixes to solve the growing problem of mass extinction (Minteer, 2014; Pimm, 2013). Unmoved by these concerns, Revive & Restore continues to promote efforts to bring back the passenger pigeon. It projects that it will be in a position to begin breeding the species in captivity by 2022 and releasing it into the wild a decade later.

Collections of cryogenically preserved genetic materials (e.g., sperm, eggs, embryos, and tissues) provide another source of DNA from endangered and extinct organisms for de-extinction efforts and scientific research. In 1975, the pathologist and geneticist Kurt Benirischke convinced the San Diego Zoo to establish the Frozen Zoo and hired the molecular biologist Oliver Ryder to head the initiative (Ryder, in press; Peterson, 2016). Over the next four decades, the Frozen Zoo would collect more than 10,000 samples of genetic material from over 1,000 species and subspecies, including the cells used to clone the endangered guar and the banteng in the early 2000s. The excitement surrounding the revolution in genomics and assisted reproductive technologies of the 1980s and 1990s led to the creation of numerous other biobanks. In 2001 the American Museum of Natural History in New York began the Ambrose Monell Cryo Collection, which currently houses around 10,000 specimens and has a capacity of over a million (O’Conner, 2015, pp. 121–132). Three years later, the British Natural History Museum, the Zoological Society of London, and Nottingham University teamed up to launch the Frozen Ark to store genetic material from endangered animals. By 2015, the collection contained 48,000 specimens from more than 5,000 species (, 2015). In 2011, the Smithsonian Institution created the most ambitious cryogenetic gene bank to date, the National Museum of Natural History Biorepository, designed to preserve 4.2 million specimens (Koebler, 2013).


Over the past century, scientists have increasingly come to understand how overexploitation, habitat destruction, invasive species, and pollution endanger the world’s biota, and they have mobilized to study and respond to those threats. More recently, they have also begun to realize the grave danger that climate change poses to plants and animals around the globe. According to the 2013 Assessment Report of U.N. Intergovernmental Panel on Climate Change, the world’s authoritative body on the subject, without additional mitigation the continued production of CO2 and other greenhouse gases will lead to an increase in global temperatures by as much 4.8 degrees Celsius by 2100 (IPCC, 2014, p. 20). Unless this trend is reversed, species will have to respond quickly to survive, either by adapting to warmer temperatures or migrating to more suitable environments. The challenge is even greater because individual species are bound up in an ecological web of many other organisms, each of which have differing abilities to adapt or migrate in the face of rapidly rising temperatures. At the same time, numerous other troubling effects of climate change that are already in evidence—rising sea levels; melting glaciers, sea ice, and permafrost; the loss of coral reefs; and extreme weather events like powerful storms, floods, droughts, and heat waves—also threaten not only individual species but entire ecosystems. In 2000, the Dutch, Nobel-Prize-winning, atmospheric chemist Paul J. Crutzen proposed a new name for the current geological epoch, the Anthropocene, to highlight how profoundly climate change, biodiversity loss, and other human-induced alterations have transformed the earth (Crutzen & Stoermer, 2000).

Whether or not geologists ultimately accept Crutzen’s suggestion, there is no doubt that humans have had a significant impact on the earth’s biota. While no one knows for sure exactly how severe the problem is, according to a 2014 estimate from the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services—the leading independent, intergovernmental body for assessing the state of the Earth’s biodiversity—the current extinction rate is as much as 1,000 times higher than the normal background rate (Pimm et al., 2014). Over the past century, scientists have acquired much knowledge about the scope, causes, and consequences of the biodiversity crisis. The question facing humanity in the early 21st century is whether we can muster the wisdom, courage, and political will needed to properly address that crisis.

Further Reading

Adams, W. M. (2004). Against extinction: The story of conservation. London: Earthscan.Find this resource:

Barrow, M. V., Jr. (2009). Nature’s ghosts: Confronting extinction from the age of Jefferson to the age of ecology. Chicago: University of Chicago Press.Find this resource:

Boardman, R. (1981). International organization and the conservation of nature. Bloomington: Indiana University Press.Find this resource:

Cioc, M. (2009). The game of conservation: International treaties to protect the world’s migratory animals. Athens: Ohio University Press/Swallow Press.Find this resource:

Colbert, E. (2014). The sixth extinction: An unnatural history. New York: Henry Holt and Company.Find this resource:

Dorsey, K. (2013). Whales & nations: Environmental diplomacy on the high seas. Seattle: University of Washington Press.Find this resource:

Farnham, T. (2007). Saving nature’s legacy: Origins of the idea of biological diversity. New Haven, CT: Yale University Press.Find this resource:

Foreman, D. (2004). Rewilding North America: A vision for conservation in the 21st century. Washington, DC: Island Press.Find this resource:

Franco, J. L. de A. (2013). The concept of biodiversity and the history of conservation biology: From wilderness preservation to biodiversity conservation. Historia (São Paulo), 32(2), 21–48.Find this resource:

Fraser, C. (2009). Rewilding the world: Dispatches from the conservation revolution. New York: Metropolitan Books, Henry Holt and Company.Find this resource:

Greenberg, J. (2014). A feathered river across the sky: The passenger pigeon’s flight to extinction. New York: Bloomsbury.Find this resource:

Hagen, J. B. (1992). An entangled bank: The origins of ecosystem ecology. New Brunswick, NJ: Rutgers University Press.Find this resource:

McCormick, J. (1989). Reclaiming paradise: The global environmental movement. Bloomington: Indiana University Press.Find this resource:

Meine, C., Soulé, M., & Noss, R. E. (2006). “A mission-driven discipline”: The growth of conservation biology. Conservation Biology, 20(3), 631–651.Find this resource:

O’Conner, M. R. (2015). Resurrection science: Conservation, de-extinction and the precarious future of wild things. New York: St. Martin’s Press.Find this resource:

Petersen, S. (2002). Acting for endangered species: The statutory ark. Lawrence: University Press of Kansas.Find this resource:

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Shapiro, B. (2015). How to clone a mammoth: The science of de-extinction. Princeton, NJ: Princeton University Press.Find this resource:

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