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Home , American cheetah, Pleistocene rewilding

Donlan CJ. 2007. Restoring America's big wild . Scientific American June 2007:72-7.

Keywords: 1NAm/////megafauna/Miracinonyx trumani/Panthera leo/Panthera onca// concolor/saber-toothed cat

Abstract: -a proposal to bring back animals that disappeared from 13,000 years ago-offers an optimistic agenda for 21st-century conservation. In the fall of 2004 a dozen conservation biologists gathered on a ranch in New to ponder a bold plan. The scientists, trained in a variety of disciplines, ranged from the grand old men of the fi eld to those of us earlier in our careers. The idea we were mulling over was the reintroduction of large vertebrates-megafauna-to North America. Most of these animals, such as and , died out roughly 13,000 years ago, when humans from Eurasia began migrating to the continent. The theory-propounded 40 years ago by Paul Martin of the University of - is that overhunting by the new arrivals reduced the numbers of large vertebrates so severely that the populations could not recover. Called Pleistocene overkill, the concept was highly controversial at the time, but the general thesis that humans played a significant role is now widely accepted. Martin was present at the meeting in , and his ideas on the loss of these animals, the ecological consequences, and what we should do about it formed the foundation of the proposal that emerged, which we dubbed Pleistocene rewilding. Restoring America’s

n the fall of 2004 a dozen conservation biologists gath- Although the cheetahs, and mammoths that once ered on a ranch in New Mexico to ponder a bold plan. roamed North America are extinct, the same or close The scientists, trained in a variety of disciplines, ranged relatives have survived elsewhere, and our discussions fo- from the grand old men of the fi eld to those of us earlier cused on introducing these substitutes to North American I in our careers. The idea we were mulling over was the reintro- . We believe that these efforts hold the potential to duction of large vertebrates—megafauna—to North America. partially restore important ecological processes, such as pre- Most of these animals, such as mammoths and cheetahs, dation and browsing, to ecosystems where they have been died out roughly 13,000 years ago, when humans from Eur- absent for millennia. The substitutes would also bring eco- asia began migrating to the continent. The theory—propound- nomic and cultural benefi ts. Not surprisingly, the published ed 40 years ago by Paul Martin of the University of Arizona— proposal evoked strong reactions. Those reactions are wel- is that overhunting by the new arrivals reduced the numbers come, because debate about the conservation issues that un- of large vertebrates so severely that the populations could not derlie Pleistocene rewilding merit thorough discussion. recover. Called Pleistocene overkill, the concept was highly controversial at the time, but the general thesis that humans Why Big Animals Are Important played a signifi cant role is now widely accepted. Martin was our approach concentrates on large animals because present at the meeting in New Mexico, and his ideas on the they exercise a disproportionate effect on the environment. loss of these animals, the ecological consequences, and what For tens of millions of years, megafauna dominated the globe, we should do about it formed the foundation of the proposal strongly interacting and co-evolving with other species and that emerged, which we dubbed Pleistocene rewilding. infl uencing entire ecosystems. Horses, , lions, ele-

70 SCIENTIFIC AMERICAN JUNE 2007 COPYRIGHT 2007 SCIENTIFIC AMERICAN, INC. Big, Wild Animals

Pleistocene rewilding—a proposal to bring back animals that disappeared from North America 13,000 years ago—offers an optimistic agenda for 21st-century conservation

By C. Josh Donlan

COPYRIGHT 2007 SCIENTIFIC AMERICAN, INC. phants and other large creatures were Questions from Readers everywhere: megafauna were the norm. A summary of this article on our Web site invited readers to But starting roughly 50,000 years ago, ask questions about rewilding. Many of the questions helped to the overwhelming majority went ex- shape the article, and answers appear throughout the text. tinct. Today megafauna inhabit less The author replies to a few others here. than 10 percent of the globe. Over the past decade, ecologist John Won’t African elephants and lions freeze to death in the Montana Terborgh of Duke University has ob- winter? Will these animals be kept indoors in served directly how critical large animals the winter, as they are in zoos? —Jason Raschka are to the health of ecosystems and how The reader brings up an important point—that many their loss adversely affects the natural questions, including climate suitability, would have world. When a hydroelectric dam fl ood- to be answered by sound scientifi c studies during the ed thousands of acres in Venezuela, Ter- process of Pleistocene rewilding. One could imagine borgh saw the water create dozens of is- scenarios in which animals would be free-living (albeit lands—a fragmentation akin to the vir- intensively managed) year-round in expansive reserves in the southwestern U.S. and perhaps other tual islands created around the world as scenarios in locales farther north, where animals humans cut down trees, build shopping would be housed indoors during the coldest months. ELEPHANT at the Copenhagen zoo. malls, and sprawl from urban centers. Even the latter alternative is arguably better than a The islands in Venezuela were too small zoo. Many zoos are struggling to fi nd appropriate space for elephants and are to support the creatures at the top of the choosing to abandon their elephant programs. food chain—predators such as , pumas and eagles. Their disappearance Aren’t there big-game hunting ranches in ? Could we learn sparked a chain of reactions. Animals anything from them? —Foster such as monkeys, leaf-cutter ants and There are many big-game hunting ranches throughout Texas, and some of them other herbivores, whose populations hold animals such as cheetahs that we know, from the fossil record, once lived were no longer kept in check by , in North America. To my knowledge, no conservation biologists have studied the ranch-held animals, but these ranches might present excellent research thrived and subsequently destroyed veg- opportunities if they are willing to collaborate. Other ranches, however, have etation—the ecosystems collapsed, with animals that are not supported by the fossil record and as such offer no biodiversity being the ultimate loser. potential for study. Similar ecological disasters have oc- curred on other continents. Degraded Hasn’t this general idea been around for a while? —Kevin N. ecosystems are not only bad for biodi- Both the concept of rewilding and the term itself have been around for some time. versity; they are bad for human econo- Rewilding is the practice of reintroducing species to places from which they have mies. In Central America, for instance, been extirpated in the past few hundred years. Pleistocene rewilding, in contrast, involves introducing species descended from creatures that went extinct some researchers have shown that intact trop- 13,000 years ago or using similar species as proxies. ical ecosystems are worth at least $60,000 a year to a single coffee farm because of the services they provide, ecosystems by keeping herbivores that habitat in which certain plants and ani- such as the pollination of coffee crops. eat kelp, such as sea urchins, in check. In mals can fl ourish. Lions and other preda- Where large predators and herbivores , elephants are keystone players; as tors control the populations of African still remain, they play pivotal roles. In they move through an area, their knock- herbivores, which in turn infl uence the Alaska, sea otters maintain kelp forest ing down trees and trampling create a distribution of plants and soil nutrients. In Pleistocene America, large preda- Rewilding Our Vision tors and herbivores played similar roles. Overview/ Today most of that vital infl uence is ab- Peter Arnold, Inc. Arnold, Peter ■ A group of conservationists has proposed reintroducing to North America sent. For example, the American chee- large animals that went extinct 13,000 years ago. tah (a relative of the African cheetah) ); DONA DONA ); ■ Close relatives of these animals—elephants, camels, lions, cheetahs—survived dashed across the in pursuit elsewhere; returning them to America would reestablish key ecological of antelopes for millions of processes that once thrived there, as well as providing a refuge for endangered years. These chases shaped the prong- species from Africa and Asia and creating opportunities for ecotourism. horn’s astounding speed and other bio- preceding pages preceding ■ The proposal has, understandably, evoked strong reactions, but the bold logical aspects of one of the fastest ani- suggestion has spurred debate and put a positive spin on conservation mals alive. In the absence of the cheetah, biology, whose role has been mainly a struggle to slow the loss of biodiversity. the pronghorn appears “overbuilt” for

its environment today. ( FELDER LARRY

72 SCIENTIFIC AMERICAN JUNE 2007 COPYRIGHT 2007 SCIENTIFIC AMERICAN, INC. Pleistocene rewilding is not about re- When the West Was Really Wild creating exactly some past state. Rather Soon after humans crossed the Bering land bridge into North America some it is about restoring the kinds of species 13,000 years ago, almost 75 percent of the continent’s large (those interactions that sustain thriving eco- weighing more than 45 kilograms) disappeared (color). One of the goals of systems. Giant tortoises, horses, camels, Pleistocene rewilding is to restore some of these species or close proxies to the cheetahs, elephants and lions: they were American West. For example, the same species of lion and cheetah that once lived in North America survive today in Africa; the African or all here, and they helped to shape North could substitute for the extinct ; and Bactrian camels might stand American ecosystems. Either the same in for the extinct . species or closely related species are available for introduction as proxies, Large Mammals of Pleistocene North America and many are already in captivity in the U.S. In essence, Pleistocene rewilding Xenarthra would help change the underlying prem- Glyptodont (Glyptotherium fl oridanum) ise of conservation biology from limit- Harlan’s (Paramylodon harlani) ing to actively restoring natu- Jefferson’s ground sloth (Megalonyx jeffersonii) Shasta ground sloth (Nothrotheriops shastensis) ral processes. At fi rst, our proposal may seem outra- Carnivores () geous—lions in Montana? But the plan Dire (Canis dirus) deserves serious debate for several rea- Gray wolf (Canis lupus) sons. First, nowhere on Earth is pristine, Black (Ursus americanus) at least in terms of being substantially (Ursus arctos) free of human infl uence. Our demograph- Giant short-faced bear (Arctodus simus) ics, chemicals, economics and politics Saber-toothed cat ( fatalis) pervade every part of the planet. Even in (Panthera leo) our largest national parks, species go ex- Jaguar (Panthera onca) tinct without active intervention. And (Miracinonyx trumani) Mountain lion (Puma concolor) human encroachment shows alarming signs of worsening. Bold actions, rather Elephants () than business as usual, will be needed to American (Mammut americanum) reverse such negative infl uences. Second, Columbian mammoth (Mammuthus columbi) since conservation biology emerged as a Dwarf mammoth (Mammuthus exilis) discipline more than three decades ago, Woolly mammoth (Mammuthus primigenius) it has been mainly a business of doom and gloom, a struggle merely to slow the Horses (Perissodactyla) loss of biodiversity. But conservation Mexican horse (Equus conversidens) need not be only reactive. A proactive ap- Western horse (Equus occidentalis) proach would include restoring natural Other extinct horses and asses (Equus spp.) processes, starting with ones we know Even-Toed (Artiodactyla) are disproportionately important, such Western (Camelops hesternus) as those infl uenced by megafauna. Long-legged (Hemiauchenia macrocephala) Third, land in North America is Long-nosed peccary (Mylohyus nasutus) available for the reintroduction of mega- Flat-headed peccary ( compressus) fauna. Although the patterns of human (Odocoileus hemionus) land use are always shifting, in some ar- White-tailed deer (Odocoileus virginianus) eas, such as parts of the Mountain deer (Navahoceros fricki) and the Southwest, large private and Woodland caribou (Rangifer tarandus) public lands with low or declining hu- (Alces alces) man population densities might be used Wapiti (Cervus elaphus) Pronghorn (Antilocapra americana) for the project. Fourth, bringing mega- Harrington’s (Oreamnos harringtoni) fauna back to America would also bring Mountain goat (Oreamnos americanus) tourist and other dollars into nearby (Ovis canadensis) communities and enhance the public’s Shrub ox (Euceratherium collinum) appreciation of the natural world. More Bonnet-headed musk ox (Bootherium bombifrons) than 1.5 million people visit San Diego’s Bison (Bison bison)

SHAWN GOULD GOULD SHAWN Wild Park every year to catch a Extinct bison (Bison spp.)

www.sciam.com SCIENTIFIC AMERICAN 73 COPYRIGHT 2007 SCIENTIFIC AMERICAN, INC. 74 SCIENTIFIC AMERICAN JUNE 2007 COPYRIGHT 2007 SCIENTIFIC AMERICAN, INC. Past, Present and Future glimpse of large mammals. Only a vate lands and could begin immediately. These artist’s conceptions illustrate some handful of U.S. national parks receive They will be critical in answering the of the ways that large animals infl uence the that many visitors. Last, the loss of some many questions about the reintroduc- and how Pleistocene rewilding of the remaining species of megafauna tions and would help lay out the costs could be benefi cial. David Burney of the National Tropical Botanical Garden, who is a in Africa and Asia within this century and benefi ts of rewilding. member of the Pleistocene rewilding group, seems likely— Pleistocene rewilding One of these experiments is already worked with artist Larry Felder in conceiving could help reverse that. under way. Spurred by our 2004 meeting, the illustrations. biologists recently reintroduced Bolson How It Might Be Done tortoises to a private ranch in New Mex- we are not talking about backing ico. Bolson tortoises, some weighing up a van and kicking some cheetahs out more than 100 pounds, once grazed parts LATE APRIL DURING THE into your backyard. Nor are we talking of the southwestern U.S. before disap- PLEISTOCENE, 14,000 YEARS AGO about doing it tomorrow. We conceive of pearing around 10,000 years ago, vic- Humans have not yet appeared on the landscape, but glaciers have receded and the Pleistocene rewilding as a series of staged, tims of human hunting. This endangered climate of what is now the western U.S. is carefully managed ecosystem manipula- tortoise now clings to survival, restricted similar to modern conditions, though a little tions. What we are offering here is a vi- to a single small area in central Mexico. cooler. Dense snow covers the Rocky Mountains. The river, fl ushed by snowmelt, is sion—not a blueprint—of how this might Thus, the reintroduction not only repatri- fairly high and a milky blue because of the fi ne be accomplished. And by no means are ates the tortoise to the U.S., it increases particles carried from the young, postglacial we suggesting that rewilding should be the species’ chance for survival. Similar soils. The banks are grazed down short but are generally green. A Columbian mammoth, a a priority over current conservation pro- experiments are also occurring outside Camelops, a Shasta ground sloth, long-horned grams in North America or Africa. Pleis- North America [see box on page 77]. bison, horses and pronghorn antelope tocene rewilding could proceed along- The reintroduction of wild horses browse, while an American lion and two watch from the opposite bank and a side such conservation efforts, and it and camels would be a logical part of bald eagle glides over the river. would likely generate conservation dol- these early experiments. Horses and lars from new funding sources, rather camels originated on this continent, and than competing for funds with existing many species were present in the late conservation efforts. Pleistocene. Today’s feral horses and The long-term vision includes a vast, asses that live in some areas throughout securely fenced ecological history park, the West are plausible substitutes for ex- APRIL 2007 encompassing thousands of square tinct American species. Because most of The Rocky Mountains have snow on their miles, where horses, camels, elephants the surviving Eurasian and African spe- upper summits only. The river, muddied by spring rains and by too many on the and large carnivores would roam. As cies are now critically endangered [see banks, has widened, and the banks are happens now in Africa and regions sur- “Endangered Wild Equids,” by Patricia eroded and broken down by hoofprints and rounding some U.S. national parks, the D. Moehlman; Scientifi c American, cattle wallows. Grass is short, patches of soil are bare, thorny shrubs predominate. Bison ecological history park would not only March 2005], establishing Asian asses graze in the background, and a wary attract ecotourists but would also pro- and Przewalski’s horse in North Amer- stands on the opposite shore. vide jobs related both to park manage- ica might help prevent the extinction of ment and to tourism. these animals. Bactrian camels, which To get to that distant point, we are critically endangered in the Gobi would need to start modestly, with rela- Desert, could provide a modern proxy tively small-scale experiments that as- for Camelops, a camel. sess the impacts of megafauna on North Camels, introduced from captive or do- APRIL 2027 American landscapes. These controlled mesticated populations, might benefi t Vegetation is more summerlike—global experiments, guided by sound science U.S. ecosystems by browsing on woody warming conditions are in full swing. The mountains have no snow on the peaks. and by the fossil record, which indicates plants that today are overtaking arid Drought, low humidity and absence of what animals actually lived here, could grasslands in the Southwest, an ecosys- mountain snow have lowered the level of the occur fi rst on donated or purchased pri- tem that is increasingly endangered. river dramatically; the water is clear and dark. Erosion is less apparent; grass looks manicured from diversifi ed grazing and C. JOSH DONLAN holds a Ph.D. in ecology and evolutionary biology from Cornell University, browsing. An Asian eletphant, Bactrian where he is a research biologist. Founder and director of Advanced Conservation Strategies, camel, bison, antelope and Przewalski’s horses graze under the watchful eye of an he serves as an adviser to the Galápagos National Park and to Island Conservation and is a African lion; a cheetah approaches from the senior fellow at the Robert and Patricia Switzer Foundation and Environmental Leadership distance. In the background a monorail Program. He was highlighted in the New York Times Magazine’s “Big Ideas of 2005” issue and carries tourists. A high-tech electrifi ed fence with solar panels is out of view. THE AUTHOR named one of 25 “all-star” innovators for 2005 by Outside magazine. He spends much of

LARRY FELDER FELDER LARRY his time in Tasmania, Australia and Santa Cruz, Calif., trying to halt on islands.

SCIENTIFIC AMERICAN 75 COPYRIGHT 2007 SCIENTIFIC AMERICAN, INC. Another prong of the project would pronghorn, facilitating ecotourism as A real-world example of how the re- likely be more controversial but could an economic alternative for ranchers, introduction of a top predator might also begin immediately. It would estab- many of whom are struggling fi nancial- work comes from the wolves of Yellow- lish small numbers of elephants, chee- ly, and helping to save the world’s fastest stone National Park [see “Lessons from tahs and lions on private property. carnivore from extinction. the Wolf,” by Jim Robbins; Scientifi c Introducing elephants could prove Lions are increasingly threatened, American, June 2004]. The gray wolf valuable to nearby human populations with populations in Asia and some parts became extinct in and around Yellow- by attracting tourists and maintaining of Africa critically endangered. Bring- stone during the 1920s. The loss led to grasslands useful to ranchers (elephants ing back lions, which are the same spe- increases in their prey—moose and — could suppress the woody plants that cies that once lived in North America, which in turn reduced the distribution threaten southwestern grasslands). In presents daunting challenges as well as of aspens and other trees they eat. Lack the late Pleistocene, at least four ele- many potential benefi ts. But private re- of vegetation destroyed habitat for mi- phant species lived in North America. serves in southern Africa where lions gratory birds and for beavers. Thus, the Under a scientifi c framework, captive el- and other large animals have been suc- disappearance of the wolves propagated ephants in the U.S. could be introduced cessfully reintroduced offer a model— a trophic cascade from predators to her- as proxies for these extinct animals. The and these reserves are smaller than some bivores to plants to birds and beavers. biggest cost involved would be fencing, private ranches in the Southwest. Scientists have started to document the which has helped reduce conflict be- If these early experiments with large ecosystem changes as reintroduced tween elephants and humans in Africa. herbivores and predators show promis- wolves regain the ecological role they Many cheetahs are already in captiv- ing results, more could be undertaken, played in Yellowstone for millennia. An ity in the U.S. The greatest challenge moving toward the long-term goal of a additional insight researchers are learn- would be to provide them with large, se- huge ecological history park. What we ing from putting wolves back into Yel- curely fenced areas that have appropri- need now are panels of experts who, for lowstone is that they may be helping the ate habitat and prey animals. Offsetting each species, could assess, advise and park cope with . As win- these costs are benefi ts—restoring what cautiously lead efforts in restoring ters grow milder, fewer elk die, which must have been strong interactions with megafauna to North America. means less carrion for scavengers such What the Critics Say Since we proposed the rewilding idea in print, in Nature in 2005, the U.S. and Canada where DDT had wiped it out. Those birds were commentators have pointed out a number of concerns, some certainly different genetically from the ones that once soared legitimate, some not. “We all remember ‘ Park,’” wrote over the Midwest, yet they have done well in their new homes. Dustin Rubenstein and his colleagues in the journal Biological “One can only imagine ... farmers coping with crop destruction Conservation. “Pleistocene re-wilding of North America is by herds of elephants, or lions and cheetah attacking cattle, or only a slightly less sensational proposal.” We disagree even children,” Rubenstein and his colleagues warn. One with this assessment because the majority of need not merely imagine the challenges of coexisting dinosaurs went extinct 65 million years ago, whereas with large predators and herbivores. Africa and Asia many of North America’s original megafauna or very have been struggling with them for centuries, and close relatives are alive today elsewhere in the world substantial progress has been made; in our plan the and can be both studied and saved. animals will not be unrestrained. Rubenstein and colleagues go on to say, “Modern “Global climate change since the Pleistocene day proxies species are wrong ... different genetically extinctions makes the restoration of vanished from the species that occurred in North America during the ecosystems through large- introduction quite Pleistocene.” True, but not that different. Available evidence unlikely,” wrote Christopher Smith in a letter to Nature in the weeks indicates that the lions in Africa and Asia today are the same following the initial proposal. Many have expressed concern about species, albeit of smaller stature, as the lions that prowled North the fact that North America’s ecosystems are not the same today as America 13 millennia ago. Recent studies of ancient DNA have they were 13,000 years ago and that reintroduced animals might elucidated close relationships between extinct elephant and therefore be unable to survive in them. Whereas habitats are and horse species and those alive today. Further, introduction of will continue to be dynamic on a timescale of thousands of years, proxies for now extinct species has proved successful in other very few plants or small mammals went extinct during the late experiments. Hundreds of peregrine falcons from Australia, Pleistocene. The major missing component of North American Europe and South America were used, for example, in captive- ecosystems today compared with the Pleistocene is the megafauna,

breeding programs to reintroduce the peregrine falcon to parts of which we can infer are critical cogs in the wheels. —C.J.D. CHOWDHURY TINA

76 SCIENTIFIC AMERICAN JUNE 2007 COPYRIGHT 2007 SCIENTIFIC AMERICAN, INC. Elsewhere in the World as , ravens and bald eagles. Wolves provide carcasses throughout In other parts of the world, preliminary efforts have begun to reintroduce the winter for the scavengers to feed on, species to places from which they have long been absent. bestowing a certain degree of stability. ■ In April 2006 a team of Canadian and Russian biologists flew 30 from Canada’s Elk Island National Park to reserve in the The Challenges Ahead Republic of Sakha, , where the closely related vanished as our group on the ranch in New 5,000 years ago. Mexico discussed how Pleistocene rewil- ■ At the 15,000-acre nature reserve Oostvaardersplassen in the Netherlands, ding might work, we foresaw many chal- conservationists are restoring horses, roe deer and . lenges that would have to be addressed ■ Beavers are being reintroduced throughout Europe, in some cases in areas and overcome. These include the possibil- where they have been absent for thousands of years. ity that introduced animals could bring ■ In the tropical Pacific, endangered birds from the Marquesas and Tongan islands have been reintroduced to nearby islands where fossils indicate novel diseases with them or that they they once lived. might be unusually susceptible to diseas- ■ In the Indian Ocean, scientists from the Mauritian Wildlife Foundation are es already present in the ecosystem; the using giant tortoises from Aldabra Island to replace two extinct species of fact that habitats have changed over the tortoise on the Mascarene Islands. Scientists have already documented millennia and that reintroduced animals increased seed dispersal for many of the island plants. The tortoises have might not fare well in these altered envi- also brought increased tourism. —C.J.D. ronments; and the likelihood of unantic- ipated ecological consequences and un- expected reactions from neighboring human communities. Establishing pro- grams that monitor the interactions among species and their con sequences for the well-being of the ecosystem will require patience and expertise. And, of course, it will not be easy to convince the public to accept predation as an impor- tant natural process that actually nour- ishes the land and enables ecosystems to thrive. Other colleagues have raised ad- ditional concerns, albeit none that seems fatal [see box on opposite page]. HORSES stand in for the extinct horse at the Oostvaardersplassen Many people will claim that the con- reserve in the Netherlands. cept of Pleistocene rewilding is simply not feasible in the world we live in today. to go back to the past; it is about using tion carries risks as well. In the face of I urge these people to look to Africa for the past to inform society about how to tremendous uncertainty, science and so- inspiration. The year after the creation maintain the functional fabric of nature. ciety must weigh the costs and benefi ts of Kruger National Park was announced, The potential scientifi c, conservation of bold, aggressive actions like Pleisto- the site was hardly the celebrated main- and cultural benefi ts of restoring mega- cene rewilding against those of business stay of southern African biodiversity it fauna are clear, as are the costs. Al- as usual, which has risks, uncertainties is today. In 1903 zero elephants, nine though sound science can help mitigate and costs that are often unacknowl- lions, eight buffalo and very few chee- the potential costs, these ideas will edged. We have a tendency to think that tahs lived within its boundaries. Thanks make many uneasy. Yet given the appar- if we maintain the status quo, things

to the vision and dedication of African ent dysfunction of North American eco- will be fi ne. All the available informa- conservationists, 7,300 elephants, 2,300 systems and Earth’s overall state, inac- tion suggests the opposite. lions, 28,000 buffalo and 250 cheetahs roamed Kruger 100 years later—as did MORE TO EXPLORE 700,000 tourists, bringing with them The Ghosts of Evolution: Nonsensical Fruit, Missing Partners, and Other Ecological tens of millions of dollars. Anachronisms. Connie Barlow. Basic Books, 2000. In the coming century, humanity will Twilight of the Mammoths: Ice Age Extinctions and the Rewilding of America. Paul S. Martin. Foto Natura/Minden Pictures Natura/Minden Foto decide, by default or design, the extent University of California Press, 2005. to which it will tolerate other species and Pleistocene Rewilding: An Optimistic Agenda for Twenty-First Century Conservation. C. J. Donlan, J. Berger, C. E. Bock, J. H. Bock, D. A. Burney, J. A. Estes, D. Foreman, P. S. Martin, thus how much biodiversity will endure. G. W. Roemer, F. A. Smith, M. E. Soulé and H. W. Greene in American Naturalist, Vol. 168, No. 5,

STEVEN RUITER RUITER STEVEN Pleistocene rewilding is not about trying pages 660–681; November 2006.

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