DRAFT COVER LETTER TO SCIENCE

We are pleased to submit the manuscript Pleistocene Rewilding: An Optimistic Vision for 21st Century

Conservation. In this review article, we justify using extant conspecifics and related species as functional

analogs of recently extinct North American vertebrates to partially restore ecological roles and evolutionary

potential that were lost with the end-Pleistocene megafaunal extinction. Pleistocene rewilding, conceived as

a series of carefully managed ecosystem manipulations, would counter the pest-and-weed biotas promoted

by human impact, facilitate the persistence and evolution of large vertebrates on a global scale, and change

the underlying premise of conservation from managing extinction to actively restoring biological processes.

Our vision is supported by ecological, economic, esthetic, ethical, and evolutionary considerations.

Because of the twelve author’s unusually diverse experiences, expertise, and pre-existing biases regarding

Pleistocene rewilding, we sought pre-publication outside reviews from only four people: S. Dobrott, M.

Phillips, and J. C. Truett of the Turner Endangered Species Program, and C. Vriesendorp, a conservation

biologist at the Field Museum of Natural History. None of the material has been published or under review

elsewhere.

We recognize that our paper will almost certainly be highly provocative, and we are confident that R. K.

Colwell and D. J. Erwin of your board of editors are highly qualified to give our manuscript a fair and

scholarly consideration. Suggestions for external reviewers are below.

David Western Email: dwestern@africaonline.co.ke Email: dwes107394@aol.com Wildlife Conservation Society & African Conservation Centre, Box 62844, Nairobi, Kenya James H. Brown Email: jhbrown@unm.edu, Biology Department University of New Mexico Albuquerque, NM 87131

Terry Chapin Email: terry.chapin@uaf.edu Department of Biology and Wildlife Institute of Arctic Biology, University of Alaska Fairbanks, Alaska, 99775

Paul K. Dayton Email: pdayton@ucsd.edu Scripps Institution of Oceanography, UCSD 9500 Gilman Drive La Jolla CA, 92093-0227 Tim Flannery Email: Flannery.tim@saugov.sa.gov.au South Australia Museum, North Terrace Adelaide, South Australia 5000, Australia Jeremy Jackson Email: jbcj@ucsd.edu Scripps Institution of Oceanography, UCSD 9500 Gilman Drive La Jolla CA, 92093-0244

For the authors, C. Josh Donlan Department of Ecology and Evolutionary Biology, Corson Hall, Cornell University Ithaca, NY 14853 U.S.A. cjd34@cornell.edu Voice: 607.254.4269 Voice: 607.227.9768 Fax: 607.255.8088

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Pleistocene Rewilding: An Optimistic Vision for 21st Century Conservation C. Josh Donlan1,*, , Joel Berger2, Carl E. Bock3, Jane H. Bock3, David A. Burney4, James

A. Estes5, Dave Foreman6, Paul S. Martin7, Gary W. Roemer8, Felisa A. Smith9, Michael

E. Soulé10, and Harry W. Greene1

Running Head: 21st century conservation

Word Count: 4860 1Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY 14853 U.S.A. 2Teton Field Office, North American Program, Wildlife Conservation Society, Moose, WY 83012 U.S.A. 3Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO 80309 U.S.A. 4Department of Biological Sciences, Fordham University, Bronx, NY 10458 U.S.A. and National Tropical Botanical Garden, Kalaheo, HI 96741 U.S.A 5U.S. Geological Survey, University of California, Santa Cruz, CA 95060, U.S.A. 6The Rewilding Institute, P.O. Box 13768, Albuquerque, NM 87192 U.S.A. 7Desert Laboratory, Department of Geosciences, University of Arizona, Tucson, AZ 85721 U.S.A. 8Department of Fishery and Wildlife Sciences, New Mexico State University, Las Cruces, NM 88003 U.S.A. 9Department of Biology, University of New Mexico, Albuquerque, NM 87131l U.S.A. 10P.O. Box 2010, Hotchkiss, CO 81419 U.S.A. *Correspondence: Department of Ecology and Evolutionary Biology Corson Hall, Cornell University Ithaca, NY 14853 U.S.A. cjd34@cornell.edu Voice: 607.254.4269 Voice: 607.227.9768 Fax: 607.255.8088

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We propose to partially restore ecological functionality and evolutionary potential

to North America that were lost with the late Pleistocene megafaunal extinction.

From Bolson tortoises (Gopherus flavomarginatus) and feral equids to Asian

elephants (Elephas maximus) and Holarctic lions (Panthera leo), our vision for this

restoration begins immediately and spans the coming century. We justify using

extant conspecifics and related species as functional analogs of recently extinct

vertebrates on ecological, evolutionary, economic, esthetic, and ethical grounds.

Pleistocene rewilding, conceived as a series of carefully managed ecosystem

manipulations, would counter the pest-and-weed biotas promoted ever more widely

by human impact, facilitate the persistence and evolutionary potential of large

vertebrates on a global scale, and change the underlying premise of conservation

from managing extinction to actively restoring ecological and evolutionary

processes.

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Africa’s large mammals are dying (1-4), stranded on a continent where wars are

being fought over scarce resources (5). With the loss of most large mammals and their

commensals, much of North America died c. 13,000 years ago (6-8). More than any other

species in the history of life, humans cause extinctions, change ecosystems, and affect the

very future of evolution (9-13), and they surely will continue to do so by default or

design (14). Here, we outline an alternative vision for 21st century conservation biology

that is based on our late Pleistocene heritage, one that is bold, optimistic, and ethically

defensible. There has been no single, non-arbitrary conservation benchmark since the end

of that epoch. Thus, we propose Pleistocene rewilding—re-instituting ecologically and

evolutionary processes that were transformed or eliminated by megafaunal extinctions—

as a conservation priority in North America. We first discuss the ecological, evolutionary,

economic, esthetic, and ethical justifications for this proposition, then describe six case

histories to inform the debate we aim to provoke.

Our vision is based on the following observations. First, Earth is nowhere pristine

in the sense of being substantially free from human influence (1-7,9-15). Human

economics, politics, demographics, and chemicals pervade every ecosystem; even our

largest parks require management and suffer extinction (16). These human impacts are

unprecedented in their magnitude, cosmopolitan in their distribution, and show alarming

signs of worsening (9,10,12-14). Second, conservation biology is largely characterized as

doom and gloom; conservationists have largely accepted this losing battle to slow

biodiversity loss, and with few exceptions struggled only to diminish its rate. Third,

future human demographic and land use patterns will be dynamic and uncertain. Many

areas in the U.S., such as parts of the Great Plains, are depopulating (17), and may offer

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future conservation opportunities (18). Fourth, humans probably were responsible to

some significant degree for the late Pleistocene extinctions in North America (6,7) and

our subsequent activities have curtailed survival prospects and evolutionary potential for

most large vertebrates (19). We therefore bear an ethical responsibility, as citizens and

scientists, to redress these problems insofar as possible.

JUSTIFICATIONS FOR PLEISTOCENE REWILDING IN NORTH AMERICA

Our prevalent conservation benchmarks dictate which taxa are regarded as native

and which are not, irrespective of ecological and historical insights (20). In North

America we routinely turn to Columbus and 1492 as a de facto restoration baseline (21),

thereby discounting significant, earlier ecological impacts by humans (22,23). The arrival

of the first Americans, the Clovis culture (c. XX,XXX YBP, 24), constitutes a less

arbitrary benchmark that is justifiable from multiple perspectives.

Prior to the late Pleistocene extinctions, mammal body size distributions were

remarkably similar across all continents, despite little overlap in species composition

(25), and the subsequent extinction of most large mammals in Australia and the Americas

drastically altered those distributions to favor smaller forms (Fig. 1, 26). Given that body

size appears to be highly conserved across taxa (25), these losses are significant with

respect to ecology and vertebrate evolutionary potential, particularly in the Americas

where the losses were greatest (8,27,28)

Ecology and conservation have recently become more process-oriented (30).

Prehistoric, historic, and contemporary evidence lend credence to the premise large

carnivores and herbivores often play important ecological roles in the maintenance of

biodiversity (8,27,29-35). It follows that many now extinct large mammals must have

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shaped the life histories of extant species through the selective forces of strong species

interactions (8,27,35,36). In some cases interaction loss leads to extinction (37,38), others

result in disequilibria (8,39,40). Numerous North American species are now believed to

be anachronistic on modern landscapes due to losses of species interactions c. 13,000

YBP (41). Osage orange (Maclura pomifera) and American pronghorn (Antilocapra

americana) provide two notable examples – extinct proboscidians and other large

herbivores likely dispersed seeds of the former and the American cheetah (Acinonyx

jubatus) likely preyed on the latter (8,36,41). The inferred ecological roles of Pleistocene

megafauna and their modern conspecifics or analogs imply specific hypotheses that

would be tested as prerequisites during the stages of rewilding described below.

While evolutionary perspectives have been raised in conservation planning

(42,43), the bold actions needed to preserve evolutionary potential in the wake of the

drastic decline in biodisparity have not been addressed. Must we accept the end of

speciation of large vertebrates (19), or shall we take responsibility for partially restoring

that potential? Africa and parts of Asia are now the only land-based places where the

Pleistocene megafauna remain intact, and the loss of many of these species within this

century seems likely (1,3,4,13,44,45). Given the demonstrable extinction risks for the

Earth’s remaining megafauna and the possibility that North American sites could serve as

additional refuge and help preserve evolutionary potential, regional rewilding (46,47)

carries global conservation implications.

Humans have an emotional relationship with large predators and herbivores that

extends back into the Pleistocene (48). A public understanding of ecological and

evolutionary history, inspired by tangible esthetic experiences, would strengthen overall

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support for the conservation of biodiversity and wilderness (49). More than 1.5 million

people annually visit San Diego Zoo’s Wild Animal Park to catch a glimpse of large

mammals, exceeding visitation for 80% of U.S National Parks (50). Per capita visitation

to U.S. National Parks has been declining since 1987—the first time since the 1930s (51).

Pleistocene rewilding would bring timely economic incentives to both private and public

lands.

CASE STUDIES: FROM THE TORTOISE TO THE LION

The Bolson tortoise (Gopherus flavomarginatus) was probably widely distributed

across the Chihuahuan Desert until the late Pleistocene (52). Today, it is critically

endangered and restricted to a small area in north-central Mexico. Once part of the North

American megafauna, weighing up to 50 kg, and susceptible to human overkill, the

Bolson tortoise disappeared from more than 90% of its range by the end of the

Pleistocene (53). A number of appropriate sites exist for reintroduction, including Big

Bend National Park and large private ranches in the Southwest. Along with clear

conservation benefits for the species, tortoise reintroductions may increase local

biodiversity by adding heterogeneity to the landscape via burrow construction, as has

been documented for other Gopherus species (54). Repatriating the continent’s largest

surviving temperate terrestrial reptile could precipitate a variety of ecological,

evolutionary, economic, and cultural benefits, with no apparent costs (Table 1, Fig. 2).

Feral equids (Equus caballus, E. asinus) have been abundant in North America

since they were introduced by Europeans 500 years ago (55). From an evolutionary and

ecological perspective, horses are native to North America: they were present there for

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most of the last 50 million years, radiated from that continent, and were diverse on it until

the late Pleistocene (Table 1,56). Feral horses and burros are widely viewed as ecological

pests, but in the context of historical ecology they are plausible analogs for extinct equids

(35). Although the ecological role and impacts of feral horses are variable over temporal

and spatial scales (55,57-59), they disperse large-seeded plants and thus may compensate

for certain large Pleistocene mammals now absent in North America (8). Wild asses (E.

hemionus) and Przewalski’s horse (E. przewalskii) are critically endangered or extinct in

the wild (60), so free-roaming North American populations would help curtail extinction

and repatriate equids to their evolutionary homeland. Whether the overall impact of

rewilding with equids would be positive or negative in local ecological and economic

contexts might depend on temporal and spatial dynamics, and thus perhaps on the

presence of appropriate predators.

The center of camelid evolution was North America, where four species of camels

and llamas were present in the late Pleistocene (61,62). Only four species remain today

worldwide, restricted to the Gobi desert and South America. Wild Bactrian camels

(Camelus bactrianus) are on the verge of extinction, less than 1000 animals remain (63).

Domesticated or captive Bactrian camels could be reintroduced to parts of North

America, further assuring the evolutionary potential of camelids and serving as browser

analogs for the closely related Pleistocene Camelops (64). Camels potentially offer

biodiversity benefits to arid and semi-arid North American ecosystems by browsing on

woody species that now often form homogeneous landscapes (65). In the 1850s, when Lt.

Beale led the Camel Military Corps from Texas to California, his animals browsed on

creosote (Larrea tridentata) and other brush species that today often dominate

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overgrazed desert lands (66). Camels might bring economic benefits as well; in Australia,

well-managed co-grazing programs of cattle and camels produce additional markets for

meat, milk, and fiber without negatively impacting the landscape or cattle production

(67).

The American cheetah was the likely principal agent of selection for the

pronghorn, whose astounding speed evolved in the context of four million years of

predation on North American grasslands (36). The African cheetah (A. jubatus), a close

relative of the American cheetah (68), was once found throughout Africa and

southwestern Asia; its current distribution has been greatly reduced and it has only a

modest chance of persisting in the wild into the next century (44,69). Breeding programs

are not self-sustaining and wild populations have continued to sustain captive ones (68).

Nonetheless, some of the over 1,000 animals in captivity (69,70) could replace the North

American cheetah as ecological and evolutionary analogs. Conservation scenarios for

cheetah are unique in that the majority of the remaining individuals are located outside of

protected areas, commonly on commercial livestock and game farmland (69,71,72).

While farmers often perceive cheetahs as threats and persecute them, environmental

education and alternative pastoral practices have recently proved useful in promoting

their coexistence with humans (71). Cheetah populations in the southwest United States

would restore what must have been strong interactions with pronghorn, help save the

world’s fastest species from extinction, and facilitate economic alternatives to ranchers

through participation in ecotourism.

Five species of proboscidians roamed North America in the Pleistocene (61,62).

Elephants (Loxodonta africana) play keystone roles on the African landscape (33,73), as

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proboscidians likely once did in the Americas (8). African elephants prevent woodland

regeneration and promote grasslands; elevated densities appear to be the primary driver

of woodland loss (75,76). Encroachment of woody and shrub plant species over the past

century now threaten the arid grasslands of western North America (65), and while the

causes are complex and debated (65,74), browsing elephants could counter shrub and tree

(e.g., Juniperus) invasion and increase landscape heterogeneity. Managed elephant

populations in North America could thus enhance biodiversity and economically benefit

ranchers through grassland maintenance and ecotourism. Further, many elephant

populations, particularly in Asia and West Africa, are in grave danger of extinction (45),

and captive populations are not sustainable for either species (77,78). With the

appropriate resources and vision, captive stock and some of the 16,000 domesticated

elephants in Asia (79) could contribute to the wild future for this flagship species by

initiating a North American population. Fencing, which has proven effective in mediating

human-elephant conflict in Africa (80), would be the main economic cost.

Lions, which prey on wild equids and other ungulates, represent the ultimate

vision for Pleistocene rewilding. The American lion (Panthera leo atrox) was conspecific

with surviving African and Asian lions, and they may collectively have once been the

most wide-ranging wild land mammal of all time (62). Lions play a pivotal role in the

Serengeti ecosystem, along with other predators, in regulating prey populations (34), as

they likely once did in the Americas and Eurasia. African lions are increasingly

threatened by habitat destruction and disease, and the Asiatic lion (P. leo persica) is now

critically endangered, with a single population in the Gir Forest of India (4,81,82).

Establishing additional populations is vital for their long-term persistence; recent

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attempts in India have failed (81). Given public attraction to large predators, the potential

esthetic and economic benefits of reintroducing lions to North America are obvious.

BUMPS ALONG THE ROAD TO PLEISTOCENE REWILDING

Potentially serious and legitimate objections to Pleistocene rewilding must be

faced candidly, with all available information and within the above-mentioned ecological,

evolutionary, economic, esthetic, and ethical contexts . With respect to genetics, the

megafaunal proxies we propose are not literally the same animals lost in the late

Pleistocene. The African lion and cheetah are somewhat smaller than their extinct

American counterparts, Camelus is different from Camelops, and so forth. “Same” is a

relative concept, however, as illustrated by the peregrine falcon (Falco peregrinus).

Celebrated as one of the single largest successful conservation efforts (83), the North

American peregrine program relied on large numbers of captive-bred birds from seven

subspecies, obtained from four continents (84,85). Despite morphological and ecological

variation among the founders, there were no differences among subspecies in breeding

success of the reintroduced birds (85), and they now serve as a collective analog for the

midwestern peregrine population that went extinct in the 1960s.

Is a pre-1492 benchmark unrealistic in our current world? Another recent

endangered species program contradicts objections that bringing back megafauna would

waste precious conservation dollars, since at least initially their population would not be

self-sustaining. The California condor (Gymnogyps californianus) was present

throughout North America until the late Pleistocene, disappearing along with the

megafauna upon which it fed, and persisting only in Pacific coastal regions. Condors last

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roamed over the Grand Canyon c. 10,000 years ago, and scavenged on now extinct taxa,

including mammoths (Mammuthus sp), horses, and camels (86,87). Nevertheless, condors

from a captive breeding program now soar over Colorado River canyons and rely on

cattle carcasses as food subsidies. While this program is unsustainable without active

management, few would now argue against efforts to save and diversify condor

populations (88). Should we not also return some of the large herbivores and their

carnivores that collectively once fed these giant scavengers?

Other potentially serious objections to Pleistocene rewilding include the

possibility of catastrophic disease transmission, the fact that habitats have not remained

static since the end of the Pleistocene, and unexpected ecological and social

consequences of species introductions (89,90). These are problems that must be

addressed in advance by sound research and management plans for each species on a

case-by-case, locality-by-locality basis. Well-designed, hypothesis-driven field

experiments will be needed to assess the impacts of potential introductions before

releases take place, especially if removal would prove difficult should unintended

problems arise. Predators offer unique challenges for conservation that must be mitigated

with protective measures and attitudinal adjustments. Just as is the case in African parks

today, we must accept the necessity for precautionary behavior in truly wild areas, as well

as the fact that carnivores kill and consume other creatures.

WHERE TO BEGIN?

We envision several stages to Pleistocene rewilding, with the first already

underway. Equids, camelids, and other ungulates are increasingly prevalent on western

landscapes, and there is widespread discussion of a buffalo commons in the Great Plains

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(18). Translocation of a captive population of Bolson tortoises to a private ranch in New

Mexico is currently under study. Small-scale experiments are urgently needed to assess

the economic, ecological, and cultural implications of more widespread re-introductions

of these and other herbivores. Large-tracts of private and public lands in the southwest

United States (91) are potentially appropriate for Pleistocene rewilding, with the fossil

record and carefully designed research as guideposts and safeguards. Private lands likely

hold the most immediate potential: more than 77,000 Asian and African large mammals

(71 species) now roam free on Texas ranches (92), although their significance for

conservation remains largely unevaluated.

A second stage can also begin immediately, with the experimental maintenance of

small numbers of cheetah, lions, and elephants on managed private property, such that

their ecological impact and sustainability can be carefully studied. The requisite animals

are already present in the U.S. or can be readily produced by captive breeding; the

primary logistical innovation at this point is to provide them with naturalistic selective

regimes, including predator-prey relationships among herbivores and carnivores. As with

other organisms targeted for rewilding, the details of planning and management for each

species should be handled by expert groups from a variety of pertinent disciplines.

The third and more distant stage would include an enormous Ecological History

Park, encompassing thousands of square miles in what are already economically

depressed parts of the Great Plains (18). Secure game fencing would limit the movements

of free-living ungulates, elephants, and large carnivores, including lions. As in Africa

today, surrounding towns would derive their livelihoods from land management and

tourism related jobs. The initiation and precise nature of each of these stages would

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depend on information derived from previous efforts, such that risks would be identified

and negative effects minimized. Two prerequisites of critical importance are rigid

adherence to established protocols, including specification of goal criteria and monitoring

regimes, and incentives for local land owners and other stakeholders.

In the coming century, we will decide, by default or design, on the extent to which

our world incorporates other species. The default scenario will surely include ever more

pest-and-weed dominated landscapes, the global loss of large vertebrates, and a

continuing struggle to slow the loss of biodiversity. Pleistocene rewilding is an optimistic

alternative vision that scales globally to other continents and oceanic islands (93-96). To

those who find the objections to Pleistocene rewilding compelling, we ask, are you

content with the negative slope of our current conservation philosophy? Are you willing

to risk the extinction of the remaining megafauna should economic, political, and climate

change prove catastrophic for Bolson tortoises, cheetah, camelids, lions, elephants and

other species within their current ranges? Are you willing to settle for an American

wilderness that is severely depauperate relative to just 100 centuries ago? The obstacles

to Pleistocene rewilding are indeed substantial and the risks are not trivial, but we can no

longer accept a hands-off approach to wilderness preservation as realistic or defensible. It

is time to not only save wild places, but to rewild and reinvigorate them.

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financially and logistically supporting the workshop on which this paper is based. Tom

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workshop participation, and constructive criticisms of this manuscript.

TABLE 1. The magnitude of biodiversity loss of North America megafauna and potential benefits and costs of Pleistocene rewilding (+ represents an increase in respective category). Late Pleistocene (LP) and current diversity of continental North American large-bodied mammals, along with potential species proxies.

Order or Family LP Current (T/E)1 Proxy2 Ecological

Benefits Ecological

Costs Economic Benefits

Economic Costs

Ease of Establishment Popularity

Predators

Felidae 13 8 (3) Cheetah predation3 ? tourism fencing; livestock

mortality?4 ++ +++

Lion predation ? tourism; hunting human conflict ++ +++

Ursidae 6 3 (2)

Canidae 9 7 (1)

Herbivores

Xenarthra 14 6 (2)

Bovidae 13 5 (2)

Equidae 11 0 Equids seed dispersal;prey5 ? tourism

fencing; compete with

cattle +++ ++

Cervidae 10 5

Antilocapridae 6 1

Proboscidea 5 0 Elephants heterogeneity; seed dispersal6

density & scale dependent effects

tourism; hunting fencing + +++

Camelidae 4 0 Camels heterogeneity; seed dispersal7

potential overbrowsing

meat, fiber production fencing +++ ++

Tapiridae 4 1

Tayassuidae 3 1

Hydrochoeridae 2 0

Castoridae 2 1

Testudinidae ? 0 Bolson tortoise heterogeneity8 none / slight tourism none +++ +

Total 102 38 (10) 1 Extant species in each taxa are significantly biased towards smaller body size (ref 21). T/E = threatened or endangered, listed by U.S. Endangered Species Act or 2001 IUCN category Near Threatened (or equivalent 1994 category LR-cd or LR-nt).

2 Potential Proxies – Camels: Camelus dromedaries, C. ferus, Lama guanicoe, Vicugna vicugna; Equids: Equus caballus, E. przewalksi, E. hemionus; Cheetah: Acinonyx jubatus; Lion: Panthera leo; Elephants: Elephas maximus, Loxodonta africana; Bolson Tortoise: Gopherus flavomarginatus 3 Predation on mule deer (Odocoileus hemionus) and elk (Cervus elaphus) would be limited latitudinally by weather; 4 Work in Namibia has demonstrated coexistence with ranchers and cheetah through education and alternative pastoral practices (ref. 70); 5 ref. 8,41,55 6 ref. 8,41,75,76; 7 ref 41,66; 8 ref 54

FIGURE CAPTIONS FIGURE 1. (A) Body size distributions (log body mass) of terrestrial North American

mammals (including bats) before (red) and after (grey) late Pleistocene (LP) extinctions.

(B) Body size distributions (kg) of four large-bodied taxonomic groups before and after

LP extinctions. Extant distribution of Perrisodactyla includes feral horses and burros.

FIGURE 2. A qualitative model for the biological, economic, and esthetic components of

Pleistocene rewilding. Iconic symbols are for condor, horses, Bolson tortoises, camelids,

cheetah, Asian and African elephants, and lions (see Table 1 for Latin names). (a) Area

requirements and likely timescale of reintroducing functional analogs of extinct

megafauna to North America (b) Conservation value and ecological role (interactivity) on

the landscape (c) Potential economic-cultural value versus conflict.

FIGURE 1.

FIGURE 2.