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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: [email protected]
Email: [email protected]
Wildlife Conservation Society &
African Conservation Centre,
Box 62844, Nairobi, Kenya
James H. Brown
Email: [email protected],
Biology Department
University of New Mexico
Albuquerque, NM 87131
Terry Chapin
Email: [email protected]
Department of Biology and Wildlife
Institute of Arctic Biology, University of Alaska
Fairbanks, Alaska, 99775
Paul K. Dayton
Email: [email protected]
Scripps Institution of Oceanography, UCSD
9500 Gilman Drive
La Jolla CA, 92093-0227
Tim Flannery
Email: [email protected]
South Australia Museum, North Terrace
Adelaide, South Australia 5000, Australia
Jeremy Jackson
Email: [email protected]
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.
[email protected]
Voice: 607.254.4269
Voice: 607.227.9768
Fax: 607.255.8088
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
1
Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY 14853
U.S.A.
2
Teton Field Office, North American Program, Wildlife Conservation Society, Moose,
WY 83012 U.S.A.
3
Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO
80309 U.S.A.
4
Department of Biological Sciences, Fordham University, Bronx, NY 10458 U.S.A. and
National Tropical Botanical Garden, Kalaheo, HI 96741 U.S.A
5
U.S. Geological Survey, University of California, Santa Cruz, CA 95060, U.S.A.
6
The Rewilding Institute, P.O. Box 13768, Albuquerque, NM 87192 U.S.A.
7
Desert Laboratory, Department of Geosciences, University of Arizona, Tucson, AZ
85721 U.S.A.
8
Department of Fishery and Wildlife Sciences, New Mexico State University, Las
Cruces, NM 88003 U.S.A.
9
Department of Biology, University of New Mexico, Albuquerque, NM 87131l U.S.A.
10
P.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.
[email protected]
Voice: 607.254.4269
Voice: 607.227.9768
Fax: 607.255.8088
1
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.
2
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
3
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
4
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
6
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
7
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
9
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
10
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
11
(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
12
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.
13
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We thank Environmental Leadership Foundation, Lichen Foundation, Turner
Endangered Species Fund, New Mexico State University, and Ladder Ranch for
financially and logistically supporting the workshop on which this paper is based. Tom
Gorton and Transmutations/S.T.F. assisted with the figures. We are especially grateful to
S. Dobrott, M. K. Phillips, and J. C. Truett for their hospitality at the Ladder Ranch,
workshop participation, and constructive criticisms of this manuscript.
21
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 largebodied mammals, along with potential species proxies.
Order or Family
LP
Current
1
(T/E)
Proxy
13
8 (3)
Cheetah
predation
Ecological
Costs
Economic
Benefits
Economic
Costs
3
?
tourism
fencing;
livestock
4
mortality?
++
+++
Lion
predation
?
tourism;
hunting
human conflict
++
+++
Equids
seed
5
dispersal;prey
?
tourism
fencing;
compete with
cattle
+++
++
fencing
+
+++
fencing
+++
++
none
+++
+
2
Ecological
Benefits
Ease of
Popularity
Establishment
Predators
Felidae
Ursidae
6
3 (2)
Canidae
9
7 (1)
Xenarthra
14
6 (2)
Bovidae
13
5 (2)
Equidae
11
0
Cervidae
10
5
Antilocapridae
6
1
Proboscidea
5
0
Elephants
Camelidae
4
0
Camels
Tapiridae
4
1
Tayassuidae
3
1
Hydrochoeridae
2
0
Castoridae
2
1
Testudinidae
?
0
102
38 (10)
Herbivores
Total
1
Bolson
tortoise
heterogeneity;
density & scale
6
seed dispersal dependent effects
heterogeneity;
potential
7
seed dispersal
overbrowsing
heterogeneity
8
none / slight
tourism;
hunting
meat, fiber
production
tourism
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.