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14 Harvard Science Review • fall 2008

By Alexandra Mushegian

Ground sloths occupy a small but beloved role in the collective

imagination. These five-ton, seventeen-foot hamster-like herbivores, which went extinct somewhere around 10,000 years ago, are commonly found as skeletons posing in natural history mu-seums. Populations of these animals were thriving in North America when humans first arrived over the Bering Land Bridge along with mammoths, lions, saber-toothed cats, twenty-pound beavers, and several species of giant tortoise, not to mention a multitude of deer and ox-like species. Shortly after humans arrived, they all died out.

The Pleistocene epoch, which ended 10,000 years ago (in archaeological terms, the end of the Paleolithic), was a golden age of animal exoticism world-wide. Why it ended—why this geologi-cal period saw a series of massive global collapses of large animal biodiversity—is a contentious topic. Many argue that the primary reason was climate change in the form of global deglaciation. Increasingly, though, studies are sug-gesting that it was human activity, in the form of hunting, habitat alteration, and the introduction of new diseases, that

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time was the major cause of these extinctions. Without exception, the timing of major biodiversity reductions on all continents and large islands coincides with the ar-rival of early humans, with many species dying out within a few hundred years after humans arrived (1).

The only continents still possessing noteworthy megafaunal diversity—Africa, and to a lesser extent, Asia—are the continents where humans coevolved with animals for the longest time. Now, due to political and socioeco-nomic instability in those regions, they are in danger there too.

Some scientists believe that the loss of large ver-tebrate biodiversity has graver consequences than merely a reduction in worldwide biodiversity. Ecological studies suggest that large herbivores and predators are crucial to maintaining healthy, robust ecosystems. Some see the loss of these species as one that will have perilous fu-ture consequences. In response, they’ve come up with a controversial plan: they want to bring them back to the USA.

REWILDINGNORTH

AMERICA

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Focus: Brave New World

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Pleistocene RewildingIn 2005, a group of scientists pub-

lished a two-page proposal in Nature. This short paper had enormous scope and ambition. In response to what they saw as the takeover of North America by “dandelions and rats,” Josh Donlan and his colleagues proposed that a vi-able solution might be to use African and Asian species as analogues of extinct Pleistocene North American species, introducing them to areas of North America set aside to become the wilderness again. If done right, they claimed, this method would restore North American ecological health and save species from extinction (2).

Ecological restoration projects are undertaken all the time. Peregrine fal-cons, for example, were on the brink of extinction from DDT contamina-tion before a program bred together several subspecies from both the New and Old Worlds and reintroduced them into the wild. Peregrines have proven to be adaptable and appear to be thriving (3). Many conservation agencies strive to restore endangered habitats, such as the National Oceanic and Atmospheric Administration’s Restoration Center, a

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Mammoths are one of the groups of animals that lived on the North American landmass before becoming extinct.

group dedicated to rescuing fish habi-tats such as mangroves and salt marshes. But the radicalism of the Pleistocene rewilding concept lies in the scale of the project in both time and space. Proponents of this plan want to restore ecosystems that existed 13,000 years ago, and they want to revert to wilder-ness areas of thousands of square miles. Stipulating carefully planned, progres-sive ecological studies and small-scaled experiments, Pleistocene rewilding would see the introduction of giant tortoises, horses, camelids (llamas), and most unexpectedly, lions, cheetahs, and elephants to the American Southwest and Great Plains regions. All of these species had close relatives living in North America during the Pleistocene (the Asian elephant, for example, is more closely related to the woolly mam-moth than it is to the African elephant) (4). These species would be set loose to pursue their normal predator-prey interactions to reshape the American environment.

This plan would represent a shift in conservation practice from focusing on cataloging and slowing biodiversity loss to actively managing and promoting

“Stipulating carefully planned, progressive

ecological studies and small-scaled

experiments, Pleistocene rewilding

would see the introduction of giant

tortoises, horses, camelids (llamas), and

most unexpectedly, lions, cheetahs, and

elephants to the American Southwest and Great Plains regions.”

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Focus: Brave New World

16 Harvard Science Review • fall 2008

productive, diverse ecosystems.

The PlanPleistocene rewilding would start

with small-scale ecological studies of the way exotic species respond to being repatriated to North American envi-ronments. These studies would start with less dramatic species—tortoises, llamas—and would progress to more challenging species like big cats. Many species, such as cheetahs, already exist in captivity in the US; they would be placed on large private properties with potential prey and the predator-prey interactions and dynamics over time would be carefully studied by experts on the species (5). In addition, informa-tion about the flora of the Pleistocene could be reconstructed from pollen in the fossil record.

Once there is a significant body of

research, the next step would be to find huge tracts of land to be rewilded. The authors of the paper suggest economi-cally depressed regions of the Great Plains as good candidates for such an endeavor. The area would be securely fenced and given the status of a national park or nature preserve. Hopefully, it would become a tourist attraction that would bring economic benefits to the surrounding region.

Similar plans are already being at-tempted in at least two spots in the world: Oostvaardersplassen, a nature reserve in the Netherlands focusing on restoration of large ungulates, and Len-skiye Stolby, Siberia, commonly known as “Pleistocene Park.” (6).

The Benefits of Rewilding

Why exactly are large vertebrates so important? Various studies show that

predators, as well as some of the more impressive herbi-vores, play a disproportion-ately large role in maintain-ing “ecological function.” “Ecological function” means, roughly, an ecosystem’s abil-ity to rebound from shocks, adapt to changing condi-tions, and evolve to greater and greater fitness—in other words, general vitality. Preda-tors help control herbivore populations, keeping them from over-expanding, de-stroying all vegetation, and subsequently starving. The importance of this “top-down” control effect has been confirmed in many situ-ations. Plant biomass declined by 40% in parts of the Arctic tundra from which predators

were excluded (7). Closer to home, many areas of the United States are facing problems because out-of-control deer populations are decimating forests and living perpetually half-starved and disease-ridden (and also carrying Lyme disease, a debilitating illness that is spread from deer to humans by ticks)(8). Deers’ previous predators, wolves and mountain lions, are largely absent in the wild because they were hunted as dangers by humans, and because their forest habitats are too fragmented to allow them to have the large areas they need to thrive. A recent review of multiple studies from Venezuelan ter-restrial and various marine ecosystems concludes that “we live in a largely top-down regulated world” (9).

Large herbivores, too, contribute to ecosystem diversity, usually by virtue

The diagram to the left maps the correlation between human arrival and extinction of indig-enous species, as well as the strength of that evidence.

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ctober 2004

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References1. Burney DA and Flannery TF, “Fifty millenia of catastrophic exinction after human contact” (2005). Trends in Ecology and Evolution 20:7.2. Donlan CJ et al, “Re-wilding North America” (2005). Nature 436:18.3. Tordoff HB and Redig PT, “Role of genetic back-ground in the success of reintroduced peregrine falcons” (2001). Conservation Biology 15:2.4. Donlan CJ et al, “Pleistocene Rewilding: An opti-mistic agenda for twenty-first century conservation” (2006). The American Naturalist 168:5.5. Greene, Harry. Personal correspondence, October 2008.6. Curry, Andrew, “Pleistocene Park: Where the au-roxen roam” (2008). Wired magazine, 22September 2008, accessed online October 2008 at http://www.wired.com/science/planetearth/magazine/16-10/mf_bison?currentPage=1.7. Aunapuu, M. “Spatial patterns and dynamic responses of arctic food webs corroborate the ex-ploitation ecosystems hypothesis (EEH)” (2008). The American Naturalist 171:2.8. Fairfax County, VA. “Deer Management Activities - Fairfax County, Virginia.” http://www.fairfaxcounty.gov/comm/deer/deermgt.htm#info. Accessed Oc-tober 2008.9. Banse K. “Do we live in a largely top-down regulated world?” (2007). Journal of Biosciences 32:791-796.10. Donlan et al 2006 Zimov SA, “Pleistocene Park: Return of the mammoth’s ecosystem” (2005). Sci-ence 308:5723.11. Rubenstein DR at al, “Pleistocene park: does re-wilding North America represent sound conservation for the 21st century?” (2006). Biological Conservation 132:232-238.12. Ibid.

—Alexandra A. Mushegian ’10 is an Or-ganismic and Evolutionary Biology concentra-tor in Currier House.

of their very size and gawkiness—for example, elephants trample vegetation and tortoises dig long burrows. These sorts of activities create heterogeneity in the environment, opening up a vari-ety of ecological niches than can then be filled by different plant and animal species. The more diverse and varied a landscape is, the greater its robustness and adaptability. By feeding on shrubs and saplings, elephants maintain grass-land ecosystems; camelids feed on the type of weedy, scrubby species that oth-erwise threaten to take over landscapes. In the Siberian “Pleistocene Park,” current studies are exploring the effects of tundra wildlife on global warming. Potentially, the activity of large mam-mals may disturb snow cover enough to keep the soil continually exposed to the cold, preventing permafrost from melting and reducing the release of additional CO2 from the soil (10).

North America today, with the disap-pearance of bison and the domination of industrial-scale agricultural mon-ocultures, is “impoverished,” according to Harry Greene, a professor at Cor-nell and one of the co-authors of the original proposal (5). Ecologically, he says, “we can likely do better” (5). The possibility of partially restoring the lost ecological and evolutionary potential of North American landscapes, he said in an email interview, was in his opinion the single best argument for Pleistocene rewilding (5).

The Objections to RewildingA natural initial objection to the

Pleistocene rewilding proposal is pure incredulity—elephants? Here? Once one has heard the justification for the proposal, concerns still remain about viability and potential consequences.

One objection is that too much time has passed since the Pleistocene for the concept of ecologically analogous species to be valid. Both the species and the environment have undoubtedly changed significantly since those times and thus the introduction of exotic species to a different continent could

have ecologically harmful effects due to unforeseen interactions with the cur-rent natives. Furthermore, introduced species could carry latent pathogens that might thrive in a new setting, bring-ing diseases that might threaten both nature and agriculture. Even if there are not dramatically deleterious effects, the desired outcome of increasing ecologi-cal function might simply not happe, if assumptions about species interactions and ecological roles turned out to be wrong (12). Extensive research is cer-tainly necessary to evaluate the threat of these sorts of problems.

Obviously, there are economic con-siderations as well. The research leading up to and during Pleistocene rewilding efforts must be funded, as well as the allocation of land, the installation of fences around Pleistocene wilderness areas, and the hiring of managers to monitor the borders. The occasional es-cape would be inevitable and elephants or lions on the loose would create a terribly dangerous situation. .

“Cost might be a big deal, possibly insurmountable,” Greene said. “But in terms of if money were not an issue, I think what really bugs people, including fellow biologists, is not ‘exotics’ and so forth, but rather [the idea of] bringing back something that can kill us” (5). The existence of Pleistocene wilderness reserves would force people to adjust their attitudes about the other animals that share their landscape and return a concern that North Americans have largely not had to face for a long time.

Indirect economic consequences and ethical problems will arise when the question of where to get the animals from is decided—currently, trade in exotic species is largely prohibited and negotiating with countries to acquire their animals is likely to be complicated. Along similar lines, there are fears that North American Pleistocene rewild-ing will divert attention and resources from conservation within Africa and Asia (11).

Questions to PonderThe debate about Pleistocene rewild-

ing raises deep and important questions about humans’ role in the biosphere. Are we responsible for atoning for the destructiveness of our thirteen thousand year old ancestors? Are we a species like any other, our capacity for destruction just part of our evolution-ary advantage? Or is it now our role to serve as large-scale gardeners, making sure the world thrives? What does it mean when we influence the course of evolution? As the idea of Pleistocene rewilding gains attention, we will have to take a look at just how well we know the species with which we share the planet.

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