Brave old world: the debate over rewilding North America with ancient animals.

For the first time in several thousand years, a lion's roar reverberates through the Grand Canyon. California condors descend into that chasm as though sliding down a spiral staircase. Bolson tortoises creep through spiky yucca plants in the Chihuahuan Desert in New Mexico. Nearby, camels and elephants munch woody shrubs. A cheetah, chasing a pronghorn toward a deep ravine, proves that you can in fact come home again.

If one group of conservation biologists has its way, this is how the western United States could look within the next century: filled with megafauna, including carnivores and herbivores imported from Africa, Asia, and other parts of the world. These animals would repopulate the area where they lived until about 13,000 years ago, when the arrival of people in the region caused them to go extinct.

The plan, called Pleistocene rewilding, suggests reintroducing into Arizona, the Great Plains, and elsewhere various species--such as Bactrian camels, peregrine falcons, and Old World cheetahs--that were once native to North America. If all goes well, these species could reestablish ecosystems that thrived in ancient times, before people began affecting the environment.

When first proposed as a brief commentary in the Aug. 18, 2005 Nature, the idea tickled the imaginations of many journalists. It even earned mention in the New York Times Magazine's "Year in Ideas" issue. However, it also aroused the tempers of some conservation biologists. Now, the same authors have published a more comprehensive follow-up, which appears in the November American Naturalist. The new version presents some compelling reasons to take the plan seriously: Pleistocene rewilding could restore lush ecosystems, curb Lyme disease, and provide a bold alternative to failing models of species conservation around the world.

"We might partially restore these lost taxa and the ecological functions that go with them," says coauthor Harry Greene of Cornell University. "One could imagine, 100 years from now, the American Great Plains turned into an ecological reserve."

But another group of researchers counters that vision. In the October Biological Conservation, a team led by Dustin R. Rubenstein, now at the University of California, Berkeley, challenges the tenets of Pleistocene rewilding, calling it only "slightly less sensational" than Michael Crichton's 1990 novel Jurassic Park (Knopf).

Rewilding could disrupt modern ecosystems just as easily as it could restore historic ones, Rubenstein argues. Once brought from Africa and Asia, genetic relatives of former inhabitants might behave differently than the original species did. A better plan would be to preserve these animals, many of which are endangered, in their native habitats, Rubenstein and his colleagues propose.

"You're putting back species that might be genetically different, and in most cases are, into ecosystems where there haven't been these species in 10,000 years, and the ecosystems have evolved without the species," says Rubenstein. "We need to do something to preserve the species on this planet, but [Pleistocene rewilding] is so bold that it requires different perspectives."

BORN TO REWILD At a New Mexico ranch, Pleistocene rewilding has already begun. Bolson tortoises have been moved there from Mapimi, Mexico--their only remaining wild habitat. During the Pleistocene, these tortoises lived in what's now these Great Plains. People who entered the region in the late Pleistocene preyed on the tortoises, and locals in Mapimi still hunt the l00-pound animals.

Soon, the tortoises will be moved to Arizona, where they will inhabit two 8.5-acre enclosures under heavy supervision, says Joe Truett of the Turner Endangered Species Fund, which runs the New Mexico ranch. Scientists will monitor the tortoises' adjustment and protect young tortoises from ravens, raccoons, and other predators. Tortoise shells don't harden until the animals are about 6 years old.

"The ranch where we're bringing the tortoises has vegetation similar to where they live in Mexico, and the climate's not that much different, either," says Truett. "If they survive the first winter, they'll be fine."

The Bolson project emerged from a meeting held at the New Mexico ranch in September 2004. Ecologist Josh Donlan of Cornell, Greene, and 10 other conservation biologists have summarized that meeting and their plan for Pleistocene rewilding in Nature last year and in the upcoming American Naturalist article.

Most conservation and rewilding efforts focus on animals that went extinct after Columbus came to America, but that approach is flawed, argue Donlan and his coauthors in those publications. It's more logical to use the Pleistocene as a benchmark for conservation, Donlan says. That's when people moved into North America across the Bering land bridge that connects Asia and Alaska. Once they arrived, people began altering habitats and exploiting natural resources. These activities eventually eliminated many species of megafauna.

Archaeological evidence showing that people directly caused these extinctions is scarce, says Paul Martin of the University of Arizona in Tucson, who is part of Donlan's team. But fossilized bones and dung, as well as remains from hunters who lived during the late Pleistocene, suggest that the disappearance of megafauna in what's now the southwest United States coincided with the arrival of people, he says.

"It's false logic to insist that if people caused these extinctions, there ought to be abundant kill-sites," says Martin. "People arrive and the extinctions occur. The field evidence is frustrating, but I think it's more than sufficient as far as ecologists are concerned."

The loss of large animals set off a chain reaction of ecological changes, Donlan's team states. It's not possible to know exactly what changes occurred, but dwindling populations of contemporary animals can show how the process might have played out.

Consider, for example, North American gray wolves. As the wolf population diminished in the 20th century, their herbivorous prey such as deer and cattle flourished, reducing the prevalence of aspen trees and other plant species throughout North America.

Reintroducing top predators from the Pleistocene, such as lions and cheetahs, could keep herbivores in check and restore lush forest structures and biodiversity that once existed in the western United States, Donlan's team argues.

This trickle-down effect on vegetation is a reasonable idea, says ecologist Deborah Letourneau of the University of California, Santa Cruz. Top predators reduce the dominance of any particular herbivore, which creates a habitat amenable to plant diversity, she explains.

If proponents of Pleistocene rewilding choose areas where a good amount of plant diversity already exists, reintroducing megafauna could shift vegetation back toward its previous state, she says. "In a desert climate, you won't get a rain forest," says Letourneau. "But it might be surprising what you do get, in terms of diversity and lushness." Moreover, she adds, the opposite technique of introducing only vegetation--a "bottom-up" approach-would likely promote less biodiversity than rewilding fauna would.

Some reintroduced megafauna could have a more direct benefit on human health. For example, forest animals that prey on deer might curb rising rates of Lyme disease. The reintroduced animals might deter deer from entering deep into the forests and picking up the ticks that convey Lyme disease to people.

Rewilding can begin right away, says Donlan, though that doesn't mean that he'll soon drive into the Colorado River Canyon with a truck full of cheetahs and pop the tailgate. The plan should progress in phases and be continually evaluated.

Start with a few Bolson tortoises heavily monitored on a ranch. Perhaps next, larger carnivores can be supervised in fenced-in areas. Down the line, the animals might coexist in an ecological history park, Donlan says, "where there's an economic revenue and science going on."

CRAZY LIKE A RED FOX In the 1850s, the European red fox was deliberately brought to Australia for recreational hunting. Since then, it has spread throughout the country, reduced bird and green turtle populations, and ravaged farmlands. Now, hunting a red fox is no longer recreational: Bounties are often placed on the species--to no avail. The animals remain largely uncontrolled.

Plenty of animal introductions have ended just as poorly, says invasive-species expert Dan Simberloff of the University of Tennessee in Knoxville. Consider reindeer in South Georgia, sea lampreys in the Great Lakes, wild boars in Hawaii, or goats in the Galapagos.

"So many different things can go wrong," Simberloff says. "It's often said that bioinvasions are inherently unpredictable. That's an overstatement, but there are an awful lot of factors to consider."

Rubenstein's group wants to avoid unpredictable outcomes. It's exciting to think that Pleistocene rewilding could reestablish lost ecosystems, says Paul Sherman of Cornell, one of Rubenstein's coauthors, but reintroducing megafauna could just as easily disrupt contemporary ones.

For example, Donlan's team argues that there haven't been any plant extinctions in North America since the Pleistocene, which implies that the tools for a reestablished ecosystem are in place. But even if Pleistocene vegetation remains extant, these plant species could be different chemically from their ancient forms or dispersed in different regions, Sherman says.

"It's not clear that you can argue that nothing's changed," he says. "Nobody really knows what things were like back then."

Moreover, he says, plant life in the western United States differs significantly from African vegetation.

The only way to ensure that an ecosystem or an animal species hasn't evolved dramatically is to introduce fauna that had gone extinct in a region in the past couple hundred years, says Sherman. This traditional type of rewilding, which is a common conservation technique, differs greatly from Pleistocene rewilding, he says.

In the traditional model, reintroducing peregrine falcons, for example, makes sense because the animal just recently became threatened by heightened amounts of the pesticide DDT in the environment, says Sherman. The Bolson tortoise passes muster, too, because it's only making a 600-mile trip to an environment similar to its current habitat.

Animals shipped from Africa and Asia, however, could complicate the situation. "Traditional rewilding has the advantage of working with the exact species that has been lost," says Ross Barnett of the University of Oxford in England, who studies cat evolution. "With Pleistocene rewilding, you are talking about taking analogous species, which have different ecological requirements, into an environment which they have not evolved to cope with."

For example, says Barnett, the closest genetic relative to the American cheetah is actually a puma--not an African cheetah. "If you bring in African cheetahs, aesthetically it seems like the same thing," he says, "but they're fundamentally not the same."

Given the uncertainties surrounding Pleistocene rewilding and the limited money that goes toward species conservation, efforts should be focused on traditional rewilding, Rubenstein says.

"We don't believe in moving species to habitats where they never existed," says Rubenstein. "We take the line that it may be better to try to preserve species facing extinction in their native habitats."

Donlan and Greene insist that they don't want Pleistocene rewilding to become a priority over on-the-ground conservation in Africa or Asia. Rather, rewilding is a chance to redress the mistaken elimination of these species and to improve biodiversity in the process.

"We're already playing God," says Donlan. "There isn't a single meter of this planet that isn't affected directly by humans. We're affecting biodiversity either by default or by design. [People] need to decide what kind of world they want to live in and how much biodiversity they're willing to coexist with."

PLEISTOCENE PARK As unique as Pleistocene rewilding sounds, Donlan's team isn't the only one to propose such a plan. Since the early 1990s, Sergey Zimov of the Northeast Science Station in Russia has discussed his version of rewilding, called Pleistocene Park.

Zimov's project takes place in the northern Siberian region of Yacutia, but his main goal is similar to that of Donlan's group: reverse a series of negative ecological changes by reintroducing Pleistocene megafauna.

At the current rate of climate change, carbon trapped in Yacutian soil will be released into the atmosphere as carbon dioxide, accelerating global warming, Zimov said in the May 5, 2005 Science. The reintroduction of megaherbivores--particularly bison from Canada--could return this terrain to a grassy ecosystem that retains carbon. So far, Zimov has introduced moose, musk oxen, and reindeer, which are being used in experiments.

A favorable environmental change in Yacutia could take place in as few as 5 to 10 years, says ecologist F. Stuart Chapin of the University of Alaska in Fairbanks, who has worked with Zimov on Pleistocene Park. The trick is not only having a high density of animals but also giving them enough room to roam comfortably.

"Human hunting contributed to the disappearance of megafauna and the difference in ecosystems that appear today," says Chapin. "Should we sit back and accept that, or are there other things to consider doing? It is a good time to open this debate and think about things in that broader context."

In the end, the debate over rewilding might elicit sympathy from its challengers, says conservation biologist Martin Schlaepfer of the University of Texas at Austin.

"It will have a positive effect of getting the public, including conservation biologists, to ask themselves what we want our wilderness to look like in the future," Schlaepfer says. "Right or wrong, Pleistocene rewilding has brought that to the forefront, and that's a very positive thing."

Even the scientists who voiced a concern about rewilding when interviewed for this article also gave reasons to try it. Simberloff likes the idea's vision and says that even if the animals did become problematic, they could be easily controlled through hunting.

Barnett says that the African lion could do well in North America, as long as it had enough space.

Rubenstein and Sherman also admit that Pleistocene rewilding is an exciting concept. The aim of their paper wasn't to silence rewilding but to incite scientific discussion, Sherman says.

"It's not the usual scientific debate," he says. "It's about a future direction of conservation biology, so you take it in the spirit of trying to figure out what's the best direction, and you go forward from there."