Moving big animals to places they don't already live is at once appealing and disturbing, a sort of adolescent environmental fantasy come to life: African lions in Nebraska! Komodo dragons in Australia! But at the beginning of the 21st century, with 7 billion humans competing for space and resources on a rapidly warming planet, exercising arguable control over the fate of nature, moving species around is a legitimate option. It's called assisted migration. Often the goal is to save endangered plants and animals, though not always. Sometimes, as with the Komodo dragon proposal, the goal is to restore ecological balance, and other proposals are motivated by an almost romantic sense of possibility: Wouldn't it be marvelous to watch cheetahs dash across the grasslands of South Dakota? As an idea, assisted migration has been around for decades, but since the millennium's turn it's moved from a mostly fringe concept to something that scientists discuss, if not argue. After all, many examples of unwittingly assisted migration show what can happen when relocation goes wrong: Cane toads swarming across Australia, brown tree snakes devouring Guam's birds, kudzu swallowing much of the southeastern United States, and of course the voracious Burmese pythons of Florida. On the flip side, however, are pheasants and sweet clover, brown trout and Norway maple, which despite their non-native origins are now considered a natural part of North American life. Sometimes relocation works fine, and an argument can be made that consciously acting as landscape-scale zookeepers and gardeners is a legitimate response to impending catastrophe. Above: Komodo Dragons to Australia? In a Feb. 1 Nature paper, biologist David Bowman of Australia's University of Tasmania raises the hypothetical possibility of introducing elephants and Komodo dragons to Australia. At first it sounds mad, but what's happening now in Australia is a form of madness, too. Massive wildfires that have become a regular and lethal fact of Australian life don't only represent climate change or natural susceptibility, but the buildup of vegetation that until 50,000 years ago would have been eaten by Australia's now-extinct megafauna. Elephants could fill that role again, writes Bowman. "The idea of introducing elephants may seem absurd, but the only other methods likely to control gamba grass involve using chemicals or physically clearing the land, which would destroy the habitat," he writes. "Using mega-herbivores may ultimately be more practical and cost-effective." Komodo dragons wouldn't do much for fires, but they would eat feral pigs and buffalo, the targets of ongoing and largely unsuccessful animal control efforts. Image: Komodo dragon (Adhi Rachdian/Flickr)

Rewilding the Great Plains Only an eyeblink of evolutionary time separates the contemporary Great Plains from one resembling the savannas of Africa. It's not a perfect resemblance, but close enough that ecologists have considered "rewilding" the Great Plains with African mammals for decades. Whether they're visionaries or crackpots is a matter of opinion; for now, most people would probably pick the latter, though the idea's survival suggests a certain persistent appeal. And for one animal, at least, it's already reality: Herds of wild horses, their disposition a matter of ongoing controversy, last roamed the west some 13,000 years ago. Bringing big mammals to North America could provide a safe haven for the species, and might also bring the environment closer to what it was before the native megafauna went extinct. Image: Wild horses outside Carson City, Nevada. (Rick Cooper/Flickr)

Bring Back the Cheetah Given the resistance of ranchers to reintroduced wolves, it's hard to imagine lions ever roaming the Great Plains -- but what about cheetahs? Considered by Tim Flannery in his 2002 book The Eternal Frontier: An Ecological History of North America and Its Peoples, cheetah pose no threat to cattle, and are less dangerous to people than mountain lions. They could also be good for pronghorn antelope, the world's fastest hoof mammal, said science writer and environmental activist Connie Barlow. Pronghorns likely evolved to outrun a now-extinct species of North American cheetah, and "will surely slow down in a few more thousands of years if we do not return the cheetah," said Barlow. Image: Keven Law/Flickr

Saving Torreya taxifolia An evergreen conifer that moved south during the last Ice Age and was stranded when the glaciers retreated, Torreya taxifolia was until recently found only along one short stretch of Florida's Apalachicola river. In the mid-20th century they experienced a severe die back. Just 500 are left in that original habitat, and like American chestnut trees they no longer live long enough to reproduce. Between the disease and regional climate change, T. taxifolia's continued wild existence seemed unlikely. Then, early this millennium, a group of activists called the Torreya Guardians transplanted healthy seedlings cloned from botanical garden survivors into a few southern Appalachian locations. So far the transplants are healthy, and the Guardians intend for the trees to not only for the lineage to continue, but for T. taxifolia to again have an ecological role. The trees could have survived in gardens, in what are called "potted orchards" -- but "potted is the equivalent of caged," wrote Torreya Guardians founder Connie Barlow in 2004. Image: United States Forest Service

Antarctic Polar Bears? At first, moving polar bears to Antarctica sounds reasonable. They're endangered. They like cold and ice. Even after global warming, Antarctica will have plenty of both. But one thing Antarctica doesn't have is large terrestrial predators, and animals there would be utterly unadapted. Transplanted polar bears would at first feast on penguins, but their defenseless populations would likely soon plummet, taking the bears down with them. Image: Alastair Rae/Flickr

Whitebark Pine Found just below the treeline in the mountains of western North America, whitebark pines have been decimated by beetles and fungal disease. Add temperatures rising faster than whitebarks can expand northward, and the situation is dire. In 2011 the U.S. Fish and Wildlife Service declared that whitebark pine would probably go extinct without help. Help may be on the way, though. A large part of northwestern British Columbia should stay cool through the 21st century, and a few pines are disease resistant. Preliminary tests suggest that transplanted seedlings will do just fine. Image: U.S. Forest Service

Richard Branson's Lemurs In 2007, billionaire entrepreneur Richard Branson bought the Caribbean island of Moskito for $19 million, pledging to turn it into a luxury eco-resort. Then, in classic Branson form, he announced a plan to populate the island with endangered lemurs relocated from Madagascar, where they're threatened with extinction. The ensuing controversy perfectly captured the tensions of assisted migration: Moving the lemurs might save them, but it could also wreck the island. "It could be a brilliant or terrible idea but we just don't know yet," said Penelope Bodry-Sanders, founder of the Lemur Conservation Foundation, to the Guardian. Image: Tambako the Jaguar/Flickr