Accounts of destruction wrought by invasive species are legion, from wild hogs in the southern United States and zebra mussels in the Great Lakes to cane toads in Australia and mongeese in Hawaii. An endangered species that now seems sympathetic could quickly become a villain.
But assisted-colonization proponents believe their animals, unlike other invasive species, would be carefully selected and their effects anticipated.
"You work out what the risks are before you take action," said Possingham. "You go through these decision trees, and start by doing some trials under very controlled circumstances, then we'll learn about it."
Things could still go wrong, said Hellmann, but the consequences pale in comparison to those of climate change and inaction. And for animals whose natural habitat has been eradicated, or who live -- as did the golden toad of Costa Rica's cloud forest -- in rapidly changing places from which they cannot escape, there may be no other option.
"If all other conservation methods fail, and evidence shows that a species is in danger of extinction, then assisted migration becomes an option that we should consider seriously," said Nature Conservancy ecologist Patrick Gonzalez.
McLachlan, however, has other reasons for opposition. Assisted colonization could be seen as a quick-fix panacea, distracting people from the necessary task of preserving habitat and braking climate change. More philosophically, there's something troubling about treating nature as a zoological theme park.
"We're destroying any semblance of the idea that a place has its own biota and history," he said. "It's not just saving a couple whooping cranes, it's redesigning the entire biota of Earth. And that's incredibly creepy to me."
Hellmann agrees that assisted colonization could be mistaken as a convenient solution. But the purity of nature, she said, is now a myth.
"You can find signatures of humanity in the deepest jungles and remote locations. This idea of pristine nature doesn't really apply," she said. "If assisted colonization will have benefits, it seems strange not to cross some arbitrary line."