Their aggressive approach to conservation featured prominently in numerous scientific articles that followed, discussing the pros and cons of assisted migration. One common theme that has emerged is the need for a framework within which to make decisions about facilitating the migration of plants and animals, not least because the unintended consequences could be irreversible and dire. Any biologist can cite a litany of disasters following the movement of plants and animals from the environments in which they evolved — from the constraints imposed there by competitors, predators and parasites — into new ecosystems. The cane toad, native to Central and South America, was released in Australia in 1935 ostensibly to control agricultural pests; it didn’t, reproduced exponentially and became a pest in its own right. The mongoose, brought to Hawaii in 1883 to control rats, instead further decimated native bird populations. Even moving something just a few miles could cause problems. The red squirrel, transported from mainland Canada onto Newfoundland in 1963 to serve as a food source for the island’s declining martens, ended up eating so many black-spruce cones — and possibly preying on nests — that a native crossbill that also relies on the tree has been in decline ever since.

These cases underscore the reality that other plants and animals are already living where you might want to introduce something in order to save it. Adding another life form could upset the ecological balance on which the native organisms rely. For these and other reasons, Anthony Ricciardi, an ecologist at McGill University in Montreal who studies invasive species and is an outspoken skeptic of assisted migration, compares the approach to playing ecological roulette. It’s impossible to predict the outcome of moving plants and animals around, he says. “I would treat assisted migration as a tool of last resort,” he told me in an email. “And it should be recognized as a technofix, rather than a sustainable conservation strategy.”

Even so, in the roughly 15 years since the scientific debate over assisted migration first emerged among academics, what’s notable today is how, naysayers notwithstanding, it’s already being put into practice. Beginning in 2016, researchers from the University of Western Australia started releasing a captive-bred swamp tortoise into seasonal wetlands about 200 miles south of its natural range; it is thought that this makes it the first animal species ever relocated to protect it from climate change. Many of the animal’s native wetlands were fragmented and shrinking, and conservationists feared that global warming would finish the tortoise off. “It’s going to literally blink out in a few years if we don’t do it,” Nicola Mitchell, an associate professor at the university and the effort’s lead scientist, told me.

For trees, the conversation has shifted from “should we do it?” to “how can we do it best?” The U.S. Forest Service, among other agencies, has experiments underway around the country to study what trees will grow most vigorously in today’s rapidly shifting climate. Some of these trees are southern varieties of species that already grow in an area. But in a few other plots, the Forest Service has planted species, like the relatively drought-tolerant ponderosa pine, that don’t yet inhabit that region of the country. By moving these trees into the area, the agency is essentially testing which, if any, of today’s nonnative trees have the best chance of thriving there in the future.

Critics argue that “nature” should be left to produce its own adaptations. The counterargument is that humanity’s global impact has become so all-encompassing that “nature,” in the sense of untouched wilderness, no longer exists (if it ever did) and that inaction could mean the disappearance of life-forms or the collapse of ecosystems. Jessica Hellmann, executive director of the University of Minnesota’s Institute on the Environment and an early thought leader on what she prefers to call “managed relocation,” likens the practice to chemotherapy. “You don’t say, ‘Oh, is chemotherapy a good idea?’ No, it’s a terrible idea,” she says. “It’s only a good idea if you’re confronted with some other terrible thing,” like cancer. Similarly, she says, assisted migration is appropriate when contrasted with the other possibility: extinction if no one intervenes.