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…