On about 3.6 per cent of the planet's surface, geographic barriers prevent wild species from migrating as effectively as the desert borderlands barred the polo-shirted young man. Take, for example, the mosaic-tailed rat, which lived on tiny Bramble Cay, an uninhabited island on the northern edge of the Great Barrier Reef off the coast of Australia. Increasingly violent storm surges steadily wiped out the island's plant life. But like other terrestrial creatures living on remote islands, or at the top of mountains, the mosaic-tailed rat had nowhere to go. The rodents' numbers diminished. By 2002 there were only 10 mosaic-tailed rats left on the island. A fisherman spotted one in 2009, but when scientists returned in 2016 to survey the island, they couldn't find even one. In 2019, with 97 per cent of the vegetation on the island destroyed, officials in Australia declared Melomys rubicola, the Bramble Cay mosaic-tailed rat, officially extinct. It was the first mammal we know of to be wiped out by climate change. Experts agreed it would not be the last.

The more potent barrier to wild species' movement is us. So far, our cities, towns, farms, and sprawling industrial infrastructure have swallowed up over half the planet's land surface. We transformed another 22 per cent of the earth's habitable land in just the last decades, mostly by cutting down forests and turning them into farms, as a recent analysis of satellite images from 1992 to 2015 showed. Our massive footprint makes life impossible for so many wild species that an estimated 150 go extinct every day, speeding up the background rate of extinction by a factor of one thousand. Species that have not lost their habitats entirely must move through a landscape disfigured by human developments. Black bears in the hardwood swamps of Louisiana must cross a highway to reach others in their population. Instead of striking out across the highway to find new mates, they've started to mate with those in their own cut-off group, becoming increasingly inbred. Cougars living in the mountains around Los Angeles must cross two freeways, including one with eight lanes of speeding traffic, to meet others of their kind. None of the cougars that scientists fitted with GPS collars could do it. Four died attempting the crossing, five turned back, and one was shot by police. Birds on the wing smash into industrial structures, each building regularly racking up corpses, like the half-dozen or so birds felled every week by the Thurgood Marshall Federal Judiciary building in Washington, D.C.