In 1995, wolves were reintroduced to Yellowstone National Park in the US. Since the wolves’ eradication by hunting 70 years earlier, the elk population had grazed uninterrupted across the land, stripping away underbrush and young trees, and damaging willow stands, which are critical to local beaver populations.

Scientists watched as the wolves set off a ‘trophic cascade’. The presence of predators kept the elk on the move, which allowed willow stands to recover. In turn, that allowed beavers to