This concept came to attention in the 1990s, when gray wolves were reintroduced to Yellowstone National Park after having been exterminated seven decades prior. Ecologists showed that the park’s elk would spend so much time watching out for the re-emergent wolves that they spent less time eating and sired fewer young. They died in numbers way beyond what the wolves were actually killing, and their losses rippled throughout Yellowstone. The trees they ate grew taller, providing more wood for beavers and nesting sites for songbirds. The entire park changed, and all thanks to fear of the big, bad wolf.

Or did it? Writing in The New York Times, Arthur Middleton said, “This story—that wolves fixed a broken Yellowstone by killing and frightening elk—is one of ecology’s most famous … But there is a problem with the story: It’s not true.” Follow-up studies suggested that the elk aren’t as afraid of the wolves as previously thought, and that other factors could have led to the elk declines including humans and drought and bears, oh my. And this dispute has fueled a broader controversy about whether it was a good idea to reintroduce the wolves at all, and whether it’s worth “re-wilding” other areas with other large predators that once patrolled them.

Meanwhile, the landscape of fear concept has since moved beyond correlative observations of wolves and elk, and into the world of experiments. In 2011, Zanette showed that song sparrows in the Gulf Islands raise 40 percent fewer chicks if they hear the calls of hawks, owls, and other predators through speakers—even if their nests are surrounded by protective nets and fences. A year later, Dror Hawlena showed that spiders with glued mouthparts can still terrify grasshoppers enough to change their metabolic rates, the chemical composition of their bodies, and the amount of nutrients they return to the soil when they die.

These studies unambiguously showed fear could affect populations and landscapes, but spiders and hawks are a far cry from the wolves, lynx, and bears at the heart of re-wilding debates. Zanette wanted evidence that these large carnivores could trigger the same kinds of effects that she saw among her songbirds. Hence: the raccoons.

Her team, including graduate student Justin Suraci, traveled to the Gulf Islands and lashed speakers to trees facing the raccoon-infested shoreline. For a month, they blasted out the sounds of either barking dogs (which kill raccoons) or seals and sea lions (which do not). For another month, they swapped. And all the while, they kept an eye on the raccoons with cameras, and combed to beach to count other tidal species.

Their results were stark. When the raccoons heard the dogs, they became more vigilant and abandoned the shorelines, spending 66 percent less time foraging in the tidal zones. This had a huge effect on their prey. After the month of barking, the team found 81 percent more fish in the rock pools, 59 percent more worms, and 61 percent more red rock crabs. And that meant falling numbers of staghorn sculpin fish (which the crabs compete with) and periwinkle snails (which they eat). Fear rippled through the entire beach, affecting everything from raccoons to snails.