When wolves were reintroduced to Yellowstone National Park in 1995, the effects were dramatic. Among other things, elk numbers fell far more than expected.

It turns out that the mere fear of wolves was having a big impact. In places where elk thought wolves might be present, they spent much more time looking out for them, leaving less time to feed. In a paper published in 2001, biologist John Laundré, who died in 2021, used the term “landscape of fear” to describe this effect.

The idea wasn’t entirely new. Previous lab experiments had shown that fear of predators alone can affect prey. Yet the prevailing view at the time was that predators affect wild prey populations only through direct predation. Laundré and others’ observations suggested this was wrong, but they didn’t demonstrate causality.

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That’s what Liana Zanette at Western University in Ontario, Canada, has done through a series of experiments over the past two decades. In British Columbia, Zanette and her colleagues played recordings of predators near wild song sparrows. Fewer eggs were laid, fewer hatched and fewer hatchlings survived. Overall, less than half as many lived compared with those played non-predator sounds. In other words, fear can have an even bigger impact than direct predation.

It is all about food, says Zanette. In addition to spending more time looking out for predators, prey animals will just completely avoid some areas, she says, “even though it might be the best food in town”.

This landscape-of-fear concept is hugely important because of knock-on effects on ecosystems. In many places on the west coast of Canada, for instance, humans have eliminated the bears, cougars and wolves that prey on raccoons. These raccoons now spend a lot of time on the shore looking for food such as crabs.

When Zanette’s team visited and played recordings of dogs barking, the raccoons mostly avoided going to the seashore and, when they did, spent much more time looking for predators. This led to a dramatic rebound in the abundance of the shore animals the raccoons feed on. Where the team played recordings of seals barking, by contrast, these effects weren’t seen.

Landscape of fear is key to fully understanding humans’ impact on wildlife. In one study, Zanette and her team used camera traps to film how wildlife responded to sounds in the Kruger National Park in South Africa. “Fear of lions should be maximal there,” she says, “but we found that humans were two times more frightening.”