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. 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 …