Lion devours zebra, so when there are a lot of lions the population of zebra declines. And if the population of lion declines, there would be fewer predators to eat zebras so their population increases--or so we thought.
For the first paper, the researchers took dragonfly larvae and put them in three separate cages suspended in three separate tanks: a tank with fish present, a tank with invertebrate predators present, and another without any predators. The predators could swim freely in the tank but could not enter the cage containing the dragonfly larvae.
For the second paper, authored by Zanette et al., the researchers used birds instead of dragonfly larvae. They fenced up the birds and set up some loudspeakers. One group got innocuous native animal sounds: geese honking, loons, seals barking, the other group got predators associated sounds (raccoons, hawks, cowbirds, etc). Then they waited for two months and checked on how the birds did.
The results are pretty striking. The birds with predator sounds laid 40% fewer eggs. But the eggs didn't hatch as often, and the babies didn't survive to adulthood as often either. Survival rates in general were lower. --scientopia.org
There's no doubt mommies were stressed.
In both experiments, the larvae and the birds weren't actually being predated, but the perception of the predators was definitely enough to decrease the population on its own. Seeing and hearing your enemy around you give you a higher sense of predation risk and fear, and that in turn increase stress level. So fear, it seems, can kill after all.
The Deadly Effect of "Nonlethal" Predators. Shannon J. McCauley et. al, Ecology, 92(11), 2011, pp. 2043–2048