Predators directly affect their prey by eating them up. But they can also influence their prey’s population dynamics in another important way, a new study reveals.
While experimenting with song sparrows, a team of researchers found that merely the perceived risk of predation—even if no actual predator was nearby—could decrease the number of offspring produced by the sparrows. And this perceived risk of predation also reduced the survival rates of the fledgling sparrows that were hatched, they say.
Liana Zanette and researchers from the University of Western Ontario, along with Michael Clinchy from the University of Victoria, first protected free-living populations of song sparrows from natural predators by surrounding their nests with netting and electric fences. The researchers then played various sounds for different groups of the sparrows.
Some of the nesting sparrows were treated to benign sounds, while others heard the sounds of predators approaching. The results of this experiment are published in the 9 December issue of the journal Science.
Liana Zanette of the University of Western Ontario describes her study on songbird hatching and the fear of predation. | Video courtesy of Liana Zanette; © Science/AAAS
“Our results show, for the first time in any wild bird, that the fear of predators is itself powerful enough to affect wildlife population dynamics,” Zanette said during a 7 December teleconference organized by AAAS. “What we show is that the fear of falling victim to a predator can traumatize the escapees so much that they have to have fewer babies, and this effect can be as important as direct killing when it comes to reducing prey numbers.”
The researchers found that birds exposed to the predator sounds chose more secluded nesting sites and made fewer foraging trips than other birds, which put their offspring at a disadvantage. In fact, according to Zanette and her colleagues, the sound of predators alone seemed to reduce the number of offspring fledged by 40%.
“The number of eggs that can be produced is largely driven by how much food is available,” said Zanette. “So, if you have a lot of food that you can shovel into your mouth, you’re going to be able to crank out a lot of eggs… What our experiment indicates is that these mothers were scanning their environment for predators, not foraging as much, and the reduction in the amount of food they were taking in is what led them to a reduction in the number of eggs they laid.”
Researchers report that the calls and sounds of predators frightened mother song sparrows so much that they laid fewer eggs. (Eggs were numbered in the order in which they were laid.) | Image courtesy of Liana Zanette; © Science/AAAS
These results mean that predators are not simply important as consumers, but that their mere presence in an environment can have dramatic effects on other species. And according to the researchers, such impacts can significantly influence trophic cascades and ecosystem stability.
“Fear effects are at the center of the debate concerning the reintroduction of wolves to Yellowstone National Park,” said Clinchy during the teleconference. “Critics of the wolf reintroduction have been able to point to two things to suggest that the wolves are not responsible for the reduction in elk numbers and thus, the restoration of the ecosystem: Wolves don’t directly kill enough elk to account for the reduction in numbers, and most of the reduction is due to a decrease in pregnancy rate.
“But our results show that the fear of predators can affect the number of offspring produced by wildlife… The disturbance of native ecosystems due to the loss of native predators has probably been greater than we previously thought, and the adverse effects of introduced predators are likely worse than we previously imagined.”
Read the abstract, “Perceived Predation Risk Reduces the Number of Offspring Songbirds Produce per Year,” by Liana Zanette et al.
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Listen to a 7 December teleconference with the Science authors.