Predators’ threats alone cut into animal populations: study

Even when predators can’t reach their prey, the fear of a potential attack is enough to significantly affect wildlife populations, a new study finds.

Researchers at the University of Victoria say their study’s results confirm years of speculation, finding that wild animals, believing themselves to be at risk of being torn apart by predator species, experience a condition "similar to post-traumatic stress in humans" and it affects the animals’ reproductive abilities.

The study looked at song sparrows nesting in British Columbia’s southern Gulf Islands, fenced off from predators but made to listen to predator noises.

"Our results suggest that the perception of predation risk is itself powerful enough to affect wildlife population dynamics and should be given greater consideration in vertebrate conservation and management," UVic biology professor Michael Clinchy said in a release.

"This has important implications for conservation and wildlife management because it suggests that the total impact of predators on prey populations will be underestimated if the effect of fear itself is not considered," he said.

"This means that the adverse effects of introduced predators are likely worse than previously imagined and the disturbance to native ecosystems due to the loss of native predators has probably been greater than we previously thought."

"Sound of fear"

Clinchy and lead study author Liana Zanette of the University of Western Ontario protected every song sparrow nest in the study with both electric fencing and fish netting, preventing attacks from natural predators such as hawks and raccoons.

Cameras were also trained on the nests 24/7, showing no eggs or nestlings were directly killed by predators during the observation period.

The researchers then played different sounds to different groups of birds throughout the four-month breeding season. One group would hear sounds associated with natural predators, while the other heard "non-threatening natural sounds."

"The birds that were hearing the predator sounds produced 40 per cent fewer offspring — simply due to the sound of fear — that’s a very significant decrease," Clinchy said.

"What this shows is that predators significantly affect the population sizes of their prey not just by killing prey but by scaring them as well."

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