Sunday, March 11, 2012

Fear Can Kill

Predators impact prey populations through well, predation. It's a fairly simple concept, just like the supply-demand concept of the business world.

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.
A group of researchers from the University of Toronto, Canada have recently published a paper that proves that the mere presence of predators in the vicinity could have an adverse effect on prey population. And yesterday there's another paper published by Liana Zanette et al. that shows the offspring produced by stressful animals have less chances of surviving to adulthood.
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.
After forty days, the results were clear: survival was significantly lower (2.5-4.3 times) in both predator treatments than in the no-predator treatment, but the two predator treatments did not differ from each other. Also, in the predator treatment, 11% of individuals that survived the larval stage died during emergence to the adult stage, while only 2% of the larval survivors died at emergence in the fishless treatment.

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.

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

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