In the wild, a predator that eats too much of its prey can drive that species toward extinction. But there are other, less understood influences that predators can have on their prey’s survival. Take, for instance, odor: New research shows that the very smell of predators may be enough to increase the chances of a whole population of animals going extinct. Fear alone, it suggests, can shape the fate of a species.
Traditional ecological theory holds that smaller populations of any creature will usually breed more productively than larger, denser populations, since individuals have less competition and more luck in mating, says Ryan Norris, a biologist at the University of Guelph. But when hungry predators hover around, small populations of prey may not be as insulated to stress as larger ones.
To test this, Norris and his colleagues recently conducted an experiment using fruit flies and one of the flies’ natural predators: praying mantises. They put a group of flies into a cage with a mantis underneath, in a separate partition. The fruit flies couldn’t see the mantis and it couldn’t attack them, but they could smell it. Days later, during the following breeding period, female fruit flies that had been in the cage laid fewer eggs than ones that had been placed in a separate cage with no mantis below. Those eggs subsequently hatched into insects that weighed less on average, too.
The researchers speculate that part of the reason females laid fewer eggs and had lighter offspring is because worrying about a lurking mantis takes away from the time fruit flies would normally spend foraging for food and having sex. Less food for fruit mothers could mean smaller eggs, though the study, which was published in Proceedings of the Royal Society B, didn’t specifically test to see whether a lack of food caused lighter offspring and less eggs. “I suspect they are eating less because they are being more vigilant of predators,” Norris says.
This only occurred with low-density populations, he notes; at high densities, other factors like competition probably overwhelms the effect of fear. But the researchers then found through modeling that when a mantis was around to strike fear into the hearts of low-density fruit-fly populations for several generations, the risk of extinction increased sevenfold over 10 generations. In the wild, the pressure predators put on a small population of prey could be even stronger, Norris says, since the models they ran didn’t include losses from direct attacks.
Justin Suraci, an ecologist at the University of California, Santa Cruz, says that Norris’s study is one of the few that approaches the effects of fear on the population level. As he sees it, it provides a break from the traditional view of predator-prey interactions, which only involves direct killing. “There’s some evidence that the fear effects outweigh the effects of actual killing and eating,” he says, adding that killing often only affects a single individual, while the fear of predators can affect entire groups.
These results aren’t limited to fruit flies. While laboratory tests using the insects may be particularly good at honing in on the specific factors that cause population declines, some researchers have identified similar effects on wild birds. A 2011 study exposed song sparrows to the sounds of predators like ravens, hawks, and raccoons, and found that just like the worried fruit flies, the birds laid fewer eggs, and fewer of the eggs produced birds that made it to adulthood.
Suraci, too, conducted a study earlier this year to investigate how fear might affect predators themselves. He and his coauthors tracked pumas in the Santa Cruz Mountains via radio collars and determined when the big cats killed deer. Pumas often return to feed on their kills for several consecutive nights, so the scientists came in during the day and set up video-camera traps with speakers that played the sounds of humans—a large source of mortality for pumas—when triggered by motion around the deer kill. For comparison, other motion-triggered speakers were set up to play more natural sounds, like tree frogs.
The human sounds appeared to make the fierce mountain lions nervous. “They are much more likely to flee immediately and abandon their kill completely,” Suraci says.
The study only tracked the effect on individual pumas, so Suraci isn’t certain what disruptions like this would do at a population level. But the human fear factor clearly seems to drain cats’ energy levels, because they will waste more time on killing more deer and spend less time eating their fill.
Norris says that more research will have to be done to determine the extent of the effect that fear can have on small populations of prey, but it may be an important component in wildlife conservation. Government agencies and conservation groups spend an enormous amount of time and effort on wildlife-management plans, many of which are based on ecosystem models to predict the effect predators will have on a given prey population. If the mere sounds and smells of a predator are enough to offset the success of subsequent generations of animals, models that ignore the power of fear might be more likely to fail.
“[If] you make the wrong decision, you not only waste a lot of money, you risk the species going extinct,” Norris says.
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