Fear of predation may be enough to push small groups of animals to extinction

Fear of predation may play a role in pushing small populations of vulnerable species to extinction, a new Canadian study has found.

Flies living in small populations who smell the praying mantis have fewer, smaller offspring

The scent of a Chinese mantis made fruit flies in small populations have fewer and smaller offspring. (University of Guelph)

Fear of predation may play a role in pushing small populations of vulnerable species to extinction, a new Canadian study has found.

That could have implications for almost any species that is prey, but particularly for some migratory bird species that are at risk, according to Ryan Norris, an ecologist and research chair at the University of Guelph in Guelph, Ont.

In the study published in Proceedings of the Royal Society B, Norris studied the effect of the presence of a predator by introducing the scent of a praying mantis into populations of fruit flies.

He found that flies living in smaller populations were more adversely affected by the scent of the predator than flies living in larger groups.

Fear makes the difference

In the smaller populations, the female flies laid fewer eggs which grew into smaller flies if exposed to the scent of a mantis ahead of breeding season. If they smelled the mantis during breeding season, their offspring were smaller.
Fruit flies in populations of varying size were exposed to the scent of the mantis, but the mantis didn't get to eat them. (University of Guelph)

The actual mantis was not present, so it was not predation, but fear of predation that made the difference, Norris said.

"What we think is going on — and we think this is psychological across many populations — is that when you have fewer individuals, any given individual in that small population has to spend more time being vigilant of predators," he told CBC News.

In a larger group, individuals don't have to worry as much because others are also looking out for predators.

"They're smelling predators and they're being quite vigilant, and when you're spending time being vigilant, you're not spending time feeding. You see that the flies are in poorer condition when they're at lower densities."

For small populations of any bird or animal, this could mean that fear is playing a role in pushing them toward extinction.

"Fear can drive the population lower and then once the population gets lower, then it gets in real jeopardy," Norris said.

The study highlights the differences in the behaviour of animals in populations of different sizes.

"Conventional wisdom about how animals respond to density is the higher the density, the worse off individuals tend to be in their ability to survive, their ability to reproduce," Norris said.

Small populations at risk

That's a good thing for animal populations as it drives numbers lower when food supplies are in short supply. In large populations, a bad winter or decline in food supply might lead to the death of many individuals, but could make the overall population stronger.

"But there's an effect proposed by Allee many years ago that is the opposite of that. ...When populations get low, they actually do worse and that's dangerous because when populations get lower, then they risk extinction," he added.

"The challenge has always been what causes it."

Norris's work suggests a new mechanism for the Allee effect, a long-known biological phenomenon that found that animals living in smaller populations tend to have lower levels of reproduction and less robust health.

There have been suggestions the Allee effect is caused by predation, difficulty finding a mate, social dysfunction or inbreeding.

But Norris's findings suggest fear can be compounded in populations with low densities and interfere with the health of individual animals.

"Just the lingering scent of a predator is sufficient to cause the population to continue to decline."

Norris is an ornithologist and his particular interest is migratory birds, including birds at risk such as the insectivores.

"I study fruit flies in the lab to stand in for birds, because I can do things in the lab that I couldn't do for birds because I can look at different types of manipulations and I can't do that in the wild," he says.

He hopes this line of study will help develop models that are realistic for bird populations whose numbers are in decline in North America.