On paper, predators and prey have a pretty straightforward relationship when it comes to population totals. A lion kills a zebra, so there’s one less zebra in a herd. However, new research suggests that predators might have a deeper, longer-lasting effect on their prospective meals: fear.
This fear of predators can impact the reproductive success of prey animals, a team from Canada’s Western University argues. The team, headed by wife and husband researchers Liana Zanette and Michael Clinchy at Western’s biology department, came to its conclusions after performing an experiment on free-living wild song sparrows. The study’s authors say there’s reason to believe that the phenomenon they found among the sparrows would be present in other species as well—at least in birds and mammals that care for their offspring.
“The presence of the predator is actually dramatically changing the behavior of the prey to a degree and over a period that it can actually affect the prey population,” Clinchy told Ars.
When the songbirds thought there was a predator nearby, they stopped foraging for food, meaning they could not provide as well for their young. During the breeding season, the birds also produced fewer eggs. The number of offspring were cut in half, and many of the young of parents that were exposed to fear were less healthy than their non-traumatized peers. “It’s as simple as ‘scared prey eat less,'” Zanette told Ars.
The team began researching this in 2010 in the Gulf Islands, just off the coast of the Canadian province of British Columbia. First, they found 11 different populations of the songbirds spread across a few of the islands. Half of the populations were used as a control on which the team used speakers to project sounds of non-predator species, like geese. The other half weren’t so lucky. The team used the speakers to broadcast the sounds of their natural predators—ravens, for instance—to induce fear. “We would do caws of those animals as the predator treatment,” Zanette said.
The team then located the nests from each group and counted the number of eggs females were able to lay. They also affixed radio tags and colored bands to the offspring after they hatched so they could identify them later in life. The team tracked them through their whole lives.
“It has effects over generations. We’re following one generation from the egg to the adult, and then [measured] … the quality of that adult, how long it’s likely to live, and, hence, what its role is likely to be in subsequent generations,” Clinchy said, adding that, to avoid any data skewing deaths from the offspring actually being killed by predators, the team set up cameras and protective nets near the nests.
From this, Clinchy and Zanette were also able to tell how many of the birds were able to become breeders themselves and how many offspring the birds—whether they were exposed to predator noises or not—produced. The team also gauged how healthy the offspring were based on the number of songs they sang.
The team performed this test again in 2013 and 2014. In all, the team found a nine percent decrease per year in the population growth rate among the group exposed to the fear, compared to a six percent increase in the populations that didn’t get the predator sounds blasted at them. In all, fewer young were born, and fewer survived into adulthood. Those that did survive showed evidence of impaired brain development, which would likely hinder their chances of survival in the long run.
The only thing to fear is fear itself, and predation
Past research from Clinchy and Zanette suggests that these issues aren’t likely to be limited to one specific songbird, as the fear of predators can impact the hunting and feeding habits of other species that care for their young. In a previous paper, the two were part of a team that performed a similar experiment on raccoons in the Gulf Islands. They found that the sounds of predators also impacted foraging among the raccoons and that this, in turn, resulted in more crabs in the area, as the raccoons ate fewer of them.
Humans can also produce this response in wildlife. In a similar experiment in the United Kingdom, a team of researchers found that human noises caused badgers to eat less. The effect of humans can even have some unexpected consequences. Another experiment showed that exposure to humans caused cougars in California to actually kill more deer. Normally, cougars will return to a kill over several days. But, when they sense humans are around, the fear makes them think better of it and just kill more deer to make up for it. In all, the researchers believe this work could have broad implications for conservation work.
“It makes sense. Who is deadly? Who has the capacity to kill you?” Zanette said. “Humans are the scariest thing out there.”