How do wild animals view us? It's fairly clear they don't consider us completely harmless, because otherwise we might have bluebirds perching on our fingers and fawns trotting after us on our way to work like in some Disney movie. But prey animals aren't the only ones that hightail it as soon as a group of humans crashes through the underbrush. Even the top predators in an ecosystem will give humans in the area an inordinate amount of space. We are, in the eyes of animals the world over, the ultimate predator.
A new study published in the journal Proceedings of the Royal Society B experimentally demonstrates that large carnivores — cougars, for instance — perceive humans as predators, and they fear us to such an extent that the caution they use to avoid us distorts the way ecosystems function. Previous research shows large carnivores are essential in shaping ecosystems, and that messing with large carnivores will set off big changes to the food web in an ecosystem. The group of researchers from Western University in Ontario and University of California Santa Cruz is the first to show that these top predators' fear of humans affects their feeding behavior, which has measurable ecological consequences further down the food chain.
So, we're a superpredator. What other superpredators are there out there? Lions? Killer whales? What?
According to study co-author Liana Zanette, professor in the department of biology at Western University, there aren't any others. There's just humans.
"There are predators, and then there is the superpredator, which is us," says Zanette. "Research has shown that globally humans kill large carnivores at nine times the rate at which they are naturally killed. We also kill middle-of-the-food-chain animals at four times the rate they are killed by their large carnivore predators. So, no matter how you slice it, we're off the scale in terms of the threat that we pose even to apex predators. We're not just any old predator, we are the superpredator at the very top of the food chain."
The research team studied fearfulness of humans in cougars living in suburbs and rural areas outside Santa Cruz, California, by setting up a specially designed motion-triggered camera at known cougar kill sites, and playing recordings of either people talking or frogs chirping to the big cats while they ate. In this way, they could gauge the cougar's responses to each type of sound and at the same time, measure the foraging cost of fear in these animals.
"We found that cougars almost invariably fled from their dinner when they heard humans talking," says Zanette. "About half never returned, and for the other half that did, it took them longer to do so. The overall effect was that over a 24-hour period, the cougars spent half as much time feeding as when exposed to the control."
The perceived presence of humans made the cougars act not like top predators, but more like middle-of-the-food-chain species. When one of these animals — a raccoon, say — thinks there are predators around, they spend their time avoiding the predator and focusing on staying alive, which means they stop doing virtually everything else, including eating. And because scared prey eat less, this affects the next tier of the food chain, and the next, and the next.
This effect can obviously be observed in more than just California cougars. Virtually every ecosystem in the world is flooded with superpredators (that's us humans, remember) at this point. Large carnivores are increasingly forced to live in human-dominated landscapes, and they evidently live in terror of us. This research shows that their behavior in response to us impacts our landscapes. This information alone can be helpful in the future in protecting a class of animals whose presence is essential to ecosystem function, and whose numbers continue to decline across the world due to human activities like hunting, persecution and habitat loss. Apparently, our existence in the landscape isn't doing them any favors, either.
"This research emphasizes the fundamental role of fear in determining how nature functions," says Zanette. "As such, it is clear that a full understanding of fear effects is necessary, and though we have learned a lot in recent years, we are just at the beginning of the learning curve."