Are Prey Hard-Wired to Fear Predators?



Brodie Farquhar / Jackson Hole Star-Tribune / August 23, 2007

Do prey animals learn to fear predators, or have thousands of years of natural selection hard-wired them to fear the carnivores that hunt, kill and eat them?

That question is at the core of research by Wildlife Conservation Society scientist Joel Berger, who believes that large prey species such as moose, caribou and elk only fear predators they've encountered before.

"If you take away the predator, you take away the fear," said Berger, whose research is published in a recent issue of "Conservation Biology," a scientific journal. Berger's study looked at 19 areas around the world, including the Greater Yellowstone Ecosystem, and found that prey species n and their surrounding ecosystems -- are all affected by the loss or reintroduction of predators.

Berger found prey species who've lived with and without predators, testing both populations by playing recordings of Siberian tigers roaring and wolves howling. Native prey species did not show the degree of agitation, vigilance, clustering for safety or flight, seen in the same species that have experience with tigers, wolves and bears.

Yet one of Berger's colleagues, someone who has co-authored several papers with him, isn't sure he agrees with Berger's interpretation of the data.

Doug Smith, who leads the Yellowstone National Park wolf research team, emphasized that he regards Berger as a friend and solid scientist. Nor does Smith question Berger's field experiments, observations and data.

"I haven't done my own experiments," said Smith, "but I do have concerns and questions about Joel's interpretation of the data."

For example, said Smith, consider Wyoming pronghorn antelope, capable of speeds up to 65 mph. Yet the fastest land-predators the antelope have to worry about might hit 35 mph.

Biologists believe that antelope evolved in the presence of an equally-fast predator n- Ice Age cheetahs that haven't been alive in North America for 10,000 years, said Smith. "If antelope have retained this ability to run ridiculously fast, why wouldn't they retain the behavior to fear predators?" asked Smith.

Smith closely observed the earliest interactions of wolves and elk after wolves were reintroduced into the Yellowstone ecosystem in 1995. Elk quickly adjusted to wolves within a few months, said Smith.

Berger said there's just not enough good data to figure out the thresholds between prey that are native and have never encountered a predator, and how quickly prey figure out that predators are deadly and must be avoided when predators come back into the picture.

Like Smith, Berger has been a keen observer of how wolves and grizzly bears have expanded into new territories in the Yellowstone region. Berger said the presence or absence of predators causes vast ripple effects in surrounding ecosystems.

For example, biologists in Yellowstone found that wolves changed elk feeding behavior. With more time spent moving around in fear of wolves, elk spent less time feeding on willows along stream banks. The revived growth of willows led to benefits for beavers (a new, abundant food source) and a variety of birds who nested in the willow stands.

Conversely, said Berger, where major predators were absent in the 1970s for southern Grand Teton National Park, migratory birds like warblers and hummingbirds were fewer in number because moose over-browsed vegetation used by those birds for food or shelter.

"Trophic cascades can run from the top down or bottom up, depending on the presence or absence of major predators," Berger said.

He gives quite a bit of credit to prey species for their ability to figure out that predators are to be feared and avoided. Yet the mere presence and activity of a reintroduced predator in the neighborhood does not automatically mean that all the prey species understand the danger. Berger noted that when wolves started preying on Yellowstone elk, they pretty much ignored the bigger and more dangerous moose. "Yet the moose mothers quickly figured things out after they'd lost a few calves to the wolves," he said.

When wolves were first reintroduced to Yellowstone, opponents of the reintroduction project predicted that the region's elk herds would be quickly eradicated. That hasn't happened, said Berger, because the prey species have adjusted their behavior to the presence of predators -n to the benefit of other species and ecological balance.

One of the questions raised by his research, and with wolves to be delisted from federal protection under the Endangered Species Act, is how many wolves are needed to keep prey species on their toes (or hooves), so to speak? While Montana has placed no cap on wolf populations, Idaho and Wyoming have plans that could allow 85 percent of Northern Rockies wolves to be killed. Just as wolves have taught prey species to act more wild and have restored natural function and balance to the Greater Yellowstone Ecosystem, will there be enough wolves five, 10 years down the road?

"If we can't make it work here, where can we?" Berger asked.