Karen Holt

John Gittleman

Prey lose fear in absence of predators

By Fariss Samarrai

Imagine an elk or a moose standing still, without fear, as a pack of wolves or a grizzly bear moves in for the kill. It seems unlikely, but this is a common occurrence in areas where predators are reintroduced after several generations of absence.

Prey animals tend to forget their fear over time and will often stand vulnerable to attack. This could threaten their survivability as a population if their numbers are already stressed. For this reason, wildlife biologists should carefully assess prey populations before introducing predators into an ecosystem, says John Gittleman, associate professor of biology, in an article published last week in the journal Science.

Wolves kill more prey when reintroduced into an ecosystem after several generations of absence.

Of 165 carnivore reintroductions worldwide, none assessed how prey populations respond to the reintroduction of predator species, he says.

"This lack of assessment could be inviting disaster, and may even greatly increase the risk of extinction for some at-risk prey species," he says. "When carnivores are introduced by wildlife managers, the emphasis is almost always on the survivability of the predator rather than the prey. But more study is likely needed to make sure the prey population can be sustained, which in turn, will sustain the predator population."

"These species become highly vulnerable when suddenly exposed to a carnivore," Gittleman says. "They don't know how to respond when a predator approaches because they've never experienced an attack." They often behave as if their evolutionarily developed fear has been completely lost.

Prey animals in this situation, such as elk and moose, have been observed standing still allowing a newly introduced wolf population to simply walk up and make the kill, Gittleman says. Studies have shown that a disproportionately high level of predation occurs immediately following the initial introduction of predators to areas where they had been nonexistent for generations. This ignorance of danger, when combined with habitat destruction, and overexploitation of resources in some areas, could reduce some prey populations to dangerously low levels. Prey animals on islands, which generally have fragile ecosystems, are particularly vulnerable to predator introductions.

"It can often take some time for prey animals to become aware of what's happening, and to relearn their fear and sense of caution," Gittleman says. "Many of them die, initially, as this learning process takes place."

Gittleman suggests that prey could possibly be trained to relearn fear by conducting a limited predator introduction before releasing large numbers of carnivores into the wild.

"It might make sense to send in one lone wolf, so to speak, to give prey an adjustment period to develop vigilance," Gittleman says. "Once the prey learns the sound of a wolf call, for example, and has the proper fear response, then a full introduction of predators might be appropriate. An assessment of the prey population would be an important step toward assuring the prey population is able to sustain the first wave of predation."

Gittleman, an evolutionary biologist who came to U.Va. in 1998, specializes in problems of biodiversity and extinction. The author of numerous papers in leading scientific journals, most recently he has co-authored three papers in Science in less than a year. This Science article was co-authored with Mathew E. Gompper, a biologist at Columbia University and a former Gittleman graduate student. They wrote the article as an invited Perspectives column, putting into broader context a peer-reviewed study appearing in the same issue.