"I have lived to see state after state extirpate its wolves. I have seen every edible bush and seedling browsed, first to anemic desuetude, and then to death."
-- Aldo Leopold

Predators change the behavior of prey.

Research about wolves that began in Yellowstone National Park has been replicated in an adjacent area, and a growing body of evidence leads scientists to conclude that this historic predator may have an ecological impact far more important than realized in the American West.

The near extinction of the gray wolf across most of the West in the past century now appears to have removed the natural element of "fear" from these ecosystems. It has triggered a cascade of ecological effects on everything from elk populations to beaver, birds, fish, and even stream systems - and helped lead directly to the collapsing health of aspen and some other tree species and vegetation...

The ecological and historical significance of wolves is only partly due to the actual impact they have by preying on other animals, both large and small, the OSU researchers have found. Just as important is the fear that many larger animals have of wolves, and the resulting behavioral changes in elk and some other grazing animals.

"Prey species will alter their use of space and their foraging patterns according to the features of the terrain and how that affects the risk of predation," Ripple and Beschta noted in their study. "They forage or browse less intensively at high-risk sites."

Some of those sites, the researchers say, are streamsides rich in aspen, cottonwood, willow and other edible vegetation. When healthy and normal, such areas naturally grow large trees and other streamside vegetation that provides the basis for supporting beaver, other wildlife, fish populations, native bird communities, and stable channel banks.

The role of fear, while emphasizing the value of wolves, is not exclusive to them, the scientists said. Even the fear of human sport hunters has a role.

One study in Montana showed that elk adjusted their foraging behavior by browsing far from roads to avoid human contact and possible predation. And research in Colorado has found that aspen was far more heavily browsed, and used year-round by elk, where sport hunting was excluded.

Ultimately, however, the value of large predators needs to be reconsidered, the reports conclude. The body of evidence has become compelling, the OSU researchers say, that predation by top carnivores, especially wolves, may be pivotal to maintaining biodiversity in some ecosystems.

Though the researchers note that humans alter prey behaviors too I think that they understate the historic role of humans. Like wolves humans took prey of all sizes in all seasons and knew the habits of prey animals. They were patient and lethal even at a distance so they were difficult to detect and avoid. Sport hunters are blundering, smelly avoidable threats compared to those who hunted to live.

I also think that they understate the effects of fear. The damage to streamside Aspens isn't due to selective browsing so much as loafing near water. Elk and deer are ruminants and must spend time loafing. They regurgitate forage that has been partially digested and chew it again to expose more surfaces to digestive bacteria. Cud chewing is an integral part of their digestion process, an evolved ability that allows them to get much more nutritional benefit from forage than non-ruminant ungulates such as horses. They need to relax to do this and they like to be near water. When they loaf near water they also trample and browse more in the area. They won't loaf there if it's dangerous, they'll find a place where they aren't so anxious about attack so that they can relax and digest.

Ruminant digression:

See Golden Calves for a somewhat speculative account of the contribution of ruminants to the fall of Rome.

The recorded history of the final fall of Rome in its Eastern capital of Constantinople includes a great plague soon followed by decades of incursions by Mongolian horsemen, the Avars, which sapped the empire of wealth paid as bribes to avert attack. The Avars had been dominant in Mongolia, hundreds of miles north of China, but by the mid sixth century had been driven from the area by the Turks, a mountain people long dominated by the Avars. The Avars fled west toward Constantinople and warmer climes where their superior horsemanship gave them an edge over the weakened Romans.

What allowed the Turks to rise up against the Avars and force them west and so bring down the remnants of Rome? Keys speculates that it was that same volcanic winter that devastated Constantinople. What interests me about this was the differential experiences of the Turks and the Avars which Keys explains was a consequence of their economic and cultural practices. The Avars were horsemen. They didn't just ride horses, they milked them and ate them too. Horses were the foundation of their economy as well as their military. The Turks were cattlemen.

Cattle, as well as all other ruminants such as goats and sheep, have marvelous digestive systems that allow them to efficiently extract nutrients from forage, even rank forage grown in stressful conditions. Horses have less efficient metabolisms that require more and better forage to sustain them. In the bad years of volcanic winter the Turks suffered much less than the Avars and roles reversed due to economic rather than military reasons. The Avars were starving because their horses were starving. They had no choice but to migrate to warmer climes for better forage.