Landscape of fear
MICHAEL MILSTEIN
Be afraid.
It’s the message a cat sends when stalking a mouse, an eagle sends when swooping toward a squirrel and a wolf sends when chasing down an elk.
And the way wild animals react to the basic fear of being caught and eaten sculpts the landscape of the West. It controls how wildlife such as elk and deer move and feed, and may go so far as to help trees grow, give songbirds places to alight and even keep streams cold and clear.
Take fear away, and carefree elk chew down the plants that hold streams in place, fraying the landscape’s fabric. Put fear back and they eat more warily. The fabric holds together.
Researchers from Oregon State University are charting those connections. Their laboratory has been the Lamar River Valley in Yellowstone National Park, where wolves were introduced in 1995, decades after their historic extermination. But the findings also have meaning in Oregon, where wolves spreading from Idaho and Yellowstone are expected to arrive in increasing numbers.
“Everything we’re looking at keeps going back to the point that the presence or absence of this keystone predator has made an incredible difference in the landscape of this valley in Northern Yellowstone,” said Robert Beschta, a retired Oregon State forestry professor and co-author of much of the research with professor William Ripple. “Could they play the same role or a similar role in other places? Possibly.”
Not all scientists agree the connection is so clear. But Beschta and Ripple’s results parallel conclusions in Canada’s Banff National Park and as far away as Africa, where the threat of lions and jaguars appears to keep zebras and wildebeests from gnawing away all their food. The more they scan for attackers, the less they nibble the hardest-to-reach blades of grass.
They live in a landscape of fear. Just as Americans altered their travel habits and hiked security measures after Sept. 11, wild animals take great pains to reduce the risk of attack.
“The reason a cow overgrazes a pasture and a zebra doesn’t is the cow isn’t frightened,” said Joel S. Brown, a professor at the University of Illinois at Chicago who has studied the question around the world. “Fear, which is basically a behavioral response, in fact has profound influences on the ecology of animals.”
Brown coined the term “ecology of fear” to describe the effect. Relatively few wolves prey on vast herds of prey such as elk, he said. So they may well exert as much or more control over prey animals merely by frightening them than by actually killing them.
Ecosystem out of balance
The two OSU researchers had no special interest in wolves when they first visited Yellowstone. Rather, they were puzzled by the relative absence of two tree species they figured should be there in abundance. For Ripple and his doctoral student, Eric Larsen, it was aspen. For Beschta, it was cottonwood.
Beschta was also perplexed by the Lamar River. It cuts a wide valley across Yellowstone’s northern reaches, and had spread smooth plains across the valley floor over many centuries. Willows, cottonwoods and other streamside plants should have been flourishing, holding the ground in place, he thought.
But the plants were sparse, and the river in recent decades had started carving the soil away.
When he examined cottonwoods along the Lamar in 2001, he found a strange pattern. Hundreds of older trees measured more than a foot around. But there were almost no midsize, middle-age trees.
Whatever new cottonwoods had sprouted since the 1920s, few had survived. From the 1960s on, almost none had made it.
“The young trees just couldn’t grow,” Beschta said. “Literally nothing was able to make it through. Nothing.”
Ripple and Larsen had already found a matching pattern among the groves of aspen dotting the valley slopes. Just one of every 20 trees they looked at had sprouted since the 1920s.
The timing was striking. The 1920s, the same time new cottonwoods and aspens stopped surviving, was when the last of Yellowstone’s native wolves were killed off.
“That was kind of our ‘aha’ moment,” Ripple said.
Key player hunted down
Beschta and Ripple were not the first to wonder what had happened to plants on Yellowstone’s northern range. Most scientists and managers before 1970 agreed that herds of thousands of elk and hundreds of bison were overgrazing the range, wiping out aspen and streamside willows.
Beavers, left with nothing to eat, declined, and barren streamsides eroded.
National Park Service bosses in Yellowstone responded by gunning down thousands of elk, bison and pronghorn. But the slaughter outraged the nation in the 1960s, and managers turned to a policy of “natural regulation.” Elk numbers were left to rise and fall on their own.
Some still argued the range could not support so many elk.
But it also seemed clear that natural regulation was not truly natural when a key Yellowstone predator was missing. Hundreds of wolves once roamed the region. But park managers killed them off so elk, moose and other animals deemed more tourist-friendly could flourish.
That changed in 1995, when a U.S. government restoration program imported wolves from Canada to Yellowstone and Central Idaho. By 2001, nearly 80 of the predators roamed the northern range.
Fear was back.
Scientists documented that where wolves had colonized the park, female elk and bison spent at least 20 percent more time scanning for threats. Elk with young calves spent nearly half the time watching their surroundings — and, probably, less time eating.
Elk numbers also fell an average of 6 percent a year in the last decade. That’s in part the result of wolves, scientists say, but also human hunting outside Yellowstone, a hard winter in 1997 and possibly drought.
Where Beschta and Ripple found no willows taller than their knees before wolves arrived on the Lamar, suddenly they were standing among plants twice as tall as they are.
Bite scars on willows outside the park revealed that elk were eating less near ledges and stream banks, where they might feel cornered if a wolf appeared. Instead they tended toward open spaces, with many escape routes.
Beaver colonies on the northern range multiplied from one in 1996 to seven in 2003.
It struck researchers that wolves had changed the landscape. The disappearance of wolves left elk lazing along streams mowing down seedlings. The return of wolves made fewer elk far more vigilant. And the seedlings took off.
Not wolves alone
Other scientists say it’s not so simple. Just because wolves returned when plants began flourishing does not mean one caused the other. There probably is not one straightforward cause, they say.
“Everybody’s in a rush to grab the thunder and attention of wolves,” says Robert Crabtree, chief scientist at the Yellowstone Ecological Research Center in Bozeman, Mont. “Certainly wolves are a factor, but the whole story is not being told. Ecosystems are complex, and you can’t ignore other explanations for one pet theory.”
For instance, floods swept the valley about the same time wolves reappeared. Cottonwoods and willow flourish after flooding, so that may have helped their resurgence as much as wolves, he said.
Elk also turn to aspen and willow in late winter when other food grows scarce. But recent winters have been so mild they may not have had to.
Ripple and Beschta say they tried to find other explanations, but none fit. The Lamar flooded while wolves were gone, but plants never flourished. Even the massive elk slaughters of the 1960s didn’t give plants enough of a break from grazing for a resurgence.
Ripple agrees wolves may not be the only factor. But he’s convinced they are a big one.
“We had all of those other things happening before wolves, and nothing made a difference,” Ripple said. “I see wolves as the key ingredient that changed the picture.”
Wolves similarly recolonized Banff National Park in the 1980s, but remained so wary of people they avoided the town within the park. Elk hovering around the town avoided the wolves. Outside the town, elk numbers fell 30 percent to 50 percent.
Willows rebounded. Warblers and other songbirds multiplied, lured by thriving wetlands.
“You do see these cascades ripple through the system like you do in Yellowstone,” said Mark Hebblewhite, a doctoral student at the University of Alberta and lead author of a new study in Banff.
He cautions against judging wolves as good or bad. They may have consequences no one has detected yet.
“Evidence is accumulating they are an important element of the ecosystem,” he said. “They do things people don’t necessarily expect. It’s a lot more than eating elk.”