| Yellowstone Bison Slaughter
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| News
Article 2/11/05 |
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| Elk
and bison wariness of predators seen as check on plant
damage
By Michael Milstein, Newhouse News Service, Seattle
Times
2/11/05 |
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 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.
"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."
Cows vs. zebras
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.
Because they live in a landscape of fear, 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 Brown,
a professor at the University of Illinois, 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.
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 also was 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 had started
carving the soil away in recent decades.
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. Almost none had made it from the 1960s
until now.
"The young trees just couldn't grow," Beschta
said. "Literally nothing was able to make it through.
Nothing."
Ripple and Larsen already had found a matching pattern
among the groves of aspen dotting the valley slopes. Only
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.
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.
"Natural regulation"
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 past 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, they suddenly
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.
Researchers were struck 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.
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," said 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."
Flooding as factor?
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 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."
Parallel findings
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. "It's a lot more
than eating elk."
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