Lessons from the Wolf
Bringing the top predator back to Yellowstone has triggered a cascade of
unanticipated changes in the park’s ecosystem
By Jim Robbins
Several scrawny cottonwood trees do not usually generate much excitement
in the world of ecology. But on a wind-whipped August afternoon in
Yellowstone National Park’s Lamar Valley, William J. Ripple, a professor
of botany at Oregon State University, stands next to a 12-foot-high
cottonwood tree and is quietly ecstatic. “You can see the terminal bud
scars,” the bespectacled Ripple says, bending the limber tree over to show
lines that mark a year’s growth of a foot or more on the broom-handle-size
trunk. “You can see that elk haven’t browsed it this year, didn’t browse
it last year and, in fact, haven’t browsed it since 1998.” Ripple gestures
at the sprawling mountain valley around us and points out that although
numerous other cottonwoods dot the landscape, this knot of saplings
comprises the only young ones–the rest of this part of the Lamar is a
geriatric ward for trees. The stately specimens that grow in the valley
bottom are 70 to 100 years old, and not a newcomer is in sight to take
their place. On the hillside, aspen trees present a similar picture.
Groves of elderly aspen tremble in the wind, but no sprouts push up in the
understory.
These trees could have died out entirely, some experts believe, if wolves
hadn’t shown up in Yellowstone. And therein lies a fascinating tale of how
ecosystems work, and how making one change can produce all sorts of
surprises.
In the dead of winter in 1995 the National Park Service and the U.S. Fish
and Wildlife Service brought 14 wolves into Yellowstone by truck and
sleigh. Gray wolves (Canis lupus) from Canada, these were the first to
call Yellowstone home since the creatures were hunted out of existence
there early in the 20th century. A year later 17 more Canadian wolves were
added.
Biologists hoped that the reintroduction would return the mix of animals
to its more natural state. They expected, for instance, that the wolves
would cull many of the elk that lived in the park. When the wolves–once
the region’s top predator–were gone, the elk population had burgeoned.
And the new generation of Canis behaved as predicted. Sixteen packs of
wolves, each composed of about 10 animals, now roam the park, and each
pack kills an average of one elk a day. The elk population, which had
swollen to 20,000 by the 1990s, is now less than 10,000.
The wolf introduction has had numerous unexpected effects as well. The
animals’ impact on the flora and fauna in the park has been profound.
Indeed, the breadth of change has been so far-reaching that researchers
from around the country have come to study the alterations. “Wolves are
shaping what you see here,” says Douglas W. Smith, leader of the
Yellowstone Wolf Project. “In 30 years, when you drive through the park,
it will look very different.”
The Ecology of Fear
Ripple, for one, is hoping for more trees. “I like aspen trees,” he
remarks over coffee in a cozy log restaurant near a cabin just outside
Yellowstone where he stays during field research. “I am
passionate about them.” Among other things, he explains, they are
biodiversity hot spots in the West, home to a variety of songbirds. When
he heard in 1997 that aspen trees were on the decline in Yellowstone and
no one knew why, he was drawn to the park to try to solve the mystery.
Ripple points to some black-and-white photographs taken of the same spot
in the Lamar Valley more than 50 years apart. “You can see that young
aspen and willow were abundant in the early 1900s. By the 1930s the trees
had stopped regenerating, and there are no young ones.
“I had a lightbulb,” he continues. He took core samples from 98 aspen
trees and discovered that only two had begun to grow after the
1920s–around the time the last substantial populations of wolves were
killed or driven off. And these two were in places that elk would be
hesitant to frequent for fear of being attacked by predators. Ripple found
big trees and tiny trees but nothing in between, because nothing new grew
from the 1930s to the 1990s. It was the first concrete evidence of a “wolf
effect.”
The wolf-effect theory holds that wolves kept elk numbers at a level that
prevented them from gobbling up every tree or willow that poked its head
aboveground. When the wolves were extirpated in the park as a menace, elk
numbers soared, and the hordes consumed the vegetation, denuding the Lamar
Valley and driving out many other species. Without young trees on the
range, beavers, for example, had little or no food, and indeed they had
been absent since at least the 1950s. Without beaver dams and the ponds
they create, fewer succulents could survive, and these plants are a
critical food for grizzly bears when they emerge from hibernation.
After the wolves’ reintroduction in 1995 and 1996, they began to increase
their numbers fairly rapidly, and researchers began to see not only a drop
in the population of elk but a change in elk behavior. The tall, elegant
mahogany-colored animals spent less time in river bottoms and more time in
places where they could keep an eye out for predatory wolves. If the
wolf-effect hypothesis is correct, and wolves are greatly reducing elk
numbers, the vegetation should be coming back for the first time in seven
decades.
Hiking along the purling Lamar River, not far from a den of one of the
wolf packs, Ripple walks by a small rise and parts a dense green curtain
of booth willows to make a point. There on the ground lie the bleached
skull, ribs and spine of an elk. And all around, the willows are much
taller than Ripple, some more than three meters high. Ripple and his
colleague Robert L. Beschta, a forester at Oregon State University, have
indeed found trees and willows rebounding in Yellowstone as wolf numbers
have climbed–but that is only part of the change occurring in the park.
Trees are coming back most dramatically in places where a browsing elk
doesn’t have a 360-degree view; these willows, for example, sit below a
rise that blocks the animals’ view. A look at the plants shows they have
not been browsed at all in several years. Elk don’t feel safe here, Ripple
contends, because they can’t see what is going on all around and are
nervous about spending time in this vicinity. Just 50 meters away,
however, where the terrain is level and wide open and the elk enjoy a
panoramic view, the willows are less than a meter tall and have been
browsed much more heavily over the past three years. “It’s the ecology of
fear,” Ripple says.
The Long Reach of the Wolf
Other changes accompany the regrowth of vegetation taking place along the
Lamar. Just upstream is a small beaver dam, one of three–the first dams
documented on the river in 50 years. Slough Creek, a tributary of the
Lamar, has six dams. Both Ripple and Smith believe that because of the
regrowth, beavers have something to eat again. “Their food caches are full
of willow,” Smith says. And other changes are in the offing. As more woody
vegetation grows along the Lamar, it will stabilize the banks and stop
some erosion. More vegetation, Ripple predicts, will also shade and cool
the stream. It means, too, more woody debris in the Lamar, which will slow
the river, cause water to pool, and improve the trout habitat, leading to
more and bigger fish.
Although the scientific focus so far has been on vegetation, the wolf
seems to have an incredibly long reach into other parts of the Yellowstone
food web as well. One of its most dramatic effects has been on coyotes.
For three years before the reintroduction of wolves, Robert Crabtree, now
chief scientist at the Yellowstone Ecological Research Center, a nonprofit
organization based in Bozeman, Mont., and his wife, Jennifer Sheldon, who
are both canid biologists, gathered baseline data on the park. Coyotes,
they have found, have sacrificed a great deal to make room for the much
larger wolves.
The number of coyotes in the park is down 50 percent and in core wolf
areas has dropped 90 percent. Male coyotes are smaller than they were
before the wolves arrived, perhaps, Crabtree says, because “the larger
ones were more aggressive and challenged the wolves and lost.” With fewer
coyotes, their prey–voles, mice and other rodents–have exploded in
number. That has benefited red fox and raptors. But red fox prey on
songbirds as well, and more foxes could mean a greater toll on birds.
Wolves have also thrown the doors to the Yellowstone meat market wide
open. Rarely do grizzly bears or cougars attack full-grown elk, although
they eat calves or feed on the winter-killed carcasses. Wolves, on the
other hand, pull down big ones all the time. After they eat their fill,
they wander away, meat drunk, to sleep it off, or they get pushed off the
kill by a grizzly. The presence of wolves has meant that much more meat is
available on the ground. All manner of scavengers make a living on these
carcasses, and an increase in numbers of everything from grizzly bears to
magpies reflects these newfound riches. The largest number of ravens on a
wolf kill ever recorded (135) was here. “We see bald eagles, golden
eagles, coyotes, ravens and magpies on every kill that’s made,” Smith
says. “I don’t know what they did before wolves showed up.”
But are wolves really the engine driving these changes? Most scientists
think so. Smith says that “wolves are to Yellowstone what water is to the
Everglades”–the primary force shaping the ecosystem. In Banff National
Park in Canada, scientists have documented changes brought by wolves that
returned on their own in the 1980s: willows reappeared, the diversity and
abundance of songbirds doubled. Now researchers are coming to Yellowstone
to tease out some of the first evidence of the impact that wolves are
having on areas near the riverbanks; at least six projects are gathering
data.
Some researchers, however, are agnostic about the effects of the wolf.
Crabtree, for example, says that yes, willows are rebounding and imaging
data show the regrowth dramatically. But a strong correlation between the
return of wolves and the new growth is far from demonstrated. “Claiming
wolves are responsible verges on bad science,” he states. “The ecosystem
in Yellowstone is a multicausal interactive system, and there’s never a
single cause. Even a predominant cause is rare. At the same time the wolf
numbers were coming back, there was flooding along the river, and the
climate is a lot warmer. Wolves probably have a role, but it is confounded
by those factors. It will take 20 years or more before we know
definitively.”
Duncan Patten is a research ecologist who served on a National Academy of
Sciences study of Yellowstone published in 2002. Yellowstone has not had a
hard winter since wolves reached high levels, he observes, and elk may not
have needed to resort to trees for food: “When winters are hard, elk take
a lot of chances to put something in their belly. Give me two hard winters
in a row, and I’ll buy the argument.”
The debate over the wolves’ influence on the elk is fanning a
long-standing argument over the proper way to manage Yellowstone’s elk. At
one time the park service also believed elk were too numerous and in the
1960s sent rangers to trap and shoot them by the thousands in a program
called “direct reduction.” By the end of the decade the total number of
elk was down to an estimated 4,000. Public outcry ended the shooting, and
in the 1970s the park service adopted a policy of natural regulation in
wilderness parks such as Yellowstone, a management philosophy that would
lift the heavy hand of humans and manage the parks as “vignettes of
primitive America.” Ever since, the elk numbers have climbed.
For decades now, critics, including the state of Montana, have denounced
the National Park Service for allowing so many elk to crowd the vast
stretch of native grasses. Letting nature take its course in what is a
decidedly unnatural situation is folly, the critics argue. Few elk would
spend the winter at such a high altitude, they add, if the animals could
migrate onto the plains. Instead hunting pressure in the surrounding area
compresses them into the park.
Some researchers assert that the return of vegetation along the
riverbanks–brought on by a reduction in the number of elk–undermines the
long-running contention of the park service that Yellowstone’s elk
population is within natural limits. But Smith defends the park’s view and
suggests that there are other ways to look at the situation. Elk numbers
are going to fluctuate wildly over time, he says, and although numbers
might have been, and still are, high, “they’re within natural limits over
the long term.”
Countering this defense, Alston Chase, author of the 1986 book Playing God
in Yellowstone, which was harshly critical of the policy of natural
regulation, says for the park service to make such an argument is absurd.
He found little evidence of large elk populations on the Yellowstone
Plateau in the past. Between 1872 and 1920, he points out, the park was
established, poaching was stamped out, the Native Americans were evicted,
and the U.S. Biological Survey was killing wolves. That is when elk
numbers started to soar–and it was a wholly unnatural irruption.
Unwitting Restoration Biologists
Although the jury is still deliberating the effects of wolves, early
evidence strongly suggests that the canids are unwitting restoration
biologists. By simply doing what they do–mainly preying on elk–they are
visiting great changes on the Yellowstone ecosystem. Many of the changes
are positive for those things humans value, and for experts to accomplish
some of these same goals would be hugely expensive.
Wolves have brought other lessons with them. They dramatically illustrate
the balance that top-of-the-food-chain predators maintain, underscoring
what is missing in much of the country where predators have been
eliminated. They are a parable for the unintended and unknown effects of
how one action surges through an ecosystem. More important, the
Yellowstone wolves are bringing into focus hazy ideas of how ecosystems
work in a way that has never been so meticulously documented. Just as the
actions of the wolf echo through Yellowstone, they will reverberate into
the future as they help to increase the understanding of natural systems.
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– JIM ROBBINS is a freelance journalist based in Helena, Mont., who
writes about the changing American West for the New York Times and other
publications.