A Top Dog Takes Over
By Sharon Levy
Exterminated from Yellowstone National Park eight decades ago, gray wolves
are back-and boosting the park’s biodiversity
FLEAS STILL HOP at the entrance to the den, dug into the side of a small
hill on the rolling surface of Blacktail Deer Plateau in Yellowstone
National Park. The Leopold Pack, with eight adolescent pups, left this
safe haven within the last couple of weeks. Standing amid the litter of a
recently active wolf den-dozens of scats, a scattering of sticks and elk
bones marked with the fine scratches of sharp puppy teeth-one is struck by
the way the site seems to disappear into the landscape. Walk a few yards
away, look back, and if you didn’t know better, you’d never guess the
wolves had been there.
Like its den, the gray wolf’s impact on Yellowstone, though profound, can
be difficult to see with the untrained eye. Exterminated from the park in
the 1920s, the species was reintroduced seven years ago. Since then,
scientists have uncovered evidence that when the wolf vanished from the
park, several other species were affected as well. Wolves, it turns out,
may shape the destinies not just of their prey and other predators but
also of plants and songbirds-a timely finding as officials consider
decreasing protection for this imperiled species. “With the wolf back in
place as the top carnivore, biodiversity is greater,” says biologist Doug
Smith, leader of the National Park Service’s Yellowstone Wolf Project.
“The return of the wolf is the best thing to happen to Yellowstone in the
past century.”
Since the arrival of Euro-American settlers, the park has experienced a
series of sometimes drastic changes. Wildlife was hunted intensively at
first, then protected piecemeal according to the prevailing attitudes of
the times. Elk, targeted by commercial hunters in the 1800s, were later
protected, while park managers mandated destruction of predators such as
wolves. Today elk are plentiful, and populations of wolves-as well as
mountain lions and black and grizzly bears-are recovering.
One major change took place after the last of Yellowstone’s wolves were
trapped and shot 80 years ago, when growth of new aspen trees came to a
halt. Photographs taken in the 1890s show aspen groves standing tall on
Blacktail Deer Plateau and elsewhere in the park, many of them gone today.
Since Yellowstone’s establishment in 1872, 95 percent of its aspen forests
have been lost. Though they never covered more than 4 percent of the
park’s area (in its northern range), aspen groves support a greater
variety and abundance of birds and understory plants than do surrounding
Douglas fir and lodgepole pine forests.
The culprits behind the decline of Yellowstone’s aspen were elk, whose
populations began to boom after the wolf disappeared. The hungry animals
devoured new growth of willow as well as aspen. For many years, the park
service controlled elk numbers by culling herds, but in 1968-in response
to a public outcry to Congress-it made a controversial decision to
minimize human meddling with the park’s wildlife, including elk. Without
culling or natural predation, there was then nothing to limit the grazers
but the carrying capacity of the land.
In a study of the trees’ growth rings, ecologists William Ripple of Oregon
State University and Eric Larsen of the University of Wisconsin-Stevens
Point found that aspen that have managed to grow to tree height-rather
than being eaten by elk as sprouts-began their lives between 1700 and the
1920s. After that, aspen continued to germinate, but they were heavily
browsed by elk and seldom managed to grow taller than six or seven feet.
Ripple says their study supports the hypothesis that wolves, through their
effects on elk, help aspen flourish. A recent National Academy of Sciences
report agrees that heavy browsing by elk has been the driving force behind
the park ‘s aspen decline, though it notes that climate change and fire
suppression also have affected the trees’ growth.
Ripple and Larsen, who launched a long-term study of aspen, elk and wolves
three years ago, are finding that even if wolves don’t change the number
of elk, they may still protect aspen by changing the way the grazers
behave. Aspen stands make good cover for wolves, and elk soon learn to
steer clear of places where wolves spend time. They may browse around the
edges of aspen forests in wolf territory, but they won’t go into the woods
where they would be most vulnerable to attack. As a result, aspen in areas
frequented by wolves are growing tall for the first time in decades.
Heavy browsing by elk during the wolf’s absence may have affected other
species too. Before the predator’s reintroduction, beavers had vanished
from the park’s northern range, although they flourished in the 1920s when
aspen groves harbored trees of all ages, including the stem sizes
preferred for dam-building. The animals themselves contributed to aspen
decline-to some degree eating themselves out of house and home-but some
ecologists believe the tree would have continued to regenerate had they
not suffered from browsing by elk. Today, following their reintroduction
on U.S. Forest Service land adjacent to Yellowstone, beavers are returning
to the parts of the park frequented by wolves. In these areas, not only
aspen but also willows are sending up tall shoots again.
Ripple believes that the wolf, the beaver and the aspen all have powerful
effects on one another. “If any of these three are missing from the
system,” he says, “problems could occur, especially if large numbers of
ungulates are present.” He enumerates the links between the species:
Wolves travel along streams and often prey on beaver; beaver need aspen
for food and dam construction; wolves control elk browsing patterns,
allowing aspen to grow; and beaver dams flood the land, creating more good
habitat for aspen.
In Banff National Park, Canada, research underway by University of Alberta
biologist Mark Hebblewhite bolsters recent findings from Yellowstone.
Before wolves recolonized the park’s Bow Valley in 1986, he says, “elk and
beaver were obviously competing for willow.” Another Banff researcher,
biologist Cliff Nietvelt, found that beavers select willow stems of a size
that vanishes when elk populations are dense. In Banff, which is a much
harsher habitat for elk than Yellowstone, the wolf’s return has reduced
the elk population by 50 to 60 percent, allowing a dramatic rebirth of
willow and aspen in wolf territories. Beaver dams built from these trees
create ponds that are biodiversity hot spots. “The only time I’ve ever
seen large numbers of boreal toads breeding in the park,” says
Hebblewhite, “was at a beaver pond about a mile from an active wolf den.”
In Yellowstone, the wolf’s reintroduction promises a cascade of effects on
other creatures as well. During the top dog’s long absence, the park’s
coyote population boomed, for example, reaching some of the highest
densities ever recorded in North America. Wolves often kill coyotes or
usurp their territories, and since the wolf’s return, biologist Robert
Crabtree of the Yellowstone Ecological Research Center estimates that
coyote numbers have dropped by 50 percent. This is good news for coyote
competitors such as red fox; sightings of these animals are already up in
areas frequented by wolves. Crabtree and Smith expect that numbers of some
other mid-sized predators, including wolverines and fishers (a rare
species of weasel), will also increase.
Just south of Yellowstone, in Grand Teton National Park and nearby
national forest lands surrounding Jackson Hole, Wyoming, moose-which
devour willow-are the problem browsers. Once rare, moose numbers began
expanding in the 1880s when humans wiped out both the wolf and grizzly
bear. Today the animal’s population density is about five times higher
inside the park than it is outside, where people are allowed to hunt
moose. Though the grizzly population is now expanding and Grand Teton is
home to two wolf packs that moved south from Yellowstone, these predators
are not yet numerous enough to have a significant impact on moose.
In 1998, biologists Joel Berger of the Wildlife Conservation Society and
Peter Stacey of the University of New Mexico studied moose effects on
willow habitat and on the birds that live there. “The willows inside the
park are heavily browsed, so they’re smaller and have many more dead stems
than those outside the park,” says Stacey. When he censused songbirds in
willow inside and outside Grand Teton, he discovered a greater variety of
species and higher abundance outside the park, where the willow habitat
was healthier and less affected by moose. MacGillivray’s warbler and the
gray catbird, two species that depend on dense willow thickets for
nesting, are absent from the park, but found outside it.
“Nobody suspected that there would be a higher diversity of birds outside
the park,” says Stacey. “We think of parks as being places that are set up
to preserve biodiversity. What we found is just the opposite, because the
park has been missing some key ecosystem components.”
Similar discoveries come out of Banff. Hebblewhite and Nietvelt found that
in areas where wolves have reduced elk density, thriving willow habitats
host a diverse array of songbirds-including the American redstart, a
species that needs mature willow to nest successfully. Near the town of
Banff, where numerous elk have taken up residence on lawns and golf
courses (safe from wolf predation), willows don’t grow above ankle height.
The birds there are grassland species, and there is less diversity than in
wolf territory. Redstarts do not nest there at all.
Hebblewhite believes that over time, the wolf will prove to have as
powerful an influence on Yellowstone’s ecology as it has on Banff’s-and
that the wolf will have the last word in a long-standing argument among
ranchers, hunters, and wildlife scientists over the best way to manage
grazing animals like elk and moose. “Restoring predators in the areas that
you can is the easiest solution,” he says. “It’s the best way of achieving
some measure of ecological integrity.”
Some argue that human hunters should control elk populations. But hunting
cannot mimic the impact of large carnivores. “Predators select different
prey than do human hunters,” says NWF Senior Scientist Steve Torbit.
People target healthy males, while top carnivores take old, young and
weakened animals as well as females. “It’s the females that control
population growth,” says Torbit. “Even where elk are hunted, populations
can continue to grow, and we are beginning to see a loss of healthy aspen
groves outside of national parks in the West. Wolves are the missing
component.”
Research results emerging from both Yellowstone and Banff are particularly
timely today. In April, the U.S. Fish and Wildlife Service, declaring its
gray wolf recovery program a success, changed the predator’s status from
endangered to threatened everywhere but in the Southwest. The agency also
said it would soon begin the process of removing the wolf from the
Endangered Species List altogether as well as abandon efforts to restore
the animals in geographic areas such as New England. Once found throughout
the United States, gray wolves today are thriving only in Alaska and the
Northern Rockies and Great Lakes regions. The plan has drawn criticism
from conservation organizations, including NWF.
Even fully protected by law, wolves remain vulnerable. In Yellowstone and
central Idaho, young adults raised in reintroduced packs have begun to
travel, looking for new places to live. Many of these pioneers will end up
being killed or relocated when they wander into areas where people live.
Despite growing evidence that wolves play a vital role in healthy
ecosystems, resistance to the predators’ presence on many ranch lands and
urban fringes remains intense. “It’s tough to find places that are wild
enough to hold wolves,” says Smith.
California writer Sharon Levy saw and heard wild wolves for the first time
when she visited Yellowstone National Park to report this story.