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A Top Predator Roars Back
Ten years ago, the gray wolf returned to Yellowstone National Park, and the place is already looking a lot better -- at least to biologists

Photo of a wolfThough the experience would terrify some people, nothing pleases Doug Smith more than to get lost in the deep woods of Yellowstone National Park within earshot of howling wolves and then come across a freshly killed elk. For Smith, a wildlife biologist in charge of the Yellowstone Wolf Project, this bit of carnage only offers further evidence that the return of the region's top predator has sparked a startling resurgence among Yellowstone's flora and fauna.

Since the reintroduction of wolves to Yellowstone nearly 10 years ago, biologists have seen an increase in animals that feed off wolf kills and the revival of several tree species. This ripple effect, known as a trophic cascade, has been documented many times in aquatic ecosystems. If the evidence in Yellowstone can be definitively linked to the reintroduction of wolves, says Smith, it would be one of the clearest demonstrations yet of a top predator's restoring a terrestrial ecosystem. "This is the Superbowl of biology," says Smith. "If we can show that ecosystems are connected top to bottom like this, that's big."

Gray wolves once roamed across large portions of North America, but the spread of ranching and hunting brought them into direct conflict with that other alpha predator, humans. Wolves had been virtually eradicated from Yellowstone by 1880, eight years after the opening of the national park. When their numbers rebounded soon after the turn of the century, the park administration undertook an active program to eliminate what it termed "a decided menace to the herds of elk, deer, mountain sheep, and antelope." Sometime in the late thirties or early forties, the last wolf pack disappeared from the park. All that remained were a few scattered sightings, which continued into the 1970s.

As wolves declined, elk populations increased prolifically. By the early 1990s, there were close to 20,000 elk in the northern end of the park and the surrounding forests -- the largest number ever recorded.

But many experts say the numbers are less important than the changes in elk behavior. Without wolves to keep them on guard, the animals were free to graze anywhere and on such delicacies as young aspens. "Aspens stopped generating into large trees in the 1920s," says Bill Ripple, a professor in the department of forest resources at Oregon State University. "If they keep disappearing without regenerating, nearly all the aspen trees could be gone from the northern range in 40 to 50 years."

Others noticed similar connections between elk grazing and the disappearance of willow and cottonwood trees, although climate may account for some of the change. One study found that no new cottonwood stands had grown in the northern end of the park for 120 years. Without branches and small trees to use for their dens, beavers abandoned streams in the north of the park, and migratory birds such as yellow warblers may have been forced to bypass the northern range in search of areas with better nesting sites. Without beaver ponds to slow their course, and without healthy vegetation to anchor their banks, in just a few decades many streams washed away nutrient-rich sediment laid down over thousands of years.

That all began to change in 1995 and 1996, when 31 gray wolves were let loose in Yellowstone. After just a few years, scientists observed the impact beyond the occasional elk brought down by the growing packs. Smith remembers walking out to the wolf pens before reintroduction and crossing patches where elk had grazed the vegetation to bare earth. Today, with more than 200 wolves roaming the nearby forests, Smith says those areas are covered in head-high thickets of young trees in which he could easily get lost.

Despite Yellowstone's surprising rebound, some biologists believe that it is still too early, and overoptimistic, to hail the return of a top predator as a panacea for an entire ecosystem. Even the strongest supporters of the reintroduction admit that analysis is complicated by such factors as six consecutive years of drought. Others point out that there are still large numbers of elk in the park -- except now they are not grazing in the open any more. As a result, they may be having negative impacts in places where no one has yet looked.

"The restoration is so young that people are very excited about it, and everyone wants to look at the effects that are positive," says David Mech, a senior scientist at the U.S. Geological Survey who has studied wolves from Yellowstone to the Arctic. "The way nature sees it, there is no positive or negative. All we're seeing is a shifting of the balance. Only humans put value on that."

Ultimately, however, the debate over whether the wolves are truly driving the changes in Yellowstone or simply playing a bit role in the park's larger dynamic may point to a much more fundamental hole in our knowledge. And this, almost everyone agrees, is the message carried by the howls echoing across Yellowstone's northern range. "Ecosystems are not only more complex than we think," says Smith, quoting the famed naturalist Frank Egler, "they're more complex than we can think."
-- Ken Kostel

Photo of a rainforest

For two decades, William Laurance of the Smithsonian Tropical Research Institute, along with a bevy of American and Brazilian scientists, kept a log of some 14,000 trees across about 115 square miles of untouched Amazonian rainforest. Their data showed a disturbing trend: The cycle of tree growth, death, and the emergence of new species is accelerating, with many of the new canopy trees thriving at the expense of smaller, old-growth trees. Reporting their findings in Nature, the scientists point to climbing global carbon dioxide levels as the cause and worry that a ripple effect could crowd out other flora and fauna critical to the ecosystem.

Photos: top, Corbis; right, Photodisc

OnEarth. Summer 2004
Copyright 2004 by the Natural Resources Defense Council