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Biologists have long recognized the power of predators in ecosystems. In the 1930s, Aldo Leopold, who advocated wolf extirpation early in his career, began to realize that the killing of predators had helped create what he called "the modern curse of excess deer and elk." In 1980, ecologist Robert Paine coined the term "trophic cascades" to describe the ripple effects of predators on herbivores, and herbivores on plants. Researchers continue to investigate and debate exactly how trophic cascades operate, but they find these so-called top-down effects at work throughout the natural world: Predators ranging from mountain lions to otters to sea stars have dramatic impacts on the ecosystems they inhabit.
In 2006, Eisenberg began her Ph.D. research with William Ripple, an Oregon State University professor who studies trophic cascades in Yellowstone National Park and elsewhere. In 2000, he and his colleagues published evidence of what had been dubbed the "ecology of fear"—that predators altered ecosystems not only by killing prey, but also by scaring it. In Yellowstone, nervous elk became less interested in eating and more interested in moving, apparently allowing more young willows, cottonwoods and aspens to sprout in some places.
Eisenberg has spent the past four years gathering data for a dissertation on the effects of wolves on elk, aspen and songbirds in Glacier and Waterton Lakes national parks. It's a demanding study that has brought her face-to-face with wolf dens, wolf kills and, of course, wolf scat. In 2007, Eisenberg spoke about her work and about trophic cascades at a Boone and Crockett meeting—to an audience that included Paul Vahldiek.
Vahldiek wasn't the only one seeing trouble in the aspen stands: Foresters throughout the Rocky Mountains had reported unusually rapid and widespread aspen die-offs, and, like Vahldiek, they'd noticed that young trees were scarce. By 2006, close to 150,000 acres of Colorado aspen were dead or damaged, according to aerial surveys. By 2008, the apparent peak of the die-off, the damaged areas exceeded half a million acres, with 17 percent of the state's aspen showing declines.
Researchers blame the die-off—now known as Sudden Aspen Decline, or SAD—on a combination of culprits, including insects and diseases emboldened by drought and higher temperatures. But hungry elk, which love to munch on tender aspen shoots, may also play a role in the trees' troubles, both recent and long-term. As Vahldiek listened to Eisenberg, he began to wonder if the aspen on his ranch could use a few more predators.
Eisenberg and Vahldiek struck up a conversation, and Eisenberg was intrigued. She was working on a book about trophic cascades, and was especially interested in the conservation of predators on private lands. After she visited the ranch, Vahldiek asked her to propose a study of aspen, elk and predators on his property. He and his business partners said they were interested in serious science, and willing to fund it.
Though Eisenberg knew of the recent wolf sightings, both rumored and confirmed, in Colorado, she assumed the animals were transients. But as she spent more time on the ranch, and as she and her field crew started searching its meadows and aspen stands for scat, she realized Colorado might already have a new resident predator.
It's easier for a wolf to get from Yellowstone to Colorado than it might sound.
"Wolves are just driven to travel," says Douglas Smith, the Yellowstone wolf biologist. "For them, it really isn't a big deal."
While wolves are wary of humans, they are able to pass through developed landscapes—even, apparently, the ranchlands and gas fields of southern Wyoming. Single wolves, or small coalitions of two and three animals, regularly strike out in search of unoccupied territory.
The risks are high, as the deaths of the two radio-collared wolves in Colorado demonstrate. But the potential re-wards—wide-open territory, abundant prey—are enormous. Even journeys of hundreds of miles "aren't in any way eyebrow-raising," says Smith.
So no matter what left the scat and tracks on the High Lonesome Ranch, wolves are likely to keep venturing into Colorado. Wolves from Idaho and Montana began showing up in eastern Oregon and Washington at least a decade ago, and now both states have breeding pairs of wolves. Utah has confirmed six sightings since 1994, but no evidence of breeding wolves.
The wolf populations in Idaho and Montana, along with wolves in eastern Washington, eastern Oregon and northeastern Utah, were taken off the federal endangered species list last April. But wolves that wander into Colorado are considered endangered species, and their management is led by the U.S. Fish and Wildlife Service. In 2004, a working group of livestock producers, wildlife advocates, scientists, sportsmen and others appointed by the Colorado Division of Wildlife developed a management plan, focused on transient wolves and on the state's responsibilities once its wolves are removed from the endangered species list. The group recommended that the state allow wolves to live where they find habitat, and permit a variety of measures—including, in some cases, lethal methods—to deal with problem wolves.
But before wolves could be delisted in Colorado, a population would have to meet recovery goals set by the U.S. Fish and Wildlife Service. Such goals don't even exist yet, and are unlikely to be considered unless and until evidence of breeding wolves emerges. "We haven't talked about what a Colorado [recovery] plan might look like," says Ed Bangs, the Western gray wolf recovery coordinator for the U.S. Fish and Wildlife Service. His agency, he adds, has "no plans for active recovery in Colorado, no active discussion to put wolves there, take them out of there, do anything with them."