Fifteen years ago, with two daughters barely out of diapers, Eisenberg and her husband, a software designer, moved from coastal California to a remote property in northwestern Montana. Eisenberg, whose father worked in the Mexican diplomatic service, had a cosmopolitan childhood -- in Tokyo, Toronto, Vancouver and elsewhere -- but she had always been drawn to cold places with mountains, big trees and what she calls "real weather." An artist by training, Eisenberg put aside her watercolors to raise her children, and in Montana, she learned about the natural world alongside them. "They always wanted to know, 'What's that bird? What's that tree?' " she remembers. "They were my teachers." In a landscape with large predators, they protected themselves by learning to recognize tracks; wildlife field guides were among the girls' first books.

In the mid-1990s, like many other local residents, the family started seeing what they thought were wolf tracks, and hearing howls that didn't sound much like coyotes. Eisenberg reported her observations to wildlife officials, who were polite but dismissive. "I was a housewife with two kids in tow," she says. "I didn't look much like a credible source."

But wolves from Canada were, in fact, recolonizing the area, and by 1996 there were an estimated 75 wolves in northwestern Montana. On the 20 acres where Eisenberg and her family live, a metamorphosis began. "In three or four years, everything changed on the land," she says. "Deer were no longer standing around and eating all day -- they were on the move. Plants that had been shrubs were suddenly six feet tall." By 2005, a three-acre meadow near their house had been overtaken by cottonwoods, conifers and shrubs such as serviceberry and wild rose.

As her daughters grew older, Eisenberg began to search for a new career, and in 2004, she enrolled in an environmental studies graduate program at Prescott College. The transformation of her backyard fresh in her mind, she focused her studies on wolves and their ecological roles.

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 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.