Patterns in an Ecosystem

Webushwhacked through an old burn at first light in a cold September rain mixed with snow, slipping on the blackened bones of downed lodgepole pines. It had been three years since I'd come this way, past a curving reach of the Flathead River and the old ranger station, through a locked gate, and into a vast, fecund meadow a few miles south of the US-Canada border in Glacier National Park. As steeped in ecological history as Yellowstone National Park's famed Lamar Valley, but far less known, the meadow lay beyond the burn, although my field crew and I couldn't see it yet. It held ecological stories plainly told as patterns in an ecosystem, which we were there to record.

When wolves (Canis lupus) recolonized northwestern Montana in the 1980s they chose Johnson Meadow, a secluded opening in a lodgepole sea, as their first home. In 1986 renowned wolf biologist Diane Boyd, then a graduate student, confirmed the first denning activity here after a sixty-year human-imposed wolf absence. Glacier National Park administrators keep this place closed to the public but occasionally allow researchers in—and then only when the resident Dutch pack, which is radio-collared, travels away from the den. Trouble is, the wolves seldom leave, lingering at the den site until long past spring whelping season, feeding on the abundant deer (Odocoileus spp.) and elk (Cervus elaphus) with which they share the meadow. When they do leave they tend to travel one or two miles from the den, remaining in the general vicinity to hunt or rest with their pups at areas called rendezvous sites.

I had last visited this den on a benign autumn day when the aspens blazed like souls on fire against a deep blue sky and thistledown floated on the wind. I had been helping with a study of how wolves select their den sites. Now I had returned to conduct research of my own: a study of trophic cascades involving wolves, elk, and aspens (Populus tremuloides) in the Crown of the Continent Ecosystem. This ecosystem spans the US- Canada border, one of two in the lower forty-eight coterminous states that contain all species present at the time of the Lewis and Clark expedition.

I had chosen to focus my research on the aspen because, although it is the most widely distributed tree species in North America, it has been declining in large portions of the intermountain West since the 1920s. Aspens reproduce clonally, sprouting from extensive root systems, and provide critical habitat for diverse species of wildlife and plants. They offer the richest songbird habitat, second only to the interfaces between streams and land, called riparian zones. Because aspens can support such profligate biodiversity, their decline has created pressing research and conservation needs. I had chosen to study elk because their impacts on aspens are greater than those of other hooved animals (called ungulates). The steepest aspen declines have occurred in areas of elk winter range, linked to predator removal and influenced by disease and climate variability.

Trophic cascades refers to the relationships among members of a biotic community : predators, prey, and vegetation. In 1980 marine ecologist Robert Paine coined this elegant term to describe this interaction web. These cascading, predator-driven, top-down effects have been reported in all sorts of ecosystems, from the Bering Sea to rocky shores to montane meadows. As in all of these systems, the fundamental three-level food web I studied indirectly touched many other members of the biotic community, which in this case included songbirds.

Rooted in flesh-and-blood encounters between predator and prey, trophic cascades involve passage of energy and matter from one species to another. Each act of predation subsumes one life so another can continue. Predation can have strong direct and indirect effects in food webs, making nutrients such as nitrogen flow through ecosystems, with significant consequences for community ecology. Wildlife corridors, such as the one I was working in, are characterized by heightened species interactions and nutrient flow. They provide natural laboratories where ecologists can learn much about trophic cascades. In 1935 preeminent American wildlife ecologist Aldo Leopold noted how predators help increase species richness and how their presence affects everything from prey to plant communities. He eloquently wrote about these relationships and the lessons he had learned from them about ethical resource management in his book A Sand County Almanac. Current trophic cascades research is adding to our awareness of these relationships. My time in Johnson Meadow was part of my effort to elucidate these dynamics.

A few feet into the lodgepole jackstraw we came upon the first wolf scat—two inches in diameter, oxidized white, filled with ungulate hair, a bold territorial marker left in a well-worn path. Generations of wolves circling the meadow and then arrowing into it had made this path as their tracks homed into the den area. The trail sped our passage through the old burn, our feet finding easier purchase where so many wolves had trod. The burn stopped abruptly at the meadow, which remained wet and marshy in some spots year-round and thus had been singed only lightly. We soon entered a network of other wolf trails that wove through tussocks of tassel-topped fescue and fireweed gone to seed, taking us deeper into the meadow.

Covering approximately ten square miles, Johnson Meadow held five large aspen stands and a long-abandoned homestead, now little more than a few boards weathered silver and a midden heap in a damp declivity. Low-lying glacier-smoothed mountains rimmed the meadow. Anaconda Peak's rocky southern face rose sharply above a series of soft green ridgelines that faded into the north. To the south Huckleberry Mountain's rounded bulk breached a fog bank, its shoulders a mosaic of burned and unburned patches. It had provided first-rate grizzly habitat until a recent fire took out much of the timber and berries. The meadow curled east, revealing its full expanse and secrets gradually. And indeed, given that few humans were allowed to enter and it was completely hidden from the road, it felt like a secret meadow. It seemed no less primordial and wild than I recalled. Bones everywhere: deer, elk, and moose. Strategically situated lays, places where the pack had rested and perhaps surveyed...