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 the landscape, matted the tawny grass. The wolf sign intensified the farther we ventured.
The wolf trails all led into a large lay. My daughter Alana, in the field with me that day, photographed a bull elk skull. It had been there so long its cranial sutures had gone mossy and the scarlet leaves of a frost-singed geranium had sprung through an eye socket. Later I would be struck by the lyrical beauty and symbolism of that image—the juxtaposition of life and death it contained and which coursed through the meadow like a leitmotif. This juxtaposition had shaped trophic cascades ideas from the beginning.
People have been noticing trophic cascades for centuries. As early as 2,500 years ago the Chinese noted cascading top-down effects and promoted the use of predators to lessen crop damage. In their orchards Chinese farmers established nests of predatory ants to reduce numbers of caterpillars and large boring beetles. in The Origin of Species, Charles Darwin documented an interactive food web, comparing the height of fir trees in fields grazed by cattle with those of firs in fields without cattle. He found that grazing cattle completely prevented forests from becoming established. In another case study he linked the relationship between predator presence (domestic cats), lower prey (mice) populations, and an increase in bee populations (whose hives the mice plundered).
Early British ecologist Charles Elton noticed these patterns, and they inspired his groundbreaking book Animal Ecology, first published in 1927. In it he described ecology as "scientific natural history" and presented concepts we have come to know as its basic tenets: food chains, the role of size in ecological interactions, niches, and the food pyramid. Influenced by Darwin, he depicted nature as an integrated economy in which food web members exchange energy. Some managers of that era, who were grappling with disease outbreaks, species invasions, unstable game populations, and degraded ranges, welcomed this community ecology perspective.
Elton conceptualized the food chain with predators at the top, followed by herbivores, and vegetation and simpler organisms at the bottom. As food chains crossed and connected in various ways they formed complex food webs, which could be seen as a map of trophic activity. In arctic systems the web would be simple; in the species-rich forests of the humid tropics the web would be vastly more complex. Today ecology continues to be built on Elton's model and on the exchange of energy across trophic levels.
Aldo Leopold, who was a close friend of Elton, observed how these ideas played out in the real world. He documented widespread sharp increases in North American populations of ungulates from the 1920s through the 1940s. The first to apply the term irruption to this phenomenon, he identified the irruption sequence as removal of a top predator, then release from predation of its herbivore prey, which leads to an increase in prey numbers, followed by overbrowsing and overgrazing—what today we call a three-part trophic cascade. He also noted cascading trophic interactions driven by wolves in Mexico's Sierra Madre Occidental, contrasting that with the extensive deer and elk irruptions throughout North America in areas where wolves had been removed, such as Arizona's Kaibab Plateau. In Mexico the intact flora, fauna, and watershed he explored harbored an abundant, but not excessive, deer herd thriving among abundant populations of all species of large carnivores present historically (e.g., grizzly bears, Ursus arctos; cougars,Puma concolor; and wolves). In his hunting journal he expressed astonishment that this was the first time he had observed land that wasn't sick, what he referred to as a "biota in its aboriginal health." But beyond recognizing food web dynamics and their significance, he identified ecologists' responsibility to utilize these emerging ideas to help solve resource management problems. He advised his wildlife students at the University of Wisconsin, "To keep every cog and wheel is the first precaution of intelligent tinkering.
Leopold's observations contributed to a debate igniting in the scientific community in the mid-1940s about the relative role of predation in shaping communities. In 1960 a landmark paper by ecologists Nelson G. Hairston, Frederick E. Smith, and Lawrence B.
Slobodkin, collectively known as HSS, created a flash point. They argued that the world is green because predators limit herbivore populations (top-down control). At the time they advanced what became known as the green world hypothesis, the scientific community commonly accepted that vegetation-driven (bottom-up) processes were the primary forces structuring populations. Since the HSS paper was published, researchers have been looking for and measuring patterns of resource use by predators and their prey. The earliest trophic cascades research was fueled by excitement and synecdoche : meticulous observations and experiments that held bigger truths.
The debate continues, exacerbated by these interactions' tangled architecture. Today most scientists agree that trophic cascades occur. According to marine ecologist James Estes, the argument now focuses more on where and why they occur. For example, Yellowstone researchers William Ripple, Robert Beschta, and others suggest that since the reintroduction of wolves in the park in the mid-1990s, cascading effects have included a reduction in the elk population, changes in herbivory patterns, a reduction of mesopredators (e.g., coyotes, Canis latrans), and recovery of woody browse species. Wolves recolonized Banff National Park on their own. Here Mark Hebblewhite and colleagues found the above associations and an increase in biodiversity, which included songbirds and beavers (Castor canadensis) However, in the similarly recolonized upper Midwest these effects are not widely evident, other than the trophic cascades observed in Isle Royale National Park by Brian McLaren and Rolf Peterson, possibly due to extensive human modification of this ecoregion. These examples illustrate that because ecosystems are highly dynamic and complex, generalizations about trophic cascades are difficult, making their application to specific management issues more challenging.
Ecologist Mary Power is well known for her studies of freshwater trophic cascades. In her 1992 landmark synthesis of top-down and bottom-up forces in food webs she argued that while top-down effects often prevail, one must consider bottom-level (plant) productivity and its potential for dynamic feedback effects on adjacent and nonadjacent (herbivores and predators) trophic levels. Power found that both top-down and bottom- up forces prevail in different settings and urged researchers to consider how increasing plant productivity in all types of systems might affect predator-prey interactions. She noted that disturbances often make ecosystems more productive and thus can further alter these relationships.
Since the 1990s ecologist Oswald Schmitz has been investigating trophic cascades involving grasshoppers and spiders. One of his objectives has been to determine whether top-down or bottom-up forces dominate. By manipulating predator presence and other variables, he found both forces present, with the former the strongest. He discovered that regardless of the strength of trophic cascades, they have important effects on plant community composition, with deep implications for biodiversity and ecosystem function.
In aquatic systems, perhaps the best-known and most dramatic example of a trophic cascade involves Estes' work in the Aleutian Islands, where he and colleagues investigated the recovery of sea otters (Enhydra lutris), the associated reduction in their sea urchin (Strongylocentrotus spp.) prey, and subsequent recovery of kelp forests (Laminaria spp. and Agarum cribrosum). Because kelp forests are highly productive components of rocky marine coastlines and support diverse communities, this otter-driven cascade has high conservation relevance. A more complex, multilayered food web occurs in South Pacific coral reefs. Here fishing by humans has been linked with a decrease in predaceous fish, an upsurge in the sea urchin population, and a reduction in biodiversity. But these relationships are not as linear as in other systems. Consequently what occurs here can be thought of as a "trophic trickle"—what happens when a three-level trophic cascade's potentially strong effects become diluted across multiple intertwined levels.
Trophic interactions such as these tell a story made all the more compelling by the breadth of our still incomplete understanding of community dynamics. It's a story that bears telling for the lessons it can teach us about sustaining the richness of life in all its forms. Even as we strive to expand our knowledge of species interactions, biodiversity is decreasing at an alarming rate. One-fifth of all the bird species in the world have gone extinct in the past 2,000 years, and 11 percent of the remaining 9,040 known species of birds are endangered. The number of plant and animal species on the earth is declining at a rate 1,000 to 10,000 times higher than in prehistoric times. Extinction estimates vary, depending on researchers' methods, with most predicting a loss of approximately 7 percent of species globally per decade. Biodiversity loss has become a crucial issue since the late 1990s as human-caused ecosystem modifications continue to precipitate extinction. Trophic cascades increase plant growth; the resulting energy surge drives nutrients across ecosystems, with significant effects that include an increase in biodiversity.
Like all aquatic and terrestrial landscapes, Johnson Meadow is a palimpsest written over for millennia by its wildlife and human inhabitants. Landscapes speak to us through these marks. Trophic cascades research involves interpreting historical marks as well as current wildlife movements and trends. To the north of the lay where the wolf trails converged in the meadow stood an aspen stand atop a low knoll. It contained a smattering of conifers. As we entered this leafy glen on a wolf trail, we found a cache of purloined toys that suggested the den was nearby: plastic soft drink bottles pocked by pups' sharp teeth, frayed strips of fiberglass insulation, and a thoroughly gnawed road sign. A shrub and sapling thicket rendered the trail nearly impassable to humans and hid the den from view until we stood directly in front of it.
Relatively unchanged since I'd last seen it, the den nestled into the root-ball of an ample spruce. On the bare earth around it we found wolf hair and pup scat, bones, ungulate hair, and several wolf beds. It had three wide openings, set flush into the ground, and was rather large, as wolf dens go, because it had been used for many consecutive years. A faint but unmistakable fetor clung to it. Just inside the shallowest opening I found the culprit: a dead juvenile spotted skunk. The bedraggled creature bore a few chew marks and a small pile of pup scat tellingly deposited on its back.