High Alert

Large carnivores promote healthy ecosystems by keeping browsers on edge

In their role as top predators, wolves and cougars nurture ecosystems. OSU researchers William Ripple and Robert Beschta have documented this “trophic cascade” in Yellowstone and Zion national parks.

In a remote corner of Zion National Park, a small herd of mule deer browse quietly. Through the sun-dappled canyon burbles North Creek, its waters cool and clear, its banks green and reedy, alive with frogs, butterflies and bird-song. But this pastoral scene in southern Utah has a dark subtext, subtle yet unmistakable in the taut posture of the animals. Their heads come up often, their eyes scan the landscape, their long ears swivel this way and that, alert for the snap of a twig, the crouch of a tawny haunch. Cougars are on the prowl.

Terra Up Close

On Red Owl Mountain

In the rural West, geography defines us. I live with my husband and teenage daughters on the shoulder of Red Owl Mountain, one of the many mountains that make up the Swan Range, which is part of the crazy quilt of ranges that forms the Rocky Mountains in northwest Montana. Read more…

In this “landscape of fear,” the highly vigilant deer move across the terrain quickly. Lingering too long beside rivers and creeks, far from escape routes, is risky. So in a predator-rife ecosystem like North Creek, sparse numbers of deer browse less intensively, leaving more uneaten vegetation behind. OSU researchers William Ripple and Robert Beschta call this behavior “risk-sensitive foraging.” The result — low disturbance to soils and organisms in fragile riparian zones — is a huge boon for the ecosystem, the scientists say. “In our research, we’re seeing a significant behavioral shift among deer and elk resulting from this fear of being preyed upon by large carnivores,” explains Ripple, a professor of forest resources in OSU’s College of Forestry. “We think this vigilant behavior, in which ungulates are constantly alert and moving, may be having a large, positive effect on ecosystems.” As evidence, he points to a second canyon just over the ridge, where cougar are rare. The teeming biodiversity of North Creek forms a stark contrast to Zion Canyon, which is close to the roads, parking lots and other amenities that draw tourists, repel cougars and attract deer. Ripple noticed the startling difference one day when he was standing along the shady streambed with a clipboard, recording data as Beschta called out measurements. He was casually observing the many colorful butterflies — the swallowtails and monarchs, the satyrs, sulphurs and spread-wing skippers — fluttering above gaudy bursts of wildflowers. The life forms around him were rich and abundant. “I was practically stepping on frogs and toads, they were so thick,” Ripple recalls. But nearby Zion Canyon, the comparison site for the study, had a relative paucity of species. At that moment, the professors decided to broaden the focus of their study from trees and streams to species abundance and biodiversity. What they found in Zion Canyon were stream banks badly eroded and largely bereft of such “indicator” species as cattails, scouring rushes, Welsh asters and cardinal flowers. At North Creek they found canyon tree frogs and red spotted toads to be 100 times more common, lizards three times more common, and butterflies five times more common than in Zion Canyon. Even populations of native fish, such as the speckled dace and the virgin spinedace, appeared to be higher in the cougar-friendly ecosystem.

Lords of the Rings

However, it was Zion Canyon’s missing trees — the cottonwoods that typically grow thick along healthy river systems — that reaffirmed for the two OSU researchers what they have been seeing in other areas of the West: the pivotal role of cougars and other big predators, particularly wolves, in maintaining ecosystems.

“A deep chesty bawl echoes from rimrock to rimrock, rolls down the mountain, and fades into the far blackness of the night….Only the mountain has lived long enough to listen objectively to the howl of a wolf.” Aldo Leopold A Sand County Almanac, 1949

The two forestry professors, independently, had long been interested in riparian tree species — cottonwoods, aspen, willow — which are diminishing in Western landscapes. In 1996, Ripple heard a presentation by fellow forestry professor Beschta about the malfunctioning ecosystem in Yellowstone National Park, as evidenced by stressed tree populations. “At that time, I was interested in aspen — not predators, not wolves, just aspen,” Ripple recalls. “Bob talked about aspen trees in Yellowstone becoming decadent, disappearing, dying out. And young ones were not growing to replace the old ones as they died. Entire groves of aspen had disappeared in recent decades.”

The mystery of the dying aspen captured him. So, with his then-Ph.D. student Eric Larsen (now a professor at the University of Wisconsin), he headed to Yellowstone. While he was there, he had an epiphany of sorts — not in the field, but in the visitor center. Hanging high on the wall was a large poster, a magnificent photo of a yellow-eyed wolf standing in a thick grove of healthy aspen. “The wolf’s stance made him look like the guardian of the aspen,” Ripple reports. “I suddenly thought, ‘Hey, wolves protect aspen.’” What he and Larsen found confirmed his hunch — a hypothesis first advanced by the father of wildlife conservation, Aldo Leopold, more than a half-century before: that large carnivores are critical to maintaining healthy ecosystems at every trophic level — that is, every link on the food chain. “We found that aspen tree regeneration has diminished since the 1930s, beginning soon after all the wolves were killed off,” he says. In the recent Zion study, the latest in nearly 10 years of cumulative research supporting their hypothesis, Ripple and Beschta turned up still more evidence. After measuring trunk diameters, taking core samples and counting rings, they found an astounding cottonwood age gap between the two canyons. The number of cottonwoods taking root and growing to maturity after 1940 was 38 times higher in North Creek than in Zion Canyon — 892 trees per kilometer compared with 23 trees per kilometer — as reported in the journal Biological Conservation in December 2006. What happened in 1940 that caused cottonwoods to crash in Zion Canyon? According to the scientists, the precursors to the collapse occurred a couple of decades earlier, causing a chain reaction all the way down the biotic pyramid. This “trophic cascade” — a top-down domino effect in which carnivores affect herbivores, and herbivores affect plant biomass — hinged on the disappearance of the big cats, which were driven to more remote reaches of the park when tourists, drawn by Zion’s sculpted cliffs and canyons, began coming by the busload. Established in 1918, the park saw visitor numbers quickly balloon. In the 10 years between 1924 and 1934, yearly visits grew eight-fold, from 8,400 to 68,800. (In 2006, more than 2.5 million people visited the park.) As crowds became bigger, cougars — notoriously shy, elusive animals — became noticeably scarcer. As early as 1938, park naturalist C.C. Presnall warned of the impending imbalance. He wrote, “Human use of the park was, and no doubt always will be, concentrated in Zion Canyon, causing profound changes in the delicate balance between deer and their natural predators.” Unafraid, the deer in Zion Canyon browsed at their leisure. Young cottonwoods, a mule-deer delicacy, were largely devoured, leaving few seedlings to sustain the groves of gnarled giants, whose leaves quake playfully in the wind, whose branches shelter birds and shade streams, and whose roots anchor soils.

The Packs Are Back

The Zion study, funded by the National Park Service, replicates the mounting body of findings by Ripple and Beschta, now an emeritus professor who has retired from the classroom but not from the field. In study after study, region after region, landscape after landscape, they have found the same ecological benefits from “apex predators” — the big carnivores at the peak of the ecological pyramid. The layered sandstone, limestone and shale of the desert southwest, the craggy timberlands of the Rockies, the vast openness of the Great Plains all share that common ecological mechanism — the trophic cascade. “The signature we’ve been seeing on all these sites,” says Beschta, “is the removal of the top carnivore, causing systems to decline.” This ecosystem unraveling happens not only because the actual killing declines, but also because the fear of being killed diminishes. Fearful foraging, quite simply, spares plants, which provide food, habitat and soil stability for the myriad organisms at the base of the biotic pyramid. In the 1990s, ecologist Joel Brown coined the term “ecology of fear” to describe the phenomenon. These kinds of top-down species interactions have been reported by scientists in diverse ecosystems from the Bering Sea to the tropics.

Yet another recent OSU study points again to that same signature. In South Dakota’s Wind Cave National Park, the scientists discovered that new cottonwoods and bur oaks virtually stopped taking hold in the late 1800s and early 1900s, when wolves, grizzlies and other big meat-eaters were wiped out. “So to the question, Do wolves have a role in maintaining ecosystem integrity?,” Beschta remarks, “the answer is ‘Yes’ in capital letters.” Ripple’s graduate students are continuing to build upon the evidence base laid by Ripple and Beschta. Jeff Hollenbeck, a recent Ph.D. student, evaluated potential trophic cascades effects on aspen condition and cavity-nesting birds in the northern Yellowstone ecosystem. Joshua Halofsky, a landscape ecologist with the Washington Department of Natural Resources, is just completing his doctoral thesis studying the potential link between plant growth and elk behavior in Yellowstone and how the presence or absence of wolves affects that link.

“Herbivore population sizes will be determined by the supply of lions, tigers, leopards, cougars, cheetahs, hyenas and wolves, not by the supply of grass.” David Quammen Monster of God, 2003

Ph.D. student Cristina Eisenberg, who lives in the wilds of Montana (see sidebar), is working with an interagency research team in Montana’s Glacier National Park and Alberta’s Waterton Lakes National Park (an area spanning the U.S.-Canada border known as the Waterton-Glacier International Peace Park) to study elk ecology and habitat maintenance. The team includes researchers from the universities of Calgary and Alberta, as well as OSU. With the help of global positioning system (GPS) radio collars, the scientists will track 50 elk and several wolf packs to learn critical details about how elk forage, travel and interact with humans and wolves. Eisenberg’s role in the $1 million study, funded by Shell Canada and operating with educational, government, industrial and conservation partners, is to investigate trophic cascades involving wolves, elk, aspen and other aspen-dependent species such as beavers and songbirds.

The Apex of Change

In Yellowstone, where the researchers got their first clear glimpse of cascading effects, recent OSU studies suggest that trophic cascades can also work as a powerful means to restore an ecosystem. Wolves have made a comeback there, reclaiming their place in an ecosystem that has declined steeply in their absence — ever since ranchers, hunters and government agents exterminated them in hopes of making rangelands safer for cattle and woodlands more bountiful for human hunters. “Putting those predators back into the system again, like the return of wolves to Yellowstone, appears to reverse the process,” Beschta says. Ripple agrees. But he remains circumspect. “Ecosystems are quite resilient,” he says. “The initial recovery of plants since wolf reintroduction in Yellowstone is impressive and exciting to see. We’re hopeful for a pretty strong recovery in the long run, but it could take many, many decades.”

One Response to “High Alert”

  1. [...] vegetation, reducing the health and biodiversity of both plants and animals. Recently, the absence of cougars in Zion National Park, and the resulting large populations of deer, has been linked to eroded stream banks and a loss of [...]