Century Without Wolves Alters Olympic Ecosystem
by Carrie Nugent, The Oregonian
Wednesday July 30, 2008, 9:23 AM
Wolves are experiencing a resurgence in the Northwest after being all but wiped out early last century. Last week, the first reproducing pack of wild wolves in decades was confirmed by biologists in northeast Oregon.
The extermination of wolves in the United States was a source of national pride in the early 20th century. Biologists thought livestock would be safe from marauders and game such as elk would flourish, leaving a bounty for hunters.
But the consequences would ripple through the decades, affecting ecosystems in ways no 1920s wolf-hunting government biologist could have foreseen — including altering rivers in Olympic National Park in Washington.
Heavy elk grazing has transformed steep-sloped rivers densely bordered with vegetation into shallow, sediment-heavy flows, Oregon State University’s Robert Beschta and William Ripple report this month in the journal Ecohydrology. These changed rivers may affect fisheries and local highways.
The chain of events that results from the loss of a top predator is called a trophic cascade. “People are studying ecosystems all over the world, from the arctic to the tropics, from the deepest sea to the highest mountains, and we think that this could be a fairly universal phenomenon,” Ripple said.
However, trophic cascades in the Olympics came as a shock. “We’ve been chasing these trophic cascades around the country,” Beschta said, “and we get into our backyard and we find it’s right there also.”
The research showed that when the wolves were extirpated from the park in the early 1900s, the elk population increased sharply. That, in turn, caused overgrazing. By the 1930s, elk were dying of starvation.
Elk numbers stabilized in the 1950s, but plant communities did not recover. Few black cottonwood or bigleaf maple trees have since reached maturity, the OSU researchers found, because nibbling elk can stunt or kill saplings, limiting the number of new trees.
Additionally, studies in Yellowstone National Park have shown that when wolves are present, elk will alter their browsing habits. They avoid areas without quick escape routes — such as streambanks. With the wolves gone, streambeds became a foraging free-for-all.
“These shrubs and trees hold the bank in place through their roots,” Ripple said. When they are gone, the rushing water erodes the banks, carrying off sediment and widening the channel. And unlike their deep, steep-banked counterparts, wide, shallow rivers can easily change course — sometimes into the path of roads.
“Now you’ve got to figure out what you’re going to do with the road — are you going to take the road out? Move it?” Beschta said. These are expensive questions.
Additionally, shallow rivers have gravel borders that are “disconnecting the stream from the forest,” Beschta said.
“If I step back 100 years, and I have these narrower, vegetation-lined channels — even during low flows the river would have been running along a vegetated streambank,” full of insects and plant litter. The insects are food for the fish, he said, and the litter feeds organisms that become food for fish.
Although the researchers did not study fish populations in Olympic National Park, a similar study in Zion National Park in Utah showed that “when there was a predator present it appeared that there were more native fish in the stream,” Ripple said. In Olympic, there’s a “big sockeye run in that system that’s been going downhill,” Beschta said Whether it is a result of a trophic cascade or other factors needs more study, Beschta added.
The effects could travel all the way downstream. Wider rivers move more gravel, sediment and silt. “That sediment will eventually end up in the ocean,” Beschta said.
However, it is hard to draw firm conclusions from ecosystems as complicated as those in national parks.
“It’s always difficult doing these retrospective historical studies because you can’t always fill in the key information that you need,” said Dave Peterson, a research biologist at the U.S. Forest Service Pacific Northwest Station. It’s clear, though, that “the elk have a tremendous impact on the forest,” he said, which today has “big trees with grassy, small shrub understory because they’ve just munched everything.”
Although it might not be feasible to reintroduce large predators to many major parks, Beschta and Ripple hope their research leads to more informed decisions. “When all the parts are present, we think there is more resilience to disturbance, for example, if there is climate change coming,” Ripple said.
“We are on the beginning of something,” Beschta said. “Where it goes — the experiment has to play out.”
— Carrie Nugent