CORVALLIS, Ore. - The reintroduction of wolves into Yellowstone National Park may be the key to maintaining groves of cottonwood trees that were well on their way to localized extinction, and is working to rebalance a stream ecosystem in the park for the first time in seven decades, Oregon State University scientists say in two new studies.
The data show a clear and remarkable linkage between the presence of wolves and the health of an entire streamside ecosystem, including two species of cottonwoods and the myriad of roles they play in erosion control, stream health, and nurturing diverse plant and animal life.
The findings of these studies were recently published in Ecological Applications, a journal of the Ecological Society of America, and the journal Forest Ecology and Management.
"In one portion of the elk's winter range along the Lamar River of Yellowstone National Park, we found that there were thousands of small cottonwood seedlings," said Robert Beschta, professor emeritus in the College of Forestry at OSU and an expert on streams and riparian systems. "There should also have been hundreds of young trees, but there were none. Long-term elk browsing had been preventing any seedlings from getting taller."
That pattern was common throughout the study area - lots of seedlings in combination with large cottonwood trees generally more than 70 years old, but little or nothing in between.
Young cottonwoods, willows, and other streamside woody species are a preferred food for browsing elk during the harsh winters in northern Yellowstone, when much of the other forage is buried under snow. But when packs of wolves historically roamed the area, food was not the only consideration for elk, which had to be very careful and apparently avoided browsing in high-risk areas with low visibility or escape barriers.
Wolves were systematically killed in the Yellowstone region and many other areas of the West beginning in the late 1800s. A concentrated effort between 1914 and 1926 finished the job - the last known wolf pack disappeared in 1926.
"I considered a variety of potential reasons that might explain the historical decline of cottonwoods that began in the 1920s and have continued up to the last couple of years," said Beschta. "I looked at climate change, lack of floods, fire suppression, natural stand dynamics, and numbers of elk. But none of those factors really explained the problem.
"Ultimately, it became clear that wolves were the answer."
While elk populations fluctuated over the decades when wolves were absent, browsing behavior appears to represent an important factor related to streamside impacts. With no fear of wolves, the elk could graze anywhere they liked and for decades have been able to kill, by browsing, nearly all the young cottonwoods. Other streamside species such as willows and berry-producing shrubs also suffered.
That in turn began to play havoc with an entire streamside ecosystem and associated wildlife, including birds, insects, fish and others. Trees and shrubs were lost that could have helped control stream erosion. Food webs broke down.
"Before the wolves came back, it was pretty clear that in some areas we were heading towards an outright extinction of cottonwoods," Beschta said.
Now, with the recent reintroduction of wolves back into Yellowstone in 1995, streamside shrubs and cottonwoods within the Lamar Valley are beginning to become more prevalent and taller, and were the focus of a second study in the same area. That study outlines how the fear of attack by wolves apparently prevents browsing elk from eating young cottonwood and willows in some streamside zones.
With the renewed presence of wolves, young cottonwoods and willows have been growing taller each year over the last four years on "high-risk" sites, where elk apparently feel vulnerable due to terrain or other conditions that might prevent escape. In contrast, on "low-risk" sites, they are still being browsed by elk and show little increase in height.
"In one case where a gully formed an escape barrier for elk, the tree height went up proportionally as the gully deepened and formed an increasing barrier to escape," said William Ripple, a professor with the College of Forestry at OSU. "Where the fear factor of wolves is high, the young trees and willows are doing much better and growing taller."
Traditionally, "keystone" predators such as wolves were known to influence the population of other animals that they preyed on directly, such as elk or antelope. What researchers are now coming to better understand is the "trophic effect," or cascade of changes that can take place in an ecosystem when an important part is removed, Ripple said.
The comparatively pristine conditions of a national park allowed this type of research to make "cause and effect" studies more feasible, the scientists point out.
"The removal of wolves for 70 years - and then their return - actually set the stage for a scientific experiment with fairly compelling results," Beschta said.
In a larger context, the studies also raise valid questions about other complex and poorly understood interactions between plants, animals, and wildlife in disturbed ecosystems across much of the American West, and perhaps elsewhere in the world, the scientists say. In some areas of the West, the disappearance of up to 90 percent of the aspen trees has been documented - another species of plant that is also highly vulnerable to animal browsing when it is young.
"The last period when aspen trees in Yellowstone escaped the effects of elk browsing to generate trees into the forest overstory was the 1920s," Ripple said, "which is also when wolves were removed from the park."
But in at least one place - America's first national park - there is now cause for hope. While it is too early to confirm the widespread recovery of cottonwoods and willows, the reintroduction of wolves appears to have put a stop to major declines in the survival of these plants, the researchers found.
"One point that should not be missed is this is actually great news for the potential recovery of cottonwood trees and mature willows in Yellowstone National Park," Ripple said. "We now have a pretty good idea why they were in decline and the return of wolves should help pave the way for their recovery.
"Even though it may take a very long time, for a change it looks like we're headed in the right direction."