By Tim Kennedy
The debate over how best to manage the deterioration of the once-robust Yellowstone willow has become a major concern for Rex G. Cates, a BYU professor who has spent 11 years investigating the cause of the decline.
Cates, a chemical ecologist, has found that fluctuations in water conditions at Yellowstone, compounded by increased elk populations, have caused the decrease in the growth potential and population size of the plants.
His research into the willows’ decline has been met with arguments that Yellowstone’s hands-off policy of natural regulation prohibits human intervention in America’s most famous national park. But Cates is concerned that if the declines continue, the once-beautiful willow habitats of Yellowstone may turn into dry, arid meadows.
“The willows used to cover the whole meadow,” says Cates, referring to a photograph taken on his summer 1998 research expedition to Yellowstone. “Now the area is being overrun by sagebrush. It would be tragic for us to lose such a beautiful part of Yellowstone.”
Using photographic records, other researchers have estimated that there has been a 60 percent decline in the number of willows covering the meadows of the northern winter range. In addition, Cates and his colleagues have demonstrated that Yellowstone’s willows, which normally reach a height of 15 to 20 feet, now stand only 2 to 3 feet high in one part of the park. The willows also have shown reduced levels of chemical defenses, known as phenolics and tannins. These weakened defenses make the plants more palatable and, consequently, more vulnerable to browsing elk and moose.
In an article accepted for publication in the Wildlife Society Bulletin, Cates and his colleagues point to a major decrease in the water levels, combined with a large population of browsing elk, as the main causes of the willow decline.
“The Yellowstone water habitat that once supported the willow population has changed. Increased temperatures have caused lower stream flows, lower water tables, and reduced flooding. The overall reduced availability of water for willows has had terrible effects on these plants,” Cates says.
A smaller beaver population has contributed to the change in the water habitats. “For the past 60 years the beaver population of the northern winter range has significantly decreased,” Cates explains. “Beaver ponds normally influence willow regeneration and persistence, and the abandoned beaver ponds are an excellent reestablishment site for willows. With a very small beaver population, willows will have less ability to grow correctly.”
Elk populations in Yellowstone have also increased because of a loss of predators and the park’s natural regulation policy. More elk have resulted in an overgrazing of the willows, impeding the plants’ ability to grow to their natural levels and contributing to the weakening of their natural defenses.
Although the researchers have been successful in establishing the causes of the willow growth dilemma, the problem is far from over.
“The controversy of how to address the willow decline is centered in the question, ‘What should the park look like?'” says Don Despain, an ecologist working on similar problems with aspens for the U.S. Geological National Survey. “ShouldYellowstone National Park be managed by human influences or by nature itself? The elusive answer to that question is hindering us from helping the willows.”
Cates feels that the natural regulation policy is important, but he admits that if the willows are to be saved, it will require human intervention.
“We’ve found in our studies of browsing animals that when natural controls are removed, like the previous removal of predators in Yellowstone National Park, then man must step in and manage these animals,” says Cates. “The reintroduction of the wolf as a natural predator may help control the elk population and help balance the ecosystem. This could give an advantage to the willows–and may help them survive.”
