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Yellowstone Willow Population Declining
PROVO, UT -- The debate over how best to manage the deterioration of the once robust Yellowstone willow has become a major concern for Rex Cates, a Brigham Young University professor who has spent 11 years investigating the cause of the decline.
Cates, a chemical ecologist at BYU, 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, important factors in the park's ecosystem.
His research into the willows' decline has been met with debate from critics who feel that Yellowstone's hands-off policy of natural regulation prohibits human intervention in assisting America's most famous national park.
But Cates' concern is that if the declines continue, the once beautiful willow habitats of Yellowstone may turn into a dry, arid meadow.
"The willows used to cover the whole meadow," said Cates, referring to a photograph taken on his last research expedition to Yellowstone last summer. "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 as a means of comparing the current population to the past population, 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 have suffered losses in other ways. The plants, which normally reach a robust height of 15-20 feet, now stand only 2-3 feet high in this part of Yellowstone National Park. The willows also have shown reduced levels of chemical defenses, known as phenolics and tannins. These weakened defenses make these plants more palatable, and consequently, more vulnerable, to browsing animals such as elk and moose.
This deterioration in the health of the willow has forced Cates and his colleagues to look at the factors contributing to the willow declines.
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 said.
A smaller beaver population has also 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 explained. "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. That is exactly the case in the northern winter range." Elk levels in Yellowstone have also increased because of a loss of predators and the park's natural regulation policy. The result of the increased population of elk has been an overgrazing of the willows. This sustained feeding on the willows has impeded these plants' ability to grow to their natural levels and has contributed to the weakening of the willows' 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?' said Don Despain, an ecologist working on similar problems with aspens for the U.S. Geological National Survey. "Should Yellowstone 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," said 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. " Cates said.
Wayne Brewster, Deputy Director of The Yellowstone Center for Resources, has followed Cates' work. "This research into the decline of the willows population has been key in helping to understand a very complex situation," Brewster said. "This piece of the puzzle is very important in helping the Yellowstone willows."
Cates agrees that the delicate period is far from over. He is, however, determined to stick with it.
"Regardless of what the end results may be, I will work as hard as I can to help Yellowstone National Park retain its grandeur."
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