College of Forestry

Global Trophic Cascades Program

Wolves linked to vegetation improvements

By Amber Travsky
March 18 ,2004, Wyoming Tribune-Eagle newspaper.

YELLOWSTONE NATIONAL PARK - Biologists had a question: what was happening to the aspen in Yellowstone National Park? Although quaking aspen is the most widely distributed tree in North America, its abundance has been shrinking over the past century. This loss has been especially apparent in our nation’s first park, where stands of the white-barked tree, with its trembling leaves and brilliant fall foliage, have decreased dramatically over the last century.

This disappearance is puzzling, especially since aspen grows rapidly and can quickly resprout from its extensive underground root systems or from seeds after fires, floods, and other disturbances.

In 1997, William Ripple, Oregon State University forest ecologist, and OSU graduate student Eric Larsen, set out to unravel the mystery.

“We had several theories on the cause of the decline,” Ripple said in an interview from his Corvallis, Ore., office. “It could be caused by the lack of fire, climate change, or other environmental factors. We weren’t sure.”

To answer the question, the researchers studied tree rings, tree size classes, and aerial photographs of the park. “We took core samples of the aspen to count the rings,” Ripple said. “From that data we developed an age structure for the aspen.”

The results were surprising. Ripple discovered the aspen quit regenerating in the 1920s. For 70 years, young aspen hadn’t faired well, with few surviving to become mature trees. About the same time that the aspen quit regenerating, wolves were eliminated from the park. Ripple said the timing begged the question: Is there a connection?

“We developed the hypothesis that there was some link among wolves, elk and aspen,” Ripple said.

The primary culprit for the loss of young aspen was elk feasting on the sprouts. The elk browsed in the willow bottoms and other open country, leisurely gobbling up tender young trees, shrubs and grasses. With wolves absent, the elk grazed anywhere they liked and for decades have been able to kill, by browsing, nearly all the young aspen. 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 and fish.

Wolves were reintroduced to Yellowstone in the winter of 1995-1996. Today, the park has 250 to 300 wolves, too many to track them all with radio collars. They are no longer classified as an endangered species, but are now "threatened," and, once a dispute between the Fish and Wildlife Service and the state of Wyoming is resolved, they may soon be "delisted" altogether.

Some ten years ago Yellowstone had the largest single elk population in the world. Weighing as much as 700 pounds apiece, they had no serious rivals. Grizzly bears, Yellowstone's top predators, are capable of bringing down an adult elk, but they mainly prey on calves. Coyotes, though numerous, were much too small to attack elk.

"The first thing that happened was that the elk ignored the wolves," said Wildlife Conservation Society senior scientist Joel Berger in a recent interview with Guy Gugliotta of the Washington Post. "The elk were treating 90- and 100-pound wolves like they were 35-pound coyotes,” Berger said. “The elk were naive. They aren't naive anymore."

Ripple said biologists are noticing improvements in the streamside vegetation. Ripple and his colleague Robert Beschta recently released the results of two studies that indicate wolves may be closely linked to recent changes in cottonwood trees and willow shrubs. Their research has focused on Lamar Valley where streamside willows and cottonwoods are beginning to become more prevalent and taller.

Ripple credits the changes to the wolf reintroduction. In some areas, elk are no longer free to browse in the willow areas unless they want to risk becoming a wolf dinner. "For 70 years, the elk congregated next to the rivers, eating the vegetation," he said. "They don't do that anymore and the changes in vegetation are a direct result of wolves being reintroduced.”

Ten years ago, Yellowstone northern herd had 17,000 elk, the largest single population in the world. Elk, not surprisingly, have suffered, both from weather and wolves, their numbers in the park shrinking to about 8,000 today.

The changes don’t just stop with wolves, elk and willows. Ripple said research is showing other improvements. In 1996 beaver were scarce and there were no known colonies in the Lamar Valley. Now there are seven colonies because the beavers can eat the low-hanging willow branches. The beavers build dams, creating marshland that brings back the otters, mink, muskrats and ducks.

Fish in the stream benefit from the improved willow and cottonwood vegetation. “Trout are attracted to shaded areas of a stream,” Ripple said. “With more shrubby vegetation, there’s more shade. It’s an ecological chain reaction that can go on and on.”

Ripple said it is the first time in 70 years, that the park has a complete suite of predators and prey. “This is just the beginning of a grand experiment,” he said. “It will take several decades to understand all of the interactions.”