New study suggests still overgrazing young aspen stands
By Bob Berwyn
SUMMIT COUNTY — U.S. Geological Survey biologists this week unveiled new research showing that aspen groves in the Yellowstone region aren’t recovering as hoped after wolves were re-introduced into the ecosystem mix.
The theory was that wolves would create a landscape of fear in which elk would move around more, reducing the amount of browsing on young aspen stands. But the study suggests that hasn’t happened yet.
“This study … confirms that none of the aspen groves studied after wolf restoration appear to be regenerating, even in areas risky to elk,” said researcher Matthew Kauffman, adding that the research also showed that elk are indeed responsible for the decline of aspen stands.
“The results were surprising and have led us to refute several previous claims regarding interactions among wolves, elk and aspen in Yellowstone,” Kauffman said.
The problem for the aspens, it appears, is simply an overwhelming number of browsing elk. Even though the Northern Range elk herd has shrunk by 40 percent since wolves returned, it hasn’t been enough to prevent the ungulates from preventing aspen recovery.
“A landscape-level aspen recovery is likely only to occur if wolves, in combination with other predators and climate factors, further reduce the elk population,” Kauffman said.
The scientists started by analyzing tree rings to establish when aspen stands stopped regenerating. The tree ring studies also showed whether aspen stands started to regenerate after wolves were brought back to the ecosystem, and whether there were differences in regeneration between areas considered safe or risky for elk. The researchers also created a control group of aspens that were fenced off to prevent grazing by elk.
The tree rings showed that the period when aspen failed to regenerate (1892 to 1956) lasted more than 60 years, spanning periods with and without wolves by several decades.
“We concluded from this that the failure of aspen to regenerate was caused by an increase in the number of elk following the disappearance of wolves in the 1920s rather than by a rapid behavioral shift to more browsing on aspen once wolves were gone from the park,” said Kauffman.
Surveys of current conditions indicated that aspen in study stands exposed to elk browsing were not growing to heights necessary to make them invulnerable to elk. The only places where suckers survived to reach a height sufficient to avoid browsing were in the fenced-in areas. In addition, aspen stands identified as risky from the predation risk map were browsed just as often as aspen growing in less risky areas.
Predators play an important role in ecosystems, Kauffman said. They can influence plants by altering how many herbivores there are (by eating the herbivores) or by changing the behavior of herbivores (deterring them from areas where predators lurk).
But he said there is still considerable scientific debate exists regarding the importance of these two ways in which predators influence their prey. And this is especially true for large carnivores.
To complicate matters, predators use different hunting strategies – there is the sit-and-wait strategy (as with a spider in a web, or a rattlesnake waiting for a mouse to leave its burrow) and the more active, go get ’em strategy (think cheetahs and wolves).
“So, given that it takes a lot of energy to avoid a predator – energy that could be used to stave off winter starvation – we wanted to find out whether the prey of active-hunting predators such as wolves demonstrated risk-induced changes in areas where they foraged for food,” Kauffman said.
“This work is consistent with much of what researchers have learned from studying wolves and elk in Yellowstone,” Kauffman said. “Elk certainly respond behaviorally to the predation risk posed by wolves, but those small alterations to feeding and moving across the landscape don’t seem to add up to long-term benefits for aspen growing in areas risky to elk.”
Filed under: Environment, Summit County Colorado, wildlife Tagged: | elk, Environment, Gray Wolf, Northern Range, Summit County Colorado, Summit County News, United States, United States Geological Survey, wildlife, Yellowstone National Park