Predator restoration stirs the ecosystem pot
By Summit Voice
FRISCO — Long term ecological monitoring in Yellowstone National Park shows a surge in aspen growth following the restoration of wolves to the ecosystem, with overall shifts in landscape conditions not seen in more than a century.
A series of studies show the recovery of vegetation as elk numbers drop, a decline driven by the return of the region’s apex predators. Biologists long hypothesized that wolves keep elk populations in check and also affect their grazing habits.
Rising grizzly bear numbers are also taking their toll on elk. As a result, lush vegetation is growing back in many but not all areas, according to Oregon State University instructor Luke Painter, who authored three recent papers that describe the results of his fieldwork monitoring vegetation growth patterns in the park.
“Without wolves, this would not have happened. Wolves caused a fundamental change, but certainly they are interacting with other factors such as bears, climate, fire and human activity,” Painter said.
Painter hiked the Yellowstone backcountry from 2010 to 2012, re-measured 87 aspen stands previously studied by his adviser, William Ripple, and former OSU student Eric Larsen in 1997 and 1998. Painter conducted a regional survey of stands across the northern part of the park and also in the Shoshone National Forest west of Yellowstone, where hunting and cattle grazing are allowed.
The findings were detailed this summer in an online report in the journal Ecology.
“This new study illustrates the powerful insights you can get from taking a view over 15 years or more,” said Aaron Wirsing, an associate professor of wildlife science at the University of Washington. Wirsing was not involved in Painter’s study.
“Wolf reintroduction was a landmark moment, but the changes vary throughout ecosystems as a function of other factors,” Wirsing added. “By taking an ecosystem point of view, this paper shows the complexity of the system and all its moving parts.”
Bison populations have also rebounded, increasing four-fold as elk numbers dwindled. In places where bison congregate, they browse on aspen, cottonwood and willow, compensating in part for the decline in elk. However, bison cannot reach as high as elk to browse, allowing more trees to escape and grow to maturity.
Overgrazing by elk led to a century long decline in aspen forests. Earlt studies by Ripple and his OSU colleague Robert Beschta pointed to the beginnings of an aspen recovery within a decade of wolf reintroduction, but other researchers reported finding little evidence of aspen regrowth.
Painter has shown that aspen recovery is widespread over much of the northern range, but where elk are still numerous, aspen stands are heavily browsed and stunted. In the famous Lamar Valley itself, bison have become the dominant herbivore, suppressing some aspen stands.
“There is a recovery of aspen happening, but it’s early and it’s not happening everywhere yet,” said Painter. “That’s the way things work in nature.”
Painter found that a quarter of all aspen stands now have five or more young aspen tall enough to escape elk browsing, a condition not seen in decades. Moreover, 46 percent of all stands have at least one tree that has grown beyond the reach of elk. Browsing rates were significantly lower in 2012 than in 1997. The greatest increases in aspen heights were in the east where Ripple and Beschta first reported signs of recovery in 2006.
Other researchers have suggested that fire and climate could be just as significant as wolves in explaining the recovery of aspen stands, but Painter found no evidence to support those possibilities. Following the severe Yellowstone fires in 1988, he said, aspen failed to recover as elk continued to browse young shoots.
In addition, aspen in northern Yellowstone showed signs of vigorous regrowth since 2000 despite relatively dry conditions, which would be likely to suppress aspen growth.
In the early 1990s, many researchers didn’t expect widespread changes to occur from wolf reintroduction, Painter said.
“The idea was that if you drop some wolves in here, everything will stay about the same, but the elk population will go down. But what happens is, it mixes up the whole pot. It’s been a surprise that there are so few elk wintering in the Lamar Valley.”