Global Warming? Blame the Wolves

Could reintroduction of wolves into Yellow Stone ecosystem have precipated climate change? Not really, but one could bend the data to prove it, reports USGS ecologist Don Despain.

Published: 06-Mar-2005

CHICO HOT SPRINGS -- Now there's one more thing you can blame on wolves: global warming.

That was the joking synopsis of a talk presented here Thursday by Don Despain, an ecologist for the U.S. Geological Survey who has studied trees and other plants in Yellowstone National Park since 1971.
Despain recalled being asked before the 1995 reintroduction of wolves to the park what impact the big carnivores were likely to have on plants.

"Not much," he predicted at the time.

However, a few years after reintroduction, willow bushes in stream bottoms in the park's northern range started growing three times faster than they had in all of Despain's experience.

"I thought I was going to have to eat my words," he told about 125 scientists gathered at a conference here to discuss climate change.

Other scientists noticed the growth as well, and some attributed it to wolves chasing elk from the creek bottoms, where they were vulnerable.

Despain grew curious and started looking at other factors.

About the same time wolves were set free in the park, a lingering drought struck the park. It isn't dry enough there to kill the willows, plants that "have always got their feet in the water," he said, but the drought also brought warmer days.

"Temperature can make a big difference in how much they grow," he said.

When the temperature rises above freezing, willows start producing new tissue. He began checking weather records and found that, between 1985 and 1996, there were an average of slightly more than 90 frost-free days between May and October.

Between 1997 and 2004, there was an average of 112 frost-free days.

The timing of those warmer days allowed for shoots to grow early in the season, Despain found, and for the plants to produce defensive chemicals late in the season. Those chemicals deter browsing by elk in the winter.

"Maybe it's not wolves exclusively," he said of the new willow growth. "It might be some kind of climatic change."

Looking at a chart outlining his findings that the weather was warmer after 1995 led to his quip: "This is the kind of data you can use to prove that wolves caused climatic change."

The strong consensus at the three-day conference is that the world is getting warmer, and that could have big impacts on the plants, animals and people of the northern Rockies.

Tree-killing insects are being invigorated by warmer summers, said Jesse Logan of the U.S. Forest Service's Forest Sciences Laboratory in Logan, Utah.

Bark beetles, he said, are a "native species, but they're invading new habitat largely as a result of climate change."

That could have major impacts on wildfires, and on the white bark pine trees of the Yellowstone region, which produce a key fall food for grizzly bears.

Another future danger could be gypsy moth, a non-native species brought to New England in the 19th century.

Their egg sacs have traveled around the nation on motor vehicles, and they've been detected in Utah.

His research indicates that by 2085, virtually all of the aspen trees in that state will be at risk.

"They can do serious defoliation," he said of the moths. "We're potentially going to be in real trouble with aspens."

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