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Scientists reexamine the effect of wolves on aspen tree recovery in Yellowstone

Aspen saplings in Yellowstone National Park.
Yellowstone National Park
Aspen saplings in Yellowstone National Park.

New research from Utah State University calls into question one of biology’s classic tales – the role of gray wolves in Yellowstone. A textbook example of keystone predators, wolves are thought to be responsible for maintaining balance in an ecosystem by keeping herbivore populations in check. Studies have shown that since the 1995 reintroduction of wolves to Yellowstone National Park, aspen growth has recovered significantly. Ph.D. candidate Elaine Brice and Dan MacNulty, Associate Professor of Wildland Resources, have now shown that earlier studies overestimated this effect.

“Aspen has been kind of in decline in the park for such a long time. So I think the reason that they started to measure just the tallest ones… the justification was that, they call it the leading edge. They were basically measuring the first Aspen to start to respond. And that was going to kind of predict what was going to happen down the line,” Brice said.

By measuring only the tallest aspen trees, researchers unintentionally biased the data. Brice and MacNulty showed that the tallest trees are the most successful and do not represent the recovery of aspen populations as a whole. Another problem with the recovery studies, Brice said, is that the data are largely recycled.

“And so you have the same datasets, the same people using them. And the snowballing of essentially, inbred citations, right, citing yourself over and over again, which really has propagated this story. So, it's not clear at all that the data is non-randomly sampled,” Brice said.

Brice’s research shows that wolves are having a positive impact on aspen recovery, but that other factors beyond overgrazing are also threatening these trees.

"As human beings, we're really predisposed to believing in sort of simple stories. And so, when these complicated details come up, the idea that, geez, maybe wolves aren't the only driver of aspen population dynamics, we kind of tune out,” MacNulty said.

Although a simple story may be compelling, the truth is often more complicated. Read more at