Capturing a True Picture of Wolves in Yellowstone: Reevaluating Aspen Recovery

The reintroduction of wolves in Yellowstone National Park in 1995 triggered a cascade of effects that ultimately restored the ecosystem, including the recovery of aspen trees. It’s an environmental success story that feels like a parable. But like many stories based on ecological realities, it’s more complex than at first glance—aspen recovery in the park is not as robust as generally believed, according to new research.

The Yellowstone story is a textbook example of a trophic cascade, in which predators help plants by eating or scaring away animals that eat the plants. When wolves were reintroduced into the Yellowstone food chain, they helped to reduce numbers of elk, which had been consuming young aspen trees. Previous research showed strong, positive growth in young aspen as the elk populations decreased—a welcome result, as aspen forests have been vanishing from the northern Yellowstone landscape for the last century.

But new research from Elaine Brice and Dan MacNulty, from Utah State University’s Department of Wildland Resources and Ecology Center, and Eric Larsen, from the University of Wisconsin Stevens Point’s Department of Geography and Geology, shows that the effect of wolves on the recovery of aspen has been exaggerated by how it was measured.

Previous studies evaluated aspen recovery in Yellowstone by measuring the five tallest young aspens within a stand. The reasoning was that the tallest young aspen trees represent a ‘leading edge’ indicator of the future recovery of the entire aspen population. But this is not the case. Sampling only the tallest young aspen estimated growth that was significantly faster than estimated by random sampling of all young aspen within the stand, according to the research.

“These are extremely complex systems and understanding them is a major challenge because they are difficult to properly sample,” said Brice. “The traditional method of sampling by only using the tallest young aspen plants to measure growth—which most research currently relies on—doesn’t capture the entire picture.”

For one, elk are picky about the aspen they consume. They tend to eat plants at shoulder height for which they don’t have to crane their necks. Once the leader stem (main trunk) of a young aspen grows past the shoulder height of adult elk, it is less likely to be eaten, said MacNulty.

Taller aspens also thrive because they tend to have the best growing conditions (sunlight, moisture, soil quality). Measuring just the tallest young trees downplays the role of these other factors that have nothing to do with elk or wolf populations. And measuring just the tallest aspen also overlooks the aspen plants that completely fail and fall off the radar.

“That’s like calculating a team’s batting average without the player who always strikes out,” said Brice.

Random sampling from the research showed an absence of aspen regeneration in some places, a vital piece missing from the initial measurements.

“The bottom line is that ecologists must stick to classic sampling principles, like randomization, to fully understand the cascading effects of large predator loss and recovery in complex ecosystems like northern Yellowstone,” said MacNulty.