Reassessing the Aspen Recovery Success Story

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New research shows that measurement has exaggerated the effects of wolves on aspen recovery.

It’s an environmental success story that feels like a parable – the reintroduction of wolves to Yellowstone National Park in the mid-1990s triggered a cascade of impacts that ultimately restored the ecosystem, including aspen recovery. But like many stories based on ecological realities, it’s more complex than meets the eye – the recovery of aspens in the park is, according to a new study published in. published, is not as robust as is generally believed Ecology letters.

The Yellowstone Story is a textbook example of a trophic cascade, in which predators help plants grow by eating or scare away herbivores that eat the plants. When wolves were reintroduced into the Yellowstone food chain, they helped reduce the number of moose that had eaten young aspen trees. Previous research showed strong positive growth of young aspens as elk populations declined – a welcome finding since aspen forests have disappeared from the northern Yellowstone landscape since the last century.

But new research by Elaine Brice and Dan MacNulty of the Department of Wildland Resources and Ecology Center at Utah State University and Eric Larsen of the Department of Geography and Geology at the University of Wisconsin at Stevens Point shows that the effect of wolves on the recovery of aspens was due to the measurement exaggerated.

Measurement of Aspen in Yellowstone National Park

Previous research showed strong positive growth of young aspens in Yellowstone National Park as elk populations declined – a welcome result. However, new research shows that aspen recovery is not as robust as previously thought. Image Credit: Photo courtesy Lainie Brice

Previous studies assessed the recovery of aspen in Yellowstone by measuring the five largest young aspen within a stand. The idea was that the tallest young aspen trees would be a “leading” indicator of the future recovery of the total aspen population. However, this is not the case – testing only the largest young aspen showed a significantly faster recovery rate than the random sample of all young aspen within the population estimated.

“These are extremely complex systems and they are challenging to understand because they are difficult to sample properly,” said Brice. “The traditional method of sampling, which uses only the largest young aspen plants to measure growth – which most research now relies on – doesn’t get the full picture.”

For one thing, moose are picky about the aspen they consume. They have a tendency to eat plants at shoulder height that don’t require them to stretch their necks. Because the main trunk of a young aspen grows past the shoulder level of an adult moose, the less likely it is to be eaten as it increases, MacNulty said. “This means that the largest young aspens grow faster because they’re bigger, not because wolves reduce moose hunting,” MacNulty said. This finding underscores the aggravating fact that the height of young aspens is both a cause and an effect of reduced moose bite.

Taller aspens also thrive because they tend to have the best growing conditions (sunlight, moisture, soil quality). Measuring only the tallest young trees downplays the role of these other factors unrelated to elk or wolf populations. And measuring only the tallest aspen also overlooks the failure of some young aspen to regenerate.

“It’s like calculating a team’s batting average without the player who always hits,” said Brice. Research samples showed the lack of aspen regeneration in some places, an important piece that was missing from the first measurements.

Understanding how ecosystems respond to changes in large predator populations is critical to resolving broader debates about the structure of food webs, determining biodiversity, and the provision of ecosystem services, the authors say. This study shows how deviations from basic sampling principles can skew that understanding. Non-random samples have overestimated the strength of a trophic cascade in this case, but can underestimate cascade effects in other situations. Randomization is one of the few safeguards against unreliable conclusions and the misguided management decisions that can spark, they said.

“The bottom line is that ecologists have to stick to classic sampling design principles like randomization to fully understand trophic cascades in complex wildlife systems like Yellowstone,” said MacNulty.

Reference: “Sampling Bias Exaggerates a Textbook Example of a Trophic Cascade” by Elaine M. Brice, Eric J. Larsen, and Daniel R. MacNulty, November 8, 2021, Ecology letters.
DOI: 10.1111 / ele.13915


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