Capturing a true picture of wolves in Yellowstone: Reevaluating aspen recovery

Research in the past showed that young aspens in Yellowstone National Park experienced strong growth as elk populations declined. This was a positive result. New research has shown that aspen recovery is less robust than previously believed. Credit: Lainie Brice
This environmental success story feels almost like a parable. The reintroduction in Yellowstone National Park of wolves in the mid-1990s set off a series of events that eventually restored the ecosystem, including the recovery and reintroductions of aspen trees. It's complicated, as many stories that are based on ecological realities. New research shows that the park's aspen recovery is less robust than previously believed.

The Yellowstone story is an excellent example of trophic cascades, where predators help plants grow through eating or scaring off herbivores. The Yellowstone food chain saw a decrease in elk population due to the introduction of wolves. Research has shown that young aspen grew strongly as elk populations declined. This is a positive result as aspen forests are rapidly disappearing from northern Yellowstone.

New research by Elaine Brice and Dan MacNulty from Utah State University's Department of Wildland Resources and Ecology Center and Eric Larsen from University of Wisconsin Stevens Point’s Department of Geography and Geology shows that the impact of wolves on aspen recovery has been exaggerated due to the way it was measured.

In Yellowstone, aspen recovery was evaluated in previous studies by measuring the height of five young aspen trees within a stand. It was believed that tallest aspen trees would be a good indicator of future aspen recovery. Research shows that sampling only the tallest aspen trees revealed a significantly higher rate of recovery than the random sampling of all the young aspen in the stand.

Brice stated that these complex systems are very difficult to understand and are difficult to sample. "The traditional sampling method that relies on only the tallest aspen plants for growth measurement, which is what most research relies upon at the moment, doesn't capture the whole picture.

One reason is that elk can be picky about what aspen they eat. Elk will eat plants that are at least shoulder height, so they don't need to strain their necks. MacNulty stated that as the leader stem (main trunk of an aspen) grows beyond the shoulders of adult elk, the chances of it being eaten decreases as it gets taller. MacNulty stated that this means that tallest young aspen grow faster not because wolves decrease elk browsing. This complicates the fact that young aspen's height is both a cause or effect of decreased elk browsing.

Aspen that are taller thrive due to the best conditions for growth (sunlight and moisture, soil quality, etc.). It is easy to overlook the importance of other factors, such as elk and wolf populations, by measuring only the tallest trees. Also, measuring only the tallest aspen leaves out the possibility that some young aspens will not regenerate.

Brice said, "It's like calculating the team's batting score without the player who always strikes it out." Random sampling of the research revealed that there was no aspen regeneration in certain places, which is a crucial piece missing from the initial measurements.

The authors believe that understanding how ecosystems react to large predator populations is crucial to solving larger debates over the structure of food webs and the delivery ecosystem services. This study shows how deviations from the basic sampling principles can skew this understanding. The strength of a trophic cascade was overestimated in this instance by non-random sampling, but cascading effects may be underestimated in other cases. They said randomization was one of the few safeguards against unreliable inferences, and the misguided management choices they might inspire.

MacNulty stated that "the bottom line is that ecologists must adhere to the classic principles of sampling design like randomization to fully understand trophic pathways in complex wildlife systems such as Yellowstone."

You can explore more Aspen is making an appearance in Yellowstone National Park because of predators

More information: Elaine M. Brice and colleagues, Sampling bias exaggerates an example of a trophic waterfall, Ecology Letters (2021). Information for Journal: Ecology Letters Elaine M. Brice and colleagues, Sampling bias exaggerates the textbook example of atrophic cascade (2021). DOI: 10.1111/ele.13915