Wolves played a key role in helping aspen trees recover in Yellowstone National Park after decades of heavy browsing by elk, researchers say. Now bison might be pushing back on the aspen recovery.
Like the elk before them, bison like to gobble aspen saplings. The lumbering bison also sometimes knock over aspen trees, Luke Painter, an ecologist at Oregon State University, told Cowboy State Daily.
Painter is the lead author of a recently-published study crediting wolves with a notable comeback of aspen trees in northern Yellowstone.
Even so, he doesn’t think that bison pose an existential threat to aspen on the scale that pre-wolf elk herds did.
While some Yellowstone wolves are getting better at hunting bison, Painter doesn’t think they’re capable of putting a serious dent in the park’s bison population.
“Bison are a lot harder for wolves to take down,” he told Cowboy State Daily.
Aspen Elkpocalypse
Yellowstone’s elk population peaked in the 1990s at about 18,000. The park now has about 2,000 elk. Some see that as a terrible decline in elk numbers and primarily blame wolves and other predators.
Others argue that the reduced number of elk is much better for the ecosystem.
Painter’s research supports the latter view and the wider idea of trophic cascade.
Trophic cascade is the concept that restoring apex predators to an ecosystem has a “cascade” of benefits all through the system.
In the case of aspen and willows, the argument is that by killing elk, as well as prompting them to move around more, wolves keep elk from camping out in one area and chowing down on saplings.
The recovered aspen and willow stands supply more food and building materials for beavers, which in turn can create more wetlands.
Painter said that the conclusions of the aspen study are based on several research trips to Yellowstone from the 1990s to the early 2020s.
At the beginning, researchers found aspen groves struggling with weak old trees, only a few young trees around 3 feet tall, and no young trees in the 6-foot range.
The later surveys found more examples of healthy aspen groves, with an array of sizes and ages, Painter said.
In addition to giving beavers more to work with, robust aspen groves provide ideal habitat for a variety of creatures, including birds and insects, he said.
Leafy aspen groves allow ample amounts of sunlight to reach the ground, creating a rich tapestry of plant life below, Painter said.
The Great ’90s Elk Die-Off
Suggesting that wolves saved aspen by killing most of the elk is over-simplified, Painter said.
The largest loss of elk in Yellowstone wasn’t because of predation but driven primarily by starvation and nasty weather, peaking around 1997, he said.
After being absent from Yellowstone for decades, wolves were reintroduced to the park in 1995. By then, elk were going through cycles of starvation, Painter said.
There was a particularly harsh winter in 1997, when wolves were still trying to get established.
Inside the park, many elk starved. Others left the park, trying to escape deep snow, only to be shot by hunters in Montana, Painter said.
Those events slashed Yellowstone’s elk population. And as the number of wolves grew, they kept the elk herds from growing back to their previously huge numbers, he said.
Wolves also keep elk moving and force them to stay more alert, so they have fewer opportunities to eat aspen saplings, Painter said.
Many of Yellowstone’s elk also established a pattern of moving north out of the park into Montana every winter, he added.
It’s not certain how much influence wolves have on more elk wintering outside of the park, he said.
There’s also debate over what factor played the biggest part in aspen recovery — whether it was due more to a reduction in the sheer number of elk or changes in elk behavior, Painter said.
Trophic Cascade Challenged
How much wolves and trophic cascade have affected Yellowstone remains debatable.
There’s some disagreement over how much influence elk and wolves actually have on the health of willow stands in Yellowstone, according to an online articlepublished by the National Park Service.
“Part of the disagreement comes from crediting wolves as the only agent, ignoring cougar recovery and increases in bear numbers, and of course elk management outside of the park,” the article says.
“Another criticism is that too much impact has been attributed to elk, that other factors like water availability need inclusion in any explanation. In dry areas with reduced elk herbivory, no willow response was observed,” the article states.
Giving Wolves Credit
Painter said that mountain lions and bears also prey on elk, but he doesn’t think nearly to the degree that wolves do.
Aspen are a vital part of ecosystems in the Rocky Mountain region, and the Yellowstone study shows how wolves protect the trees from elk, he said.
“Other things affect elk, but this big change (in aspen health) wouldn’t have happened without wolves,” Painter said.
Mark Heinz can be reached at mark@cowboystatedaily.com.
