The reintroduction of gray wolves to Yellowstone National Park in 1995 was meant to bring balance to a disrupted ecosystem. Nearly three decades later, the results are becoming clear—especially in the forests of the park’s northern range, where a new generation of quaking aspen is thriving for the first time in 80 years.
Wolves (Canis lupus) had disappeared from Yellowstone by the 1930s due to habitat destruction, hunting, and deliberate eradication programs. Without them, the elk population (Cervus canadensis) exploded—reaching an estimated 18,000 individuals. These elk heavily grazed the landscape, eating everything from grasses and shrubs to the twigs and bark of young aspen, preventing new trees from growing.
“You had older trees, and then nothing underneath,” said Luke Painter, an ecologist at Oregon State University and lead author of a new study published in Forest Ecology and Management.
A dramatic ecological shift
When wolves were brought back in 1995, elk numbers began to decline. Today, the elk population in the park has dropped to around 2,000. Painter and his colleagues returned to three areas they had first studied in 2012, looking specifically at aspen regeneration.
They surveyed 87 aspen stands and found that one-third had a widespread presence of tall aspen saplings, while another third had large patches of them. “We’re seeing significant new growth of young aspen,” said Painter. “This is the first time we’ve found it in our plots.” These new trees—defined by trunks over 2 inches (5 cm) in diameter at chest height—haven’t been seen in these areas since the 1940s.
He added, “It doesn’t mean they won’t die, but it’s a good sign we’re getting new trees. As they grow taller, they become more resilient.” These maturing trees are now large enough to reproduce—either by spreading through root shoots or by producing seeds.
The researchers found tall aspen saplings in many of the sites studied. (Image credit: Photo provided by Luke Painter, OSU College of Agricultural Sciences.)
Bison, biodiversity, and beyond
Still, Yellowstone’s aspen recovery isn’t guaranteed. While elk numbers have dropped, bison (Bison bison) populations have increased in some areas. “Bison are much harder for wolves to hunt,” said Painter, which means their growing numbers could become a new challenge for young aspen trees.
What’s remarkable about this recovery, according to Painter, is that it’s driven not by climate shifts or weather patterns, but by the return of a top predator. “Aspen are a keystone species,” he explained. “Their open canopy filters light in ways that support a wide diversity of plant life.”
That diversity includes fruit-bearing shrubs, insects, and birds—as well as species like beavers, which use aspen, willows, and cottonwoods for both food and building materials. The ripple effect of more aspen could reshape entire parts of the ecosystem.
And the benefits may extend even further. Painter noted that populations of bears and cougars in the area also appear to have increased since wolves were reintroduced, though the exact reasons remain unclear.
Rewilding and its impact
“The paper shows the important ecological benefits occurring from the restoration of wolves to Yellowstone,” said Dominick Spracklen, professor of biosphere-atmosphere interactions at the University of Leeds. Spracklen, who has studied wolf reintroduction proposals in Scotland, emphasized that ecosystems lacking apex predators often spiral out of balance.
“While reintroducing large carnivores presents real challenges—especially around human-wildlife coexistence—this work highlights the profound ecological rewards such efforts can bring,” he said.
