Yellowstone is in trouble unless we can bring back the beavers
The story starts in the early-20th century when people killed the last of Yellowstone's wolves. After the wolves disappeared, people noticed that proliferating elk were changing the landscape, as their relentless grazing turned once-lush areas barren.
When biologists reintroduced wolves to the park in 1995, the initial effect was promising. Although elk populations did not decline as much as expected, the plants they ate started to regrow. Ecologists theorized that elk altered their behavior when wolves were around and consequently spent less time grazing.
The wolves quickly became the poster child for trophic cascades, how bringing back top predators can restore out-of-whack ecosystems. In recent years, however, ecologists have realized that bringing back wolves hasn't been enough to restore plant communities in Yellowstone.
"Predators can be important," Oswald Schmitz, an ecologist at Yale University in New Haven, Connecticut, told Nature, "but they aren't a panacea."
Thus ecologists have started looking closer at the role beavers play in the ecosystem.
As beaver dams disappeared, so did the wetlands and streams they supported - and these are the areas that suffer most today.
Wolf reintroduction has helped beaver populations somewhat: Today, people again spot beavers along the larger rivers. Unfortunately, those rivers are too big for beavers to dam, and beavers remain absent from the smaller waterways that they can dam. Meanwhile, the plant communities along those streams remain damaged.
"It's problematic because the willows and beavers have a mutualistic relationship: Beavers eat and cut them down to build their dams, and the dams raise the water tables and bring water up so it's more available for plants," Kristin Marshall, an ecologist at the University of Washington who published her research on the Yellowstone beavers in the the journal Proceedings of the Royal Society B, told Business Insider.
Not only will the willows not recover, but the marshy habitats around them won't recover either. That includes not only the trees and plants but also the birds, amphibians, and any other creature that needs wetland to survive.
Some places might be too far gone for anything, even the beaver, to help.
"This has changed to a site that can't go back," Doug Smith, the head of Yellowstone's wolf-reintroduction program and the park's main beaver biologist, told High Country News about Yellowstone's Elk Creek, "There's no water."
But it's not too late to give up hope on the beavers. Wolves returned 20 years ago and much of their effect is only being seen now, so this type of recovery takes a long time. The beavers could still make a comeback and finish restoring Yellowstone.