Y: Look at these beautiful tall willows, Don. They seem bigger than they were the last time I visited Yellowstone years ago.
D: You’re right—they are bigger, which means they’re finally getting back to their normal size. The willows in Yellowstone National Park used to commonly grow up to 157 inches tall. But in the 1990s, most willows in the northern part of the park only grew to be 39 inches.
Y: What made them shrink and then grow tall again?
D: By 1930, people had hunted Yellowstone’s wolves out of existence. With no wolves to hunt elk, the elk population exploded, and all those elk browsed on vegetation including willows, preventing them from growing tall. Wolves were reintroduced to the park in 1995 in an attempt to restore the area’s natural ecosystem and, since then, there’s been a reduction in elk browsing which finally let willows grow tall again. To confirm this, scientists compared data from three periods: 1988-1993, when there were a lot of elk around and very short willows; 2001-4, when willows were just starting to recover; and 2016-2018, when signs of recovery were more visible. Their studies showed that as elk densities decreased in the 2000s, willows began to recover as elk browsing decreased. Other studies note, though, that less elk-browsing isn’t enough to completely restore willow height—because of changes to the ecosystem’s stream hydrology, it won’t be that easy for willows to ever be quite as tall as they used to be.
Y: I wouldn’t have made the connection between wolves and willows. It just goes to show how interconnected everything is.