When the U.S. Fish and Wildlife Service reintroduced 14 gray wolves to Yellowstone National Park in 1995, the animals were, in some ways, stepping into a new world.
After humans hunted wolves to near-extinction across the Western U.S. in the early 20th century, the carnivore’s absence likely altered ecosystems and food webs across the Rocky Mountains. Once wolves were reintroduced to the landscape, scientists hoped to learn if, and how quickly, these changes could be reversed.
Despite studies claiming to show early evidence of a tantalizing relationship between wolves and regenerating riparian ecosystems since the canines returned to Yellowstone, scientists are still debating how large carnivores impact vegetation and other animals, according to a new paper published this month.
The scientific intrigue centers on the degree to which carnivores have an indirect effect on other fauna and flora, a dynamic scientists call a “trophic cascade.” Beginning in the early 2000s, several studies appeared to show evidence of a transformational relationship between wolves, elk and riparian vegetation in Yellowstone.
But as more time has passed, other studies have suggested that Yellowstone wolves’ connection to the park’s riparian ecosystems may be more subtle than previously believed.
“It’s not that there’s not evidence consistent with a trophic cascade in Yellowstone,” said Chris Wilmers, a professor of wildlife ecology at the University of California Santa Cruz, and the paper’s lead author. “It’s that the effects are a lot more complicated and weaker than what was initially thought.”
The theory of wolves changing the landscapes and vegetation along streams and rivers in Yellowstone was born out of the changes scientists observed in the park after human hunting decimated beaver and wolf populations. More elk and other ungulates dominated the landscape, grazing riparian vegetation on the open banks of streams whose dams were no longer as robustly maintained by beavers. Water began moving through the park more rapidly, carving riverbeds into deep v-shapes and lowering the water table. This, in turn, diminished over-grazed riparian vegetation habitat, leaving less material for any remaining beavers to dam the waterways. All this caused a negative feedback loop that dried out areas around the park’s streams.
When wolves were reintroduced to Yellowstone, some scientists reported that they altered elk grazing patterns, causing them to become skittish about spending prolonged periods in open riparian habitat. This helped willow and aspen trees along stream banks regenerate, perhaps an early indication of a trophic cascade, one that could give beavers more material to slow flows and reduce waterside erosion.
But other scientists have questioned these theories, and for the new paper, Wilmers and four other researchers expanded the focus from wolves to include pumas and bears, other large carnivores that are recovering from the brink of human-driven extinction across the West.
In an analysis of about 170 citations published since the 1930s, Wilmers and his team found clear evidence of predator-induced trophic cascades in only limited circumstances.