When the US Fish and Wildlife Service reintroduced 14 gray wolves to Yellowstone National Park in 1995, they were venturing into a landscape that had dramatically changed in their absence.
Humans had hunted wolves to near-extinction in the Western US by the early 20th century, a move that likely disrupted ecosystems and food webs throughout the Rocky Mountains. With their return, scientists aimed to observe whether these changes could be reversed, and if so, how rapidly.
Initially, studies pointed to compelling signs of a significant relationship between wolves and recovering riparian ecosystems. However, a newly published paper highlights that the scientific debate continues over how exactly large carnivores like wolves impact vegetation and other wildlife.
Central to this intrigue is the concept of 'trophic cascades', where carnivores indirectly influence other flora and fauna. While early 2000s studies showed a notable interaction between wolves, elk, and riparian plants, more recent research suggests the impact of wolves on Yellowstone's riparian ecosystems may be more muted than originally thought.
'It's not that there's no evidence of a trophic cascade in Yellowstone,' said Chris Wilmers, a wildlife ecology professor at the University of California, Santa Cruz, and lead author of this new paper. 'It's that the effects are far more complex and weaker than previously assumed.'
Initially, the theory that wolves could alter landscape and vegetation along Yellowstone’s streams and rivers emerged from patterns observed after human activity had drastically reduced beaver and wolf populations. Increased numbers of elk and other ungulates grazed on riparian vegetation openly by streams, as beaver-maintained dams diminished. The faster water flow led to deeper riverbeds and lowered water tables, negatively affecting riparian vegetation and leaving less material for any remaining beavers to build dams. This created a feedback loop drying out stream-adjacent areas.