A Colorado State University experiment spanning more than two decades has found that removal of apex predators from an ecosystem can create lasting changes that are not reversed after they return—at least, not for a very long time.

A 20-year experiment conducted by Colorado State University researchers in Yellowstone National Park found that restoring apex predators like wolves was not a quick fix for ecosystems degraded by their absence. While wolf reintroduction lowered elk populations, willows and aspens did not recover as strongly as expected even after carnivore numbers rebounded naturally. Constructing fences and dams showed the importance of reducing browsing and increasing water access independently. The study challenges the idea that easily reversing food webs can undo lasting ecological changes.