From an article on the wolves in Yellowstone Park in today’s New York Times:
Nearly absent for decades, willows have roared back to life in Yellowstone, and the reason, Mr. Smith believes, is that 10 years after wolves were introduced to Yellowstone, the park is full of them, dispersed across 13 packs.
He says the wolves have changed the park’s ecology in many ways; for one, they have scared the elk to high ground and away from browsing on every willow shoot by rivers and streams.
“Wolves have caused a trophic cascade,” he said.
“Wolves are at the top of it all here. They change the conditions for everyone else, including willows.”