The document discusses how the reintroduction of wolves to Yellowstone National Park has impacted elk populations and riparian vegetation communities. Figures show that elk populations increased substantially during the 20th century in the absence of wolves, leading to overbrowsing of cottonwoods and willows. Following wolf reintroduction in the 1990s, elk numbers declined and riparian vegetation has recovered, as evidenced by increased willow height and aspen recruitment. The recovery of wolves has helped restore balance to the ecosystem by limiting overgrazing by elk.