Ecologists studying 'trophic cascades' have found that the reintroduction of apex predators can lead to surprising improvements in river health. In Yellowstone National Park, the return of wolves reduced the overgrazing of willow and aspen by elk. This allowed the trees to grow tall enough to provide shade for the rivers and materials for beavers. _______ these beavers built dams that slowed the water flow and created diverse habitats for fish and insects.