When the wolf was removed the coyote was top dog. Coyotes don't have enough speed to catch an adult pronghorn so they catch their offspring. This lead to low populatuons for the pronghorn. The elk was the cause of erosion, low populations of the bever, less spawning places for fish, and less habitats for aquatic insects and animals. This happened because the wolf was not there to keep they coyote in check and to prey on the elk.
When the wolf was reintrduced to the yellowstone ecosystem the coyote got in it's place. This lead to normal populations of the pronghorn. The elk returned to their normal habitat. Bever dams encourage the growth of aquatic plants. These aquatic plants provide food and habitats for aquatic insects. Aquatic insects provide food for birds and fish. Rivers and ponds act as reservoirs increasing water flows in the summer months. This happened because the wolf was there to make kills and the wolf drove the elk back to their natural habitat.