SAN FRANCISCO, Nov. 11 (Xinhua) -- The ecosystem at Yellowstone National Park is being improved with the return of large terrestrial carnivores such as wolves and cougars, according to a new study by Oregon State University (OSU) in western United States.

OSU scientists Robert Beschta and William Ripple found that the growing population of gray wolves and cougars, which nearly became extinct due to hunting in Yellowstone in the early 1990s, helped balance the number of elk, the prey of the two carnivores.

With the absence of the wolves and cougars, the population of elk expanded quickly and they ate away so many willow plants along the park's stream, leading to the deterioration of the aquatic and riparian ecosystems in the park.

The return of large predators such as wolves was decisive in controlling the elk population, which benefited the health of plant communities and streams in Yellowstone, Beschta said.

He also noted that the healing of the park's landscape is in its early stage and it may take more time for stream channels to recover in some areas.

The findings of the OSU were published in the journal Ecohydrology earlier this week.