The reintroduction of wolves to Yellowstone National Park has had a dramatic effect on its environment, helping to restore its ecological balance to a more natural state that was last seen half a century ago.

Since wolves were returned to America’s most celebrated national park in 1995 after an absence of 70 years, young aspen trees have started to grow again for the first time in more than 50 years, research has shown.

Although wolves have no direct impact on the growth of aspens – deciduous hardwood trees that are typical of the American West but in long-term decline – they have made their influence felt through what scientists have termed the “ecology of fear”.

Their return has halved the park’s elk population over the past decade, and those that remain have started to avoid browsing on young tree shoots in areas where they feel particularly vulnerable. The combination of these factors has allowed more saplings to thrive, so that some have reached heights at which they are no longer likely to be eaten by elk and other herbivores.

Scientists say that the phenomenon shows how the existence of a natural food web, complete with a top predator such as wolves, can benefit an entire ecosystem. The aspen is not the first tree to show signs of recovery since the wolves’ return.

“This is really exciting, and it’s great news for Yellowstone,” said William Ripple of the Oregon State University College of Forestry, who led the aspen study. “We’ve seen some recovery of willows and cottonwood, but this is the first time we can document significant aspen growth, a tree species in decline all over the West. We’ve waited a long time to see this, but now we’ re optimistic that things may be on the right track.

“The issue of aspen decline in the American West is huge, and their recovery will depend on local conditions and issues in many areas. In northern Yellowstone, we finally have some good news to report. It’s just a start, but it’s a pretty good start.”

Wolves were eradicated from Yellowstone, which is largely in Wyoming and takes in small areas of Montana and Idaho, in the 1920s, and the decline of aspen and cottonwood trees has been dated to precisely this period. Large trees that were at least 70 years old still stand, but few younger trees survived as new shoots were rapidly eaten by large herds of grazing animals, principally elk, that were no longer kept under control by predation. The loss of trees and shrubs had a major ripple effect throughout the ecosystem, say scientists. There was greater water erosion, a loss of beaver dams, and a breakdown of food webs. Birds, insects, fish and plants were all affected.

The new study, which is published in the journal Biological Conservation, has looked chiefly at aspen growth on land near to streams. It found that over the past decade – since wolves were reintroduced to the park in 1995 – some aspen saplings have grown more than 7ft, putting them above the height at which they can readily be browsed by elk.

The recovery has probably been influenced more by changed elk behaviour than by lower numbers – the elk population is still higher than it was in the mid1960s when aspens were in decline, even though it is much lower than it was a decade ago.

Professor Ripple said: “In riparian zones, where wolves can most easily sneak up on elk, and gullies or other features make it more difficult for elk to escape, we’ve seen the most aspen recovery.

“We did not document nearly as much recovery in upland areas, at least so far, where elk apparently feel safer. But even there aspen are growing better in areas with logs or debris that would make it more difficult for elk to move quickly.”

Robert Beschta, Professor Emeritus of Forestry at Oregon State, said: “When I first looked at these degraded ecosystems in the mid1990s in Yellowstone, I had doubts we would ever be able to bring the aspen back. There were so many elk, and the stream ecosystems were in such poor shape. The level of recovery we’re seeing is very encouraging.”