Park Yellowstone. How the return of wolves changed the ecosystem

When wolves returned to Yellowstone in the 1990s, scientists announced the beginning of a great recovery. The elk population dropped, willows reappeared along the rivers, and the park began to resemble its former landscape. But the story did not end there – and it is far more complex. The latest studies show that without appropriate water conditions and the return of beavers, the trophic cascade effect remains only partial.

When the absence of wolves disrupted the forest…

In the 20th century wolves almost completely disappeared from Yellowstone as a result of culling and human pressure. The consequences affected the entire ecosystem. Elk and deer populations grew year by year, and herbivores heavily browsed everything that had a chance to grow along the rivers: young willows, aspens, and seedlings of other trees. Riparian vegetation declined to such an extent that beavers no longer had material to build dams, causing their numbers to fall almost to zero.

The hydrology of the whole valley changed. Fewer beaver dams meant fewer shallow ponds, fewer wetlands, and increasingly impoverished habitats for many water-dependent species. After years, this degraded landscape was perceived as natural, even though it had little in common with Yellowstone’s former, diverse terrain.

The vision of revival from the 1990s

After the reintroduction of wolves in 1995 and 1996, changes began to appear: a decrease in the elk population, reduced pressure on riparian vegetation, and the emergence of young aspens and willows. Research was then conducted by, among others, William J. Ripple and Robert L. Beschta. According to an analysis published in 2025, the volume of willow crowns in the riparian zone of northern Yellowstone increased by as much as 1500 percent between 2001 and 2020, which is linked primarily to reduced browsing pressure from elk and improved soil conditions.

A new perspective, or when wolves are not enough

However, the latest studies indicate that the influence of wolves is not sufficient. Results of a 20-year experiment published in 2024 show that the cascade effect is weaker in places where hydrological conditions still do not support vegetation recovery. Although wolves limit the elk population, without a raised water level beavers do not build stable dams, and willows do not grow tall enough to restore the former valley landscape.
Additionally, researchers warn against oversimplified conclusions – many earlier findings were based on selective observations, which led to an overestimation of the wolves’ impact.

The role of all predators and the landscape mosaic

Modern analyses indicate that changes in Yellowstone are influenced not only by the presence of wolves but also by bears, cougars, beavers, as well as climatic conditions, local hydrology, and human pressure. The ecosystem therefore responds as a network of interconnections, not a simple sequence of causes and effects.

Where wetlands were restored and beavers reappeared, the landscape regained ecological functions: groundwater levels stabilized, habitats for fish and birds increased, and riparian vegetation regenerated more quickly. In places lacking these elements, the effects of reintroduction remain weaker.
For ecologists and conservation practitioners, this is a valuable lesson: rewilding efforts must take into account the entire web of dependencies, not just a single species.