Removing top predators from a food web may trigger rapid ecosystem changes, but whether predator reintroductions can undo such changes in similar timescales is unclear. Writing in Ecological Monographs in January 2024, Hobbs, Cooper and colleagues show why restoring large carnivores to Yellowstone National Park has not reversed ecosystem shifts that were initiated a century ago. Beaver dams and dense stands of tall willows were once widespread along streams in Yellowstone, but nearly disappeared during the 20th century. As Hobbs et al. explain, the elimination of wolves and cougars led to higher elk numbers, which resulted in over-browsing on willow and was followed by beaver disappearance. In recent decades, wolves were reintroduced, cougars and grizzly bears returned and elk numbers declined, raising the question of whether the system would return to its previous state, or — if not — why.
To find out, Hobbs et al. established a controlled experiment in 2001, with fenced plots to exclude herbivores and artificial dams to simulate beaver activity. Two decades of data show not only that are willows still growing below their potential, but also that they need both dams and the absence of browsing to grow tall. This implies that controlling elk is a necessary but not sufficient condition to restore willow stands in Yellowstone. The authors also measured willow growth at other sites, installed camera traps and analysed wildlife data collected annually by Yellowstone National Park, and found that elk declined but bison became abundant. Therefore, browsing intensity remained high, despite the increasing abundance of wolves. Further observations from streams where beavers returned suggest that these ecosystem engineers choose areas with tall willows, which reinforces the notion that the disappearance of both tall willows and beaver dams was caused by uncontrolled elk populations. These findings indicate that self-reinforcing processes initiated by the loss of large carnivores resulted in a stable state that carnivore reintroduction alone cannot reverse. Therefore, management interventions in Yellowstone should not assume simple wolf–elk–willow trophic cascades but account for the interplay with other species. The findings serve as warning that reintroducing large carnivores is not a quick fix to restore ecosystems.
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