Feb 7

Scientists say the reintroduction of grey wolves to Yellowstone National Park is not enough to enable a full recovery of the ecosystem. 

Photo of grey wolf running in the snow

Grey wolf running in the snow

Trophic cascade

When grey wolves all but vanished from Yellowstone National Park in the early 1900s, the absence of this keystone predator had a marked effect on the park’s ecosystem. Elk, the wolves’ natural prey, rapidly increased in numbers, escalating grazing pressure on willow trees that grow by the sides of streams. As a result, the decline in willow led to a severe decrease in the beaver population. Beavers rely heavily on willow to provide food and materials with which to build their dams. Beavers and willows have a mutual relationship whereby the willow also benefits from the beavers’ presence, due to the raised water tables caused by their dams. The loss of Yellowstone’s wolves led to a cascade of dramatic changes in the ecosystem’s structure, known as a trophic cascade.

Photo of American beaver felling a tree

American beaver felling a tree

Wolves return

When wolves returned to Yellowstone National Park in 1995, elk numbers fell and shrub recovery became evident through increased plant height and berry production. This led some scientists to predict ecosystem recovery following the return of the park’s top predator. Authors of a paper in the Proceedings of the Royal Society B have revealed a ‘recovery test’, explaining that if the ecosystem is indeed in recovery, willow trees must be at least two metres tall in order to escape being eaten by elk and to provide the beavers with necessary food and material to build dams.

Photo of male elk calling

Male elk calling

Insufficient recovery

In the study concerned, researchers measured willow trees at four sites in Yellowstone from 2001 to 2010. Some willow tree plots were fenced to prevent the elk from grazing, whereas some had simulated dams. Regardless of fencing and growth time, the researchers found that only the willows that grew in the plots with simulated dams reached heights of more than two metres. The outcome of this study suggests that riparian ecosystems are unable to recover fully due to the presence of wolves alone; tall willows cannot return without the beaver, yet the absence of tall willows inhibits the beaver’s much-needed return. It is clear that the co-existence of beavers and willow trees is able to drive the structure of riparian ecosystems, and that for the Yellowstone ecosystem to continue to recover, beavers will need to enter the equation.

Photo showing grey wolves with different coloured coats

Grey wolves can have different coloured coats

 

Read more on this story at The Guardian – The return of grey wolves ‘not enough to restore Yellowstone’s ecosystem’.

View photos and videos of grey wolves, North American elk and the American beaver on ARKive.

Kaz Armour, ARKive Text Author

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