For the first time in 80 years, aspen trees are thriving in Yellowstone National Park in the US, due mostly to the reintroduction of wolves to the area.
According to a new study the return of wolves has led to an ecological transformation.
Gray wolves disappeared from the park by 1930 because of habitat loss and hunting.
Their loss led to a type of deer species, elk, taking over.
The elk chomped grass, shrubs and even young trees which meant aspen trees didn’t grow.