Without cougars to prey on them, mule deer proliferated and devoured the shoots of young black oak trees. The oaks were replaced with pines and firs and the wild primrose became a rare find in the valley, the San Francisco Chronicle reported Friday.

"The loss of top predators, whether it's wolves in Yellowstone or cougars in Yosemite, is having a severe and degrading impact on plant communities," William Ripple of Oregon State's department of forest resources told the newspaper.

The findings are published online in the journal Biological Conservation.