Without Cougar Predators, Deer Kill Cottonwoods

CORVALLIS, OR–The disappearance of Cougars from part of Zion National Park over the past 70 years has allowed deer populations to increase, resulting in ecological damage, loss of cottonwood trees, eroding streambanks, and declining biodiversity, finds a new study from Oregon State University (OSU).

The research just published in the journal “Biological Conservation” confirms predictions made more 50 years ago by naturalist Aldo Leopold, considered the father of wildlife ecology.

“When park development caused Cougars to begin leaving Zion Canyon in the 1930s, it allowed much higher levels of deer browsing,” said Robert Beschta, an OSU professor emeritus of forest hydrology. “That set in motion a long cascade of changes that resulted in the loss of most cottonwoods along the streambanks and heavy bank erosion.”

“But the end result isn’t just loss of trees,” he said. “It’s the decline or disappearance of shrubs, wetland plants, amphibians, lizards, wildflowers, and even butterflies.”

In Zion Canyon, a popular tourist attraction for over a century, Cougars are absent, scared off by the influx of human visitors. With their natural enemy gone, growing deer populations ate young cottonwood trees, robbing streambanks of shade and erosion protection.  As a result, floodplains began to erode away.

By contrast, a nearby roadless watershed has similar native ecology but is remote, with an intact Cougar population and fewer Mule Deer. Streambanks in this watershed have nearly 50 times more young cottonwood trees as well as thriving populations of flowers, lizards, butterflies, and species of plants that help stabilize stream banks, provide food-web support, and protect floodplains.

“The documentation of species abundance that we have in this study is very compelling,” said William Ripple, a professor in the OSU Department of Forest Resources and lead author on the study.

“These two canyons, almost side by side, have a similar climate and their ecosystems should be quite similar,” Ripple said. “But instead they are very different, and we hypothesize that the long-term lack of cottonwood recruitment associated with stream-side areas in Zion Canyon indicates the effects of low Cougar and high deer densities over many decades.

The findings of this study may be relevant to other ecosystems elsewhere when key predators are gone, the researchers said, and high populations of native herbivores such as deer or elk, or domestic grazers such as cattle or sheep, affect native biodiversity.–ENS