YOSEMITE NATIONAL PARK, Calif. - A new study of Yosemite National Park concludes that the displacement of cougars in the 1920s and a resulting increase in deer populations are what set the stage for the ongoing demise of black oak trees - a key element of the park's plant and wildlife ecology.
The findings, just published online in the journal Biological Conservation by scientists from Oregon State University, are disturbingly similar to those found in studies of the effect of wolves http://oregonstate.edu/ua/ncs/archives/2004/dec/ecology-fear-wolves-gone-western-ecosystems-suffer in Yellowstone National Park, cougars http://oregonstate.edu/ua/ncs/archives/2006/oct/cougar-predation-important-wildland-ecosystems in Zion National Park, and other recent research.
What was once considered a radical and unproven theory about the ecological importance of top predators now appears to be a trend seen across the canyons and mountains of the American West.
Scientists say that "trophic cascades" of ecosystem disruption are occurring when keystone predators disappear - and an "ecology of fear" that these wolves and mountain lions once induced is being lost.
"What we and other researchers have now observed in multiple sites is that the loss of large predators is resulting in far more grazing and foraging by deer or elk," said Robert Beschta, a professor emeritus in the OSU College of Forestry. "It affects everything from plants to streams, and the wildlife dependent upon them. Whole ecosystems are being greatly altered."
The changes are not irreversible, as the return of wolves to Yellowstone National Park has shown. In studies there, river valleys were once stripped almost bare by herds of elk that were unmolested by wolves. This led to loss of aspen, willow, cottonwood and eventually beaver, as well as increased stream erosion. But when the wolves were reintroduced more than a decade ago, some streamside zones began to recover. Similarly, aspen are again establishing themselves in sites where they had been on the road to localized extinction.
The newest study in Yosemite shows a similar pattern of vegetation impacts - but no recovery yet. In this case, the problems were caused by the near elimination of cougars in the 1920s that allowed deer populations to surge. Following that, increasing crowds of visitors in the past century forced the remaining cougars largely away from the human presence. And the occurrence of black oak, a critically important tree that dominated the Yosemite ecosystem for hundreds of years, began to decrease.
In recent decades, the invasion of conifers has been acknowledged as a major factor in oak decline, through overtopping and crowding out the oaks. However, even in areas where conifer invasion was not occurring and large, established oak trees continued to live, deer ate almost all the seedlings, with the process continuing to worsen as decades passed. Other forest shrubs and the understory were also affected, leaving mostly just large oaks and a forest floor covered with grass or ferns - and almost nothing in between.
The conifer invasion had also been linked to the loss of fire, which occurred when Native Americans no longer burned the area. However, the OSU study observed virtually no successful establishment of young oak trees following recent prescribed burns by the National Park Service. The deer apparently ate the young shoots.
Lacking some control of deer browsing, burning may actually be accelerating the death of the older existing oaks while providing essentially little improvement in young oak recruitment, they said.
In areas avoided by deer, the study found that many times more young oak trees survived.
When more deer are present, everything changes. Dozens of other animal species that rely on acorns for food and oak trees for nesting or cover may be affected, the study said. Additionally, a substantial decrease is likely in shrubs and wildflowers, and the wildlife species dependent upon them.
"We also documented a decline in evening primrose, which along with other wildflowers used to carpet the Yosemite Valley," said William Ripple, a professor in the OSU Department of Forest Resources and lead author on the study. "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."
Large predators, many scientists now believe, are important not just because they prey directly on other animals and help control their populations. In addition, the very presence of large carnivores is frightening to deer and elk, and significantly changes their behavior patterns. They are far less likely to forage for extended periods in areas of high predator risk. This phenomenon, still being studied, has been called the "ecology of fear."
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