Eugene Weekly : Natural Resistance : 2.1.07

Regime Shift
What happens when the predators leave?

Have you ever driven up Zion Canyon Scenic Drive in Zion National Park? Have you gotten out of your car at the massive, glowing red wall called Temple of Sinawava and ambled up the scenic walkway, big old cottonwoods at the Virgin River to your left and The Narrows closing in on you up ahead?

I remember having no worries while small Josh and Zeke raced ahead of O’B and me on the easy path 25 years ago. It was perfectly safe: lots of happy people around, no steep drop-offs, balmy weather, stunning scenery at every step.

But there was and is something crucial missing from that drop-dead beautiful canyon: cougars.

Way back in 1938, seven years before I was born, Zion Park Naturalist C.C. Presnall told us the story. He wrote, “Human use of the park was, and no doubt always will be, concentrated in Zion Canyon, causing profound changes in the delicate balance between deer and their natural predators.”

Just how profound those changes are has recently been recorded by Drs. William Ripple, of the OSU Forest Resources Department, and Robert Beschta, retired OSU Forestry professor. In an article with the long title, “Linking a Cougar Decline, Trophic Cascade, and Catastrophic Regime Shift in Zion National Park,” they tell a story of profound contrasts between two Zion National Park stream canyons: much-visited North Fork of the Virgin River (Zion Canyon) and North Creek. North Creek doesn’t have a road in it. It sees the occasional hiker, not the tourist busloads of humans. Rarely visited North Creek has no common name like “Zion Canyon.” But it does have cougars.

The words “trophic cascade” in the title of Ripple and Beschta’s article refer to this: When cougars (predators) leave an area like Zion Canyon, populations of deer (plant consumers) increase. Young cottonwood (producers) are chewed off by the many deer and can’t grow up into large trees. In Zion Canyon, few young cottonwood saplings have become grown-up trees since the 1930s when Presnall was writing. In North Creek, by contrast, where cougars abound, there are more young cottonwood trees than old ones, which is normal for stream-side forests.

But the effects don’t stop there. The phrase “catastrophic regime shift” in the title of Ripple and Beschta’s article means this: The Virgin River in Zion Canyon is falling apart. With few cottonwood roots holding the river’s banks together, the stream is widening and deepening with every flood. Less shaded by trees, the widened stream is heated by the sun, becoming less habitable for native fish and invertebrates. The deepened stream has become isolated from its floodplain above. As a result, the floodplain’s riparian (streamside) flowers and shrubs, dependent on moisture for their lushness and diversity, are replaced by bare soil and some dry-land (upland) plants. This loss of riparian flowers, rushes, and shrubs means fewer butterflies, frogs, toads and lizards.

Meanwhile, over in North Creek, none of this is happening. With humans rare and cougars common, its producers (cottonwood) are at work holding the banks together. When floods come, water jumps the banks and replenishes North Creek’s riparian corridor. Water-loving rushes and cattails are 80 times as common as in Zion Canyon; wildflowers like scarlet cardinal flower and aster are several times more common; canyon tree frogs and red spotted toads hundreds of times more common; lizards three times more common, butterflies five times more common; native fish three times more common.


The same story, with some different characters, took place in Yellowstone National Park. Once wolves were eliminated from Yellowstone (1926), elk increased, cottonwood forest recruitment stopped, aspen declined, beaver left, hawks and eagles had little elk carrion to eat — on and on. As Beschta, Ripple and others are now documenting, all this is now reversing with the 1995 reintroduction of wolves.

Meanwhile, Idaho’s new Gov. Butch Potter is vowing to allow hunters to exterminate 60 of the 70 wolf packs that are now helping restore Idaho following the Nez Perce Tribe’s guidance of wolf reintroduction there under the Endangered Species Act. Because of the Tribe’s success, the Bush administration is removing endangered species protection for Northern Rockies wolves, and Potter wants cattle protection. Protected cows mow down willows and young cottonwood.

This is how we dismantle the world.

Mary O’Brien of Eugene has worked as a public interest scientist since 1981. She can be reached at


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