OREGON STATE UNIVERSITY

Predators help shape rivers by affecting grazing behaviors

04/04/2007

CORVALLIS, Ore. – Large carnivores not only play a pivotal role in the health of ecosystems, they can also affect the very shape of the landscape, according to recent research by two OSU forestry professors.

Where predators such as wolves and cougars are absent, river channels are apt to widen and erode as deer and other browsers, free of fear, devour and trample streamside vegetation. These riparian plant communities — willows, cottonwoods, sedges — help anchor soils, hold sediments and maintain riverbanks.

OSU researchers William Ripple and Robert Beschta have found evidence, both historical and contemporary, of significant impact from predation on the width, depth and meanders of the Gallatin River in Yellowstone National Park. A story about their research appears in the spring issue of Terra, OSU’s research magazine (on the Web at oregonstate.edu/terra).

Archival photos show dramatic — and deteriorating — changes in the river’s path from the mid-1920s, when wolves were wiped out, through the latter decades of the 20th century. In contrast, an image from the early 2000s, after wolves had once again gained a foothold in the park, show signs of renewal.

“It appears that the presence or absence of this apex predator can impart important effects upon lower trophic levels: first to elk, then to willows and finally to … processes associated with floodplain systems,” professors Robert Beschta and William Ripple explain in the journal Earth Surface Processes and Landforms (May 2006).

The ecological consequences of catastrophic channel transformations are profound. As their banks crumble, rivers flow faster and sediments get finer, while water temperatures grow warmer and water tables sink deeper. Plant communities shift from moisture-loving to dry-land species. On the over-browsed sections of the Gallatin floodplain, for instance, shrubby cinquefoil and lodgepole pines replaced the lush willow thickets and dense sedges characteristic of healthy riparian zones.

Animal communities across the food web — from birds to aquatic insects, butterflies, fish, frogs, toads and lizards — shrink or disappear along with the vegetation they depend on.

Beavers are another casualty of the top-down “trophic cascade.” Their dam-and-pond systems play a critical role in maintaining plant, vertebrate and invertebrate diversity and biomass in riparian ecosystems, the researchers stress. One Wyoming scientist found about 75 times more waterfowl in streams with beaver ponds than in similar streams devoid of the industrious rodents, Beschta and Ripple note.

In studies across the West, the researchers have been amassing evidence of the predator-vegetation-biodiversity link. The stream channel study takes the linkage still further, connecting the carving of landscapes by river systems to the prowling of these landscapes by large carnivores.

“The heavy annual browsing of willow communities after the loss of wolves ultimately generated major changes in floodplain functions and channel morphology,” they say. “To the best of our knowledge, we are the first to connect a large, highly interacting carnivore to the characteristics of a river floodplain and its channel.”

Wolves and cougars could, under certain circumstances, be important management tools for restoring riparian zones — essentially, reviving the natural stasis of a system that has been out of balance since predators were extirpated, the researchers suggest.

Read more about the research of Beschta and Ripple in the spring 2007 issue of Terra, OSU’s research magazine, oregonstate.edu/terra.