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Sea of shrubs. These cattle eat grass, forcing voles to dine on the surrounding rabbitbrush.

Voracious Voles to Ranchers' Rescue

TUCSON, ARIZONA--After shrubs invade their grasslands, there's almost nothing a rancher can do to remove them. Now an ecologist has found an environmentally friendly solution: Let native rodents do the hard work. All that's required is shifting the time of year cows are put out to pasture.

Decades of cattle grazing and fire suppression have transformed many landscapes of the North American West, allowing shrubs to take over what were once meadows and grasslands. One particularly stubborn invader is the native woody rabbitbrush. "This stuff is immune to burning, cutting, and a lot of herbicides," says biologist Dennis Bramble of the University of Utah in Salt Lake City. So Bramble and his wife Jean started experimenting on their own ranchland in southern Utah. They shifted the season during which cattle grazed from spring and summer to fall--and that made all the difference.

Without grazing cows, the spring grass grew thick and lush. The plentiful food ignited a breeding frenzy among montane meadow voles. In the fall, the Brambles introduced cattle, which promptly munched the grass. The voles had to eat rabbitbrush, and they burrowed down to gnaw at the roots. After several years of this abuse, rabbitbrush declined from 3000 plants per hectare to just over 200. The spring and summer grazing typical in the West has "probably short-circuited this natural control system," Bramble told the annual meeting of the Ecological Society of America and the Society for Ecological Restoration here yesterday.

Given the chance, rodents can greatly reduce shrub cover, says reclamation specialist David Clark of the New Mexico Mining and Minerals Division in Santa Fe. Even so, Bramble acknowledges that ranchers may be reluctant to shift grazing time. While it would pay off in the long-term by creating richer grassland for their cattle, he says many ranchers might not want to risk hungry cattle (and financial losses) in the meantime.

Related sites
Dennis Bramble's Web page
More on the vole
More on the shrub