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Griffiths (1902, 1941); M. Mc Claran (1971, 1998, 2003); Robert F. Buttery (1987).

A visitor to southeastern Arizona’s Huerfano Butte a century ago would have looked out on a sea of grass, with the occasional shrub dotting the landscape. Today native grasses are sparse, replaced by woody vegetation and desert-like ground cover. American rangelands and grasslands, and savannahs across the globe, are undergoing similar, stark conversions. The culprit is “up for debate,” says Steve Archer, a University of Arizona plant ecologist. It may be a combination of factors, such as livestock grazing or changes in climate and fire. Long-term studies at the University of Arizona’s Santa Rita Experimental Range, where Huerfano Butte is located, offer some clues. “My read of the evidence is that grazing is really at the heart of it,” Archer says. “Long-term heavy grazing opens the door for a lot of shrubs to move in and take over. Grasses never get a chance to rest and recuperate, and shrubs aren’t very palatable to livestock.” Once shrubs move in, native grasses might not come back, even if livestock grazing is halted, as it was decades ago at Santa Rita. Such ecosystem shifts can hurt wildlife, like prairie-chickens and sharp-tailed grouse, whose declines have been linked to shrub encroachment. “Protecting those species,” says Archer, “means protecting their ecosystems.” Conserving existing grasslands might require less intensive grazing and controlled burns to keep bushes at bay.—Alisa Opar

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