Dry years help the prairie dog maintain control over the vegetation, and thus favor town expansion, while wet years speed recovery of the grasses and work against the town. Towns that had spread into lush grassland during the favorable conditions of drought may be severely reduced or even eliminated with the return of adequate precipitation. In some cases, the clearing activities of prairie dogs may actually speed plant succession. As a direct result of overgrazing by domestic stock, much of the grassland of the semi-arid West was invaded by sagebrush, a tough, drought-resistant plant that tends to perpetuate itself indefinitely once introduced. By cutting down the sage, prairie dogs eliminate it from the plant community and open the land to grasses. In its struggle for control of territory, the prairie dog was aided by the bison. The vast herds that moved across the landscape like a black plague of giant locusts left choice tracts of the grasslands devastated in their wake. The effects of such a physical force are difficult for us to comprehend today. Migrating northward in huge columns that sometimes measured 80 kilometers (50 miles) across, the bison cropped and trampled the greening spring grasses. Surely when the bison and the prairie dog were finally replaced by cattle, the grasslands, free at last from such punishment, would flourish. But no, instead the grasses languished. How could this be so? Nature is always more complex than our perception of it. For countless centuries the Great Plains had survived its many moods of drought, dust storm, and wildfire, its scattered plagues of bison and grasshopper. Nature knows no “varmits,” and change is not catastrophe, but opportunity. But to the settler who shot hawks, owls, and eagles to protect his calves, poultry, and children, the lesson was a long time dawning. Ironically, the grasslands needed the periodic despoilment of bison and burrow. When the bison herd moved on, it left behind tons of fertilizer, key to the area’s future rejuvenation. But the millions of hoofs also left the ground compacted. Were it not for the ceaseless activities of countless burrowing animals, including the industrious prairie dogs, loosening the soil and undoing the damage, the bison would have eventually destroyed much of their range. For an animal so successfully adapted to life on the unrelieved expanses of the open plains, it is ironic that today prairie dogs survive in isolated colonies at the very margins of its desirable habitat. 3 Guide and Adviser Regional Map [Illustration: National Parks] [Illustration: _The map comes from the “Guide and Map, National Parks of the United States,” a brochure that may be purchased from the U. S. Government Printing Office. For information on roads leading to Devils Tower, see the next page._] Visiting the Park