Ecologically, the bison throngs of yesteryear added up to a force like wind or wildfire. Their grazing, trampling, dust wallowing and constant movement altered and renewed habitats decade after decade, creating niches for all kinds of creatures. Predators and scavengers large and small followed in the herds' wake. Pronghorns and prairie grouse favored the herbs and grass sprouts that appeared after buffalo had mowed down the taller growth in passing. An estimated six billion prairie dogs--along with black-footed ferrets, cottontail rabbits, amphibians and other animals living in or around the rodents' underground metropolises--were also tied to a bison-dominated community. The cows that followed in the wake of the bison's near-demise are, by contrast, an uneasy fit with prairie flora and fauna. They tend to wander much less than bison and will graze sites until native plants give way to weedy invaders. Since cattle need more water than bison do, they are particularly hard on streams and streamside vegetation--to the detriment of nesting birds, native fish and a host of other wildlife. Cattle are also less hardy than bison, and require more day-to-day attention--including feed grain and protection from predators--by a livestock owner.