In an unusual shake-up of traditional alliances, ranchers and environmentalists are banding together in Colorado to fight a common enemy: urban sprawl. In Custer County, at the foot of the Sangre de Cristo Mountains, three conservation groups and six ranchers have signed a covenant limiting the kind of development permissible on the land — no trophy homes, no golf courses, no condominiums. The result? An 11,000-acre swath of green that will ensure that ranchers still have the wide-open lands they need for their cattle, while also guaranteeing that subdivisions and other signs of creeping suburbia will be kept at bay. The Custer County deal is the largest and most ambitious of a recent flurry of alliances between ranchers and enviros in an effort to protect the open spaces of the West.