Natural, biodegradable, and more insulating than wool, cashmere is as close to the perfect fiber you can get. Just one catch: The goats whose downy undercoats are so prized have insatiable appetites, consuming 10 percent of their body weight in roughage every day. In China, where large herds are stripping the ground of vegetation and topsoil, desertification is a mounting problem. The good news? With the help of the Wildlife Conservation Society, a cooperative of herders in Patagonia have developed a way of raising cashmere-producing goats without running roughshod on the environment.