Shifting cultivation is an agricultural system in which a person uses a piece of land, only to abandon or alter the initial use a short time later. This system often involves clearing of a piece of land followed by several years of wood harvesting or farming until the soil loses fertility. Once the land becomes inadequate for crop production, it is left to be reclaimed by natural vegetation, or sometimes converted to a different long term cyclical farming practice. This system of agriculture is often practised at the level of an individual or family, but sometimes may involve an entire village. An estimated population exceeding 250 million people derive subsistence from the practice of shifting cultivation, and ecological consequences are often deleterious.