Peasants have been forced off their lands by government development projects, such as major dams, by colonization, by government collectivization or compulsory acquisition of land for various purposes, by the privatization of public land, by commercial landholders who change land use practices to make them incompatible for subsistence activities (e.g. clearance of forests), or for a variety of other reasons. Usually the only unallocated land onto which they can move is marginal for primary production (steep slopes, semi-arid lands etc.) and can be rapidly degraded.