The term conservation usually refers to environmental conservation or preservation of natural resources. In the United States, the term came into use in the late 19th century, when valuable natural resources such as timber, farmland, and pasture were threatened by rapid economic development. Concerns extended to include the Great Plains as frontier settlement moved westward at an increasingly rapid rate with the construction of continental railroads, which laid land open for further development. The term suggests preservation and protection of natural resources to prevent exploitation and destruction. Conservation also includes the recognition of limits to growth. Presently, the term conservation embraces the ecosystems and resource base of the Earth by protecting its capacity for self-renewal and sustainability.