A "natural resource" can be defined as anything that is useful to an organism, population, or ecosystem. The term "useful" implies that the resource is available and safe and can be obtained at a reasonable cost (Miller, 1979). However, the usefulness of resources often changes with time as social and economic systems grow and develop. For example, more than 90 percent of the energy supply for the United States in 1850 was wood. By 1920 coal had replaced wood as the primary energy source, providing nearly 75 percent of domestic energy. By 1980 U.S. energy demand had quadrupled, and more than 75 percent of it was met by oil and natural gas (Stoker et al., 1975).