Ecological Systems, Closed: Systems that provide for the maintenance of life in an isolated living chamber through reutilization of the material available, in particular, by means of a cycle wherein exhaled carbon dioxide, urine, and other waste matter are converted chemically or by photosynthesis into oxygen, water, and food. (NASA Thesaurus, 1988)Life Support Systems: Systems that provide all or most of the items necessary for maintaining life and health. Provisions are made for the supplying of oxygen, food, water, temperature and pressure control, disposition of carbon dioxide and body waste. The milieu may be a spacecraft, a submarine, or the surface of the moon. In medical care, usually under hospital conditions, LIFE SUPPORT CARE is available. (From Webster's New Collegiate Dictionary)Ecosystem: A functional system which includes the organisms of a natural community together with their environment. (McGraw Hill Dictionary of Scientific and Technical Terms, 4th ed)Conservation of Natural Resources: The protection, preservation, restoration, and rational use of all resources in the total environment.Biodiversity: The variety of all native living organisms and their various forms and interrelationships.Ecology: The branch of science concerned with the interrelationship of organisms and their ENVIRONMENT, especially as manifested by natural cycles and rhythms, community development and structure, interactions between different kinds of organisms, geographic distributions, and population alterations. (Webster's, 3d ed)Models, Theoretical: Theoretical representations that simulate the behavior or activity of systems, processes, or phenomena. They include the use of mathematical equations, computers, and other electronic equipment.Population Dynamics: The pattern of any process, or the interrelationship of phenomena, which affects growth or change within a population.Geography: The science dealing with the earth and its life, especially the description of land, sea, and air and the distribution of plant and animal life, including humanity and human industries with reference to the mutual relations of these elements. (From Webster, 3d ed)Models, Biological: Theoretical representations that simulate the behavior or activity of biological processes or diseases. For disease models in living animals, DISEASE MODELS, ANIMAL is available. Biological models include the use of mathematical equations, computers, and other electronic equipment.