A Closed Ecological Life Support System (CELSS) is a habitat unit of interdependent organisms that utilizes an energy input, but is not reliant on matter exchange from outside the system. The Earth itself is an example, but the term is typically used to describe artificial ecosystems. Such systems can not only improve understanding of the Earth's much more complex ecosystem, they can potentially serve as back up systems for maintaining survival of species in the eventuality of the changing ecosystem of the Earth. Such redundant systems can be isolated on Earth, in space, or on other planetary bodies, provided there is an energy source.
In thermodynamic terms, a closed system does not permit any exchange of matter, but energy can move in or out of the system. In a closed ecological system, waste products produced by one species must be used by another species. If the purpose is to maintain humans, waste products such as carbon dioxide, feces and urine must thus be converted into oxygen, food, and water. A closed ecological system must contain at least one autotrophic organism, capable of converting energy from the environment (sunlight, heat or inorganic chemicals) into organic energy-rich molecules (carbohydrates)