Closed ecological system

Closed ecological systems or contained ecological systems (CES) are ecosystems that do not rely on matter exchange with any part outside the system in order to help maintain life. The term is most often used to describe small, man-made ecosystems. Such systems can potentially serve as a life-support system or space habitats. In a closed ecological system, any waste products produced by one species must be used by at least one other species. If the purpose is to maintain a life form, such as a mouse or a human, waste products such as carbon dioxide, feces, and urine must eventually be converted into oxygen, food, and water. These systems still rely on constant external energy input, such as sunlight. A closed ecological system must contain at least one autotrophic organism. While both chemotrophic and phototrophic organisms are plausible, almost all closed ecological systems to date are based on an autotroph such as green algae.