Closed ecological systems are ecosystems that do not rely on matter
exchange with any part outside the system. Although the Earth itself
fits this definition, the term is most often used to describe much
smaller manmade ecosystems. Such systems are scientifically interesting
and can potentially serve as a life support system during space
flights, in space stations or space habitats. In a closed ecological
system, any waste products produced by one species must be used by at
least one other species.http://www.eco-sphere.com