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 higher life form, such as a mouse or a human, waste products such as carbon dioxide, faeces and urine must eventually be converted into oxygen, food, and water.
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 a phototroph such as green algae.
An ecosphere is a glass enclosed, self-contained and self-sustaining closed ecosystem sold primarily as a gift or experiment. It can include tiny shrimp, algae, gravel, decorative shells, and gorgonia.