A self-sustaining ecosystem is an ecosystem that does not need to make any exchange or have contact with an outside or adjoining ecosystem. These are most commonly seen as bottle ecosystems or glass jar/dome ecosystems. These glass jar ecosystems are often given as gifts.
Self-sustaining ecosystems, even the bottle type, can be important in order for scientist to study and help create systems that can be used on space stations or to help sustain the ecosystem on Earth. Some full scale self-sustaining ecosystems are projects like Biosphere 2 and the Bios programs, of which there are three versions. A self-sustaining system needs to have a few key things, such as water, plants to create oxygen and sunlight.