An ecosystem can be defined as 'a structural and functional unit of biosphere or segment of nature consisting of community of living beings and the physical environment, both interacting and exchanging materials between them'.
Besides a mechanical system or a living body, the ecosystem usually has a boundary within which the component parts function together as one unit.
An ecosystem may be natural (like forest, lake, ocean etc) or man-made (such as an aquarium, a crop field etc), temporary (like a rainfed pond) or permanent (like a lake, forest, etc), aquatic (such as pond, ocean etc) or terrestrial (like grassland, forest, etc).
Classification of Ecosystem
An ecosystem may be as small as a drop of pond water and as large as an ocean. Ecosystems can be recognised as self - regulating and self - sustaining units.