The terms ecosystem was introduction by Sir Arthur tensely in 1935. An ecosystem can be defined s "a structural and function unit of biosphere consisting of living organisms and their physical environment, both interacting and exchanging the materials between them". In other words," an ecosystem is an area that contains organisms For example Plants, animals, bacteria and human beings etc. which are interaction with one another and with their non- living environment". An ecosystem may be big or small, natural or manmade, eg.:

(i)         Micro organism such as bacterial limited to few mm2 or cm2 areas (Natural).

(ii)        Desert ecosystem may exist in hundreds of Km2 area (Natural).

(iii)       Crop field , aquarium (Man made).