→ An ecosystem can be visualised as a functional unit of nature, where living organisms interact among themselves and also with the surrounding physical environment.
→ Ecosystem varies greatly in size from a small pond to a large forest or a sea. Many ecologists regard the entire biosphere as a global ecosystem, as a composite of all local ecosystems on Earth.
→ Since this system is too much big and complex to be studied at one time, it is convenient to
divide it into two basic categories. (A) terrestrial and (B) the aquatic.
(A) Terrestrial ecosystem : Forest, grassland and desert
(B) Aquatic ecosystems : pond, lake, wetland, river and estuary.
→ Crop fields and an aquarium may also be considered as man-made ecosystems.