Have you seen lakes, ponds and pools in your surroundings? They are all called water bodies with many components in them. Can you list out the things which are found in water bodies? Mud, nutrients, clay, dissolved gases, planktons, microorganisms, plants like algae, Hydrilla, Nelumbo, Nymphaea and animals like snake, small fish, large fish, frog, tortoise and crane are the components of the water bodies which constitutes ecosystem. Further, we all know that plants and animals are prominent living components in the environment. They interact with space components such as air, water, soil, sunlight, etc. For example, you have studied in class XI, one of the life processes, photosynthesis which utilizes sunlight , water, carbondioxide, nutrients from the soil and release oxygen to the atmosphere. From this, we understand that the exchange of materials takes place between living and space components. Likewise, you can study the structure, function and types of ecosystem in this chapter. The term ‘ecosystem’ was proposed by A.G. Tansley (1935), who defined it as ‘the system resulting from the integration of all the living and nonliving factors of the environment’. Whereas, Odum (1962) defined ecosystem ‘as the structural and functional unit of ecology’.