The ecosystem is the basic structural and functional unit of ecology.
The term ecosystem was first proposed by Tansley in 1935.
Traditionally, an ecosystem is defined as any unit that includes all the living organisms (the community) in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity and material cycling between the abiotic and biotic components within (Ramakrishnan, 2001). The ecological study of ecosystems or “Ecosystem ecology” is considered as the most important aspect of ecology.
The central theme of ecosystem concept is that the living organisms of a community not only interact among themselves but also have functional relationship with their nonliving environment. This structural and functional system of communities and their environment is called an ecosystem. It comprises the interactions of the organisms and their environmental factors, involving the flow of energy and cycling of materials (Fig. 5.1).
Thus all natural ecosystems are self sustaining since there is continuous flow of energy and nutrient cycling, while all human managed systems are more or less dependent upon energy subsidy that is external to the system.
Depending upon the species diversity and the manner in which they are organized, we can recognize the ecosystem types such as aquatic, marine, forest, grassland and agro-ecosystem. These typologies are determined not only by the species composition but also by the physiognomic characteristics and soil and climatic conditions. Freshwater ecosystems are usually named after the size and nature of the freshwater body such as pond, lake, and river.
A wetland ecosystem, looking outwardly, presents an intermediate condition between an aquatic and a terrestrial ecosystem. The largest ecosystem on this earth is ‘marine ecosystem’, which consists of several subdivisions, each having its physicochemical and biological characteristics. For example, in the deepest ocean producers are absent, but many other organisms survive there as they are dependent for food on the dead organic matter coming from the upper layers of the ocean.