An ecosystem is a functional and stable unit where all living organisms interact among themselves and with the surrounding physical environment, leading to a flow of energy and nutrients. Ecosystems are usually classified as natural and artificial. Natural ecosystems exist naturally in nature and function all by themselves. These ecosystems are further classified as terrestrial and aquatic ecosystems. Forests, deserts and grasslands are some terrestrial ecosystems, while the ocean, lakes and wetlands are some aquatic ecosystems. Unlike natural ecosystems, artificial ecosystems are created and maintained by human beings. Some artificial ecosystems are croplands, orchards, gardens and aquariums. Two major components in any ecosystem are abiotic and biotic. There is a continuous interaction between the abiotic and biotic components of an ecosystem, resulting in a physical structure which forms a characteristic for each type of ecosystem. The vertical distribution of different species occupying different levels is known as stratification.
Two main processes occur in an ecosystem. One, there is a continuous cycling of nutrients between living and non-living components and two, there is a continuous input of energy from an ultimate source which is mainly the sun. This energy is tapped by the producers and is passed to other trophic levels in the ecosystem. Generally, the producers occupy the first trophic level at the base, followed by primary, secondary and tertiary consumers.
All ecosystems, big or small, perform four major functions, namely, productivity, decomposition, energy flow and nutrient cycling. There is a unidirectional flow of energy from the lower to the higher trophic levels with substantial amounts of energy being dissipated in the form of heat to the environment at various levels. Thus, the structure of ecosystems varies greatly and its functioning depends on rate of nutrient and energy flow and also the inter-relation between living organisms and the environment, thereby forming a self-sustaining system.