Ecosystem is a functional unit of nature, where living organisms interact among themselves and with the surrounding physical environment. The size of the ecosystem varies from a small pond to a large forest or a sea. The ecosystem is divided into two basic categories, namely the terrestrial and the aquatic. Forest, grassland and desert are some examples of terrestrial ecosystems; pond, lake, wetland, river and estuary are some examples of aquatic ecosystems. The two major components of ecosystem are: abiotic component and biotic component. The energy flow in an ecosystem is unidirectional that is from primary producers to tertiary carnivores. The main structural features of an ecosystem are composition of species and stratification. The four important components of an ecosystem are productivity, decomposition, energy flow, and nutrient cycling Primary productivity is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed in terms of weight (g–2) or energy (kcal m–2). The rate of biomass production is called productivity. The primary productivity can be of two types: gross primary productivity (GPP) and net primary productivity (NPP). Secondary productivity is defined as the rate of formation of new organic matter by consumers. The process of breaking down complex organic matter into inorganic substances like carbon dioxide, water and nutrients by decomposers is called decomposition. Energy flow is unidirectional. First, plants capture solar energy and then, food is transferred from the producers to decomposers. Organisms of different trophic levels in nature are connected to each other for food or energy relationship forming a food chain. The natural interconnection of food chains makes it a food web. Ecological Pyramid is a graphic representation of the total productivity at each trophic level and is of three types: pyramid of number, pyramid of biomass and pyramid of energy. The biotic community is dynamic. It changes with the passage of time. These changes are sequentially ordered and constitute ecological succession. The ecological succession begins with invasion of a bare lifeless area by pioneers which later pave way for successors and ultimately a stable climax community is formed. Ecological succession is of two types: the first type is primary succession; the second type is secondary succession. The movement of nutrient elements through the various components of the ecosystem is called nutrient cycling. The nutrient cycling is of two types—gaseous and sedimentary. Carbon and nitrogen are gaseous cycle whereas sulphur and phosphorous cycle are sedimentary cycle.

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