An ecosystem is a functional unit of nature where living organisms (biotic components) interact among themselves and with the surrounding physical environment (abiotic components). The term was coined by A.G. Tansley in 1935.
An ecosystem consists of two main components: biotic (producers, consumers, decomposers) and abiotic (temperature, light, water, soil, minerals). Ecosystems can range from a small pond to an entire ocean, from a patch of forest to the entire biosphere.
Producers (autotrophs) form the base of every ecosystem. In terrestrial ecosystems, green plants are the primary producers; in aquatic ecosystems, phytoplankton play this role. Consumers are heterotrophs that depend on producers directly or indirectly — primary consumers (herbivores), secondary consumers (carnivores that eat herbivores), and tertiary consumers (top carnivores).
Energy flow in an ecosystem is unidirectional and follows the 10% law proposed by Lindeman. Only about 10% of the energy available at each trophic level is transferred to the next level. This is why food chains rarely have more than 4–5 trophic levels.
Productivity in an ecosystem is measured as the rate of biomass production. Gross Primary Productivity (GPP) is the total rate of photosynthesis. Net Primary Productivity (NPP) = GPP − Respiration. The most productive ecosystems are tropical rainforests (on land) and coral reefs/estuaries (in water).
Decomposition is the process by which decomposers (fungi and bacteria) break down dead organic matter into simpler inorganic substances. The steps of decomposition are: fragmentation → leaching → catabolism → humification → mineralization.
Ecological pyramids represent the trophic structure of an ecosystem. Pyramids of energy are always upright. Pyramids of biomass are generally upright but inverted in aquatic ecosystems (where phytoplankton biomass is less than zooplankton at any given time). Pyramids of numbers can be upright, inverted, or spindle-shaped.
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