Chapter 12 — Ecosystem

An ecosystem is a functional unit of nature where living organisms interact among themselves and with their physical environment; it is studied through four key processes: productivity, decomposition, energy flow, and nutrient cycling.

Class 12 Biology Chapter 12 covers ecosystems as structural and functional units of nature comprising abiotic components (air, water, soil) and biotic components (producers, consumers, decomposers). The chapter explains primary productivity (GPP and NPP), the decomposition of detritus through fragmentation, leaching, catabolism, humification and mineralisation, unidirectional energy flow across trophic levels following the 10 per cent law, and ecological pyramids of number, biomass, and energy. Terrestrial and aquatic ecosystem examples, food chains, food webs, and the roles of grazing and detritus food chains are discussed in detail.

1. 01An ecosystem functions through four processes: productivity, decomposition, energy flow, and nutrient cycling
2. 02Gross primary productivity (GPP) minus respiration losses equals net primary productivity (NPP), which is the biomass available to heterotrophs
3. 03Decomposition converts complex organic detritus into inorganic substances through fragmentation, leaching, catabolism, humification, and mineralisation
4. 04Energy flow in ecosystems is unidirectional; only 10 per cent of energy is transferred from one trophic level to the next
5. 05Ecological pyramids (number, biomass, energy) represent feeding relationships; the pyramid of energy is always upright and can never be inverted
6. 06Plants capture only 2–10 per cent of photosynthetically active radiation (PAR), which is less than 50 per cent of total incident solar radiation