Chapter 12 — Ecosystem

Chapter 12 of the Class 12 Biology NCERT textbook, "Ecosystem", covers the ecosystem as a functional unit of nature where organisms interact with each other and their physical environment, studied through four key processes: productivity, decomposition, energy flow, and nutrient cycling.

• Structure and function of an ecosystem — An ecosystem combines abiotic components like air, water, and soil with biotic producers, consumers, and decomposers, and operates through the linked processes of productivity, decomposition, energy flow, and nutrient cycling.
• Productivity and decomposition — Producers fix energy as primary productivity (GPP and NPP), setting the food available to consumers, while decomposers recycle detritus into inorganic nutrients through fragmentation, leaching, catabolism, humification, and mineralisation.
• Energy flow through trophic levels — Energy moves in one direction along food chains and webs, with only about ten per cent passing to the next trophic level, which explains why food chains are short and higher levels support fewer organisms.
• Ecological pyramids — Pyramids of number, biomass, and energy depict feeding relationships between trophic levels; while some can be inverted, the pyramid of energy is always upright because energy is progressively lost at each transfer.

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