Ecosystem covers how living organisms interact with their abiotic surroundings as a functional unit, and the four key functions every ecosystem performs: productivity, decomposition, energy flow, and nutrient cycling. You learn GPP and NPP, the 10 percent law of energy transfer, food chains and webs, ecological pyramids, and biogeochemical cycles. It is a high-yield, formula-light NEET chapter where direct fact and figure-based questions are common.
Class 12 Biology · Chapter 13 – swipe through all 9 cards to understand the whole chapter.
What an Ecosystem Is
A functional unit where living organisms interact with their physical surroundings.
A pond is the classic small, self-sustaining ecosystem; sum of all = biosphere.
- Abiotic: soil, water, light, temperature, inorganic nutrients
- Biotic: producers, consumers, decomposers (saprotrophs)
- Natural (forest, pond) vs man-made (crop field, aquarium)
Structure vs Function
Every ecosystem has a structure and performs four functions.
Trap: stratification is a STRUCTURE, not a function.
- Species composition = identity + count of all species
- Stratification = vertical layering (trees on top, herbs below)
- Producers are the ONLY entry point for usable energy
Productivity (GPP, NPP)
Productivity is the rate of biomass production per area per time.
Unit: g m⁻2 yr⁻1 (or kcal m⁻2 yr⁻1). GPP is always > NPP.
- GPP = total organic matter fixed in photosynthesis
- NPP = biomass left for herbivores & decomposers after respiration
- Secondary productivity = new organic matter made by consumers
Global Productivity
NCERT expects two biosphere productivity numbers.
Oceans cover ~70% of Earth yet are far less productive per area.
- Depends on species, photosynthetic capacity, nutrients
- Sunlight and temperature also control productivity
- Don’t swap the two figures: 170 (whole) vs 55 (oceans)
Decomposition
Decomposers break complex detritus into inorganic nutrients.
Mostly aerobic; humus is the dark, slow-decomposing reservoir.
- Detritus = dead leaves, roots, bark, faeces
- Faster with N & sugars; slower with lignin & chitin
- Warm + moist speeds it; cold + anaerobic slows it
Energy Flow & 10% Law
Energy flows one way and drops sharply at each trophic level.
Unidirectional: producers → consumers → decomposers, lost as heat.
- Sun is source; PAR <50% of light, plants fix only 2–10% of PAR
- 90% lost as heat/metabolism at each transfer
- So food chains rarely exceed 4–5 trophic levels
Food Chains & Webs
Trophic levels are feeding steps counted from the producer.
Standing crop = biomass present at a level at a given time.
- Grazing food chain (GFC) starts with producers
- Detritus food chain (DFC) starts with dead matter
- Interlinked chains = food web (more realistic)
Ecological Pyramids
A graphical snapshot of trophic structure, producers at the base.
Number & biomass CAN invert; energy cannot (10% law).
- Inverted number → single tree with insects & birds
- Inverted biomass → pond/sea (phytoplankton: small crop, high turnover)
- Limitation: decomposers given no place in pyramids
Nutrient Cycles & Services
Matter cycles endlessly between organisms and environment.
Phosphorus has no gaseous phase; ~71% of carbon is in oceans.
- Succession: lichens are pioneers; primary slow, secondary faster (soil exists)
- Costanza valued ecosystem services ≈ US$33 trillion/yr
- Largest service share = soil formation (~50%)
📝 Practice Ecosystem — 10 NEET PYQs
Real previous-year questions · with answers & solutions
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Related Chapters in Class 12 Biology
- Organisms and Populations Class 12 Notes
- Biodiversity and Conservation Class 12 Notes
- Environmental Issues Class 12 Notes
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Frequently Asked Questions
An ecosystem is a functional unit in which living organisms (biotic components like producers, consumers, and decomposers) interact with their non-living physical surroundings (abiotic components like soil, water, light, and temperature). A pond is the classic example of a small, self-sustaining ecosystem, while the sum of all ecosystems on Earth forms the biosphere.
Gross Primary Productivity (GPP) is the total rate of organic matter produced by producers during photosynthesis, while Net Primary Productivity (NPP) is what remains after producers use some energy in respiration. The relation is NPP equals GPP minus respiration, so GPP is always greater than NPP, and NPP is the biomass actually available to herbivores and decomposers.
The 10 percent law, given by Lindeman, states that only about 10 percent of the energy at one trophic level is transferred to the next, with roughly 90 percent lost as heat and metabolism at each step. Because of this sharp drop, food chains usually do not exceed four to five trophic levels.
The pyramid of energy is always upright because energy flow is unidirectional and decreases at each successive trophic level due to the 10 percent law, so a higher level can never hold more energy than the level below it. Pyramids of number and biomass can sometimes be inverted (for example, a single tree supporting many insects, or phytoplankton in a pond), but the energy pyramid never inverts.
Yes, Ecosystem is part of the NEET Class 12 Biology syllabus under the Ecology unit and is considered high-yield, with the ecology section as a whole carrying significant weightage in the Botany or biology paper. Questions are usually direct and fact-based, covering productivity figures, the 10 percent law, decomposition steps, and ecological pyramids, making it a scoring chapter.