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Ecosystem
Start here: why this chapter matters
This is the twelfth chapter of NCERT's Class 12 Biology textbook (pages 205–215 of the 2026-27 reprint), the second of Unit X, Ecology. It has five sections — 12.1 Ecosystem: structure and function, 12.2 Productivity, 12.3 Decomposition, 12.4 Energy flow and 12.5 Ecological pyramids — with Figures 12.1–12.4, a summary and 11 exercises. Earlier editions also taught ecological succession, nutrient cycling (the carbon and phosphorus cycles) and ecosystem services; this reprint keeps only two Summary sentences on nutrient cycling and ecosystem services, so this page covers them briefly from those sentences and does not teach succession. That makes this a short chapter: seven concept sections.
- Neighbouring chapters: Organisms and Populations (Chapter 11) studied populations and their interactions — predation is the energy conduit used here; Biodiversity and Conservation (Chapter 13) follows. Photosynthesis in Higher Plants and Respiration in Plants (Class 11) explain GPP and the respiration (R) that turns it into NPP; Microbes in Human Welfare (Chapter 8) covers decomposers at work in sewage treatment and biogas. Geography's ecology and soil chapters (biomes, humus, soil formation) cover the same ground from the land side.
- Prelims — environment and ecology: what an ecosystem is (2015), food chains and their order (2013, 2014), decomposers and detritivores (1979, 2013, 2021), humus (1986), productivity of ecosystems (2013), producers in the ocean (2021), loss of phytoplankton (2012), the top carnivore in a food pyramid (1992), a food chain upset by new predators (1981), the phosphorus cycle (2021) and rain-forest soils (2023).
- GS3 — Environment: Mains 2022 asked what happens to the cellulose deposited each year before it yields CO₂ and water — the decomposition steps of this chapter.
| Word | What it means here |
|---|---|
| Ecosystem | a functional unit of nature: organisms interacting among themselves and with the physical environment |
| Stratification | vertical distribution of species at different levels (trees, shrubs, herbs) |
| Primary productivity | rate of biomass (organic matter) production by plants per unit area per unit time |
| GPP / NPP | gross: all organic matter made in photosynthesis; net: GPP minus the plants' respiration (R) |
| Secondary productivity | rate of formation of new organic matter by consumers |
| Detritus | dead plant remains and animal remains, including faeces — the raw material for decomposition |
| Detritivore | an animal that breaks detritus into smaller particles, e.g. the earthworm |
| Humus | dark, amorphous, colloidal matter, highly resistant to microbes; a reservoir of nutrients |
| PAR | photosynthetically active radiation — under 50% of incident solar radiation |
| Trophic level | an organism's place in the food chain according to its source of food |
| Standing crop | mass (or number) of living organisms at a trophic level at a particular time |
| Saprotroph | a decomposer (mainly fungi and bacteria) that digests dead matter outside its body and absorbs the products |
At a glance — the six ideas to carry away
- Ecosystem: functional unit of nature — organisms interacting among themselves and with the physical environment; a pond to the whole biosphere. Terrestrial (forest, grassland, desert), aquatic (pond, lake, wetland, river, estuary), man-made (crop field, aquarium).
- Structure: species composition + stratification (trees, then shrubs, then herbs and grasses). Functions: productivity, decomposition, energy flow, nutrient cycling. A pond shows all of them: phytoplankton and plants, zooplankton and fish, decomposers at the bottom.
- Primary productivity: rate of biomass made by plants (g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹). GPP − R = NPP; NPP feeds herbivores and decomposers. Secondary productivity: new organic matter made by consumers. Biosphere NPP ≈ 170 billion t dry weight/yr; oceans (≈70% of surface) only 55.
- Decomposition: detritus → CO₂, water, nutrients by fragmentation (detritivores, earthworm), leaching, catabolism (bacterial and fungal enzymes), humification (humus, colloidal, slow) and mineralisation. Oxygen-requiring; faster with nitrogen and sugars, warm and moist; slower with lignin, chitin, cold, anaerobiosis.
- Energy flow: sun the only source (except deep-sea hydrothermal ecosystems); PAR < 50% of incident radiation; plants capture 2–10% of PAR; flow is unidirectional. GFC (grass → goat → man) is the major conduit in water; on land more energy flows through the DFC (fungi, bacteria = saprotrophs). Chains join into food webs. 10 per cent law; standing crop measured as biomass (dry weight).
- Pyramids of number, biomass, energy: base = producers. Usually upright; inverted numbers on a tree, inverted biomass in the sea (phytoplankton 4 vs zooplankton 21); energy never inverted. Limits: ignore species at two levels, assume simple chains, leave out saprophytes. Summary only: nutrient cycling gaseous (carbon) and sedimentary (phosphorus); ecosystem services (forests purify air and water).
1. Ecosystem: structure and function
An ecosystem is a functional unit of nature in which living organisms interact among themselves and with the surrounding physical environment. It may be as small as a pond or as large as a forest or a sea; many ecologists treat the whole biosphere as one global ecosystem. For study, ecosystems are divided into terrestrial (forest, grassland, desert) and aquatic (pond, lake, wetland, river, estuary); crop fields and aquaria are man-made ecosystems.
Structure. The interaction of biotic and abiotic components gives each ecosystem a characteristic physical structure. Identifying and listing its plant and animal species gives its species composition; the vertical distribution of species at different levels is stratification — in a forest, trees occupy the top layer, shrubs the second and herbs and grasses the bottom. Function. The components work as a unit through four processes: productivity, decomposition, energy flow and nutrient cycling.
A pond as an example. A pond is a shallow, fairly self-sustaining water body showing all four. Its abiotic part is the water with dissolved inorganic and organic substances and the rich soil at the bottom; the solar input, cycle of temperature, day-length and other climatic conditions regulate the rate of the whole pond. Autotrophs: phytoplankton, some algae and floating, submerged and marginal plants. Consumers: zooplankton and free-swimming and bottom-dwelling forms. Decomposers: fungi, bacteria and flagellates, most abundant at the bottom. The pond does what the biosphere does: autotrophs convert inorganic into organic matter with the sun's radiant energy; heterotrophs consume the autotrophs; dead matter is decomposed and mineralised for reuse — over and over again. Energy moves one way, to higher trophic levels, and is dissipated as heat.
- Stratification is vertical layering (trees, shrubs, herbs); species composition is the list of species — together they are the ecosystem's structure.
- The four functions are productivity, decomposition, energy flow and nutrient cycling — not ‘producers, consumers, decomposers’, which are components.
- Matter cycles; energy flows one way.
- An ecosystem is the community together with its physical environment (UPSC 2015) — not the community alone, nor just the inhabited part of the Earth.
- Crop fields and aquaria are ecosystems too — man-made.
- The word ‘ecosystem’ was introduced by the British ecologist A. G. Tansley in 1935.
- Stratification also occurs in lakes, by temperature and light, and in the soil, as horizons.
2. Productivity: GPP, NPP and secondary productivity
A constant input of solar energy is the basic requirement for any ecosystem. Primary production is the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis, expressed as weight (g m⁻²) or energy (kcal m⁻²). The rate of biomass production is productivity, expressed as g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹ so that ecosystems can be compared. Gross primary productivity (GPP) is the rate of production of organic matter during photosynthesis; a considerable part of it is used by plants in respiration (R). GPP − R = NPP, the net primary productivity — the biomass available to heterotrophs (herbivores and decomposers). Secondary productivity is the rate of formation of new organic matter by consumers.
Primary productivity depends on the plant species of an area, on environmental factors, on availability of nutrients and on plants' photosynthetic capacity, so it varies between ecosystems. The annual NPP of the whole biosphere is about 170 billion tonnes (dry weight) of organic matter; the oceans, despite covering about 70 per cent of the surface, produce only 55 billion tonnes — so land produces about 115 billion tonnes, roughly two-thirds of the total, on about 30 per cent of the surface (asserted in the source file).
| Term | Formula or meaning | Who makes it |
|---|---|---|
| Gross primary productivity (GPP) | rate of organic matter made in photosynthesis | producers |
| Respiration (R) | part of GPP used by plants themselves | producers |
| Net primary productivity (NPP) | GPP − R; food for herbivores and decomposers | producers |
| Secondary productivity | rate of new organic matter formed | consumers |
- NPP = GPP − R, so NPP is always less than GPP; consumers can use only NPP.
- Production is an amount (g m⁻²); productivity is a rate (g m⁻² yr⁻¹).
- Secondary productivity is made by consumers, not by ‘secondary producers’ among plants.
- The oceans cover about 70% of the surface but make only 55 of 170 billion tonnes of NPP — about a third, not most of it.
- The main limiting factor for aquatic productivity is light (it fades with depth), with nutrients next — exercise 1(c).
- Among ecosystems, NPP per unit area is generally highest in tropical rain forests, swamps and marshes, estuaries, coral reefs and mangroves, and lowest in deserts, tundra and the open ocean (UPSC 2013 ranked mangroves, grasslands, lakes, oceans).
- Upwelling zones are productive because they bring nutrients up to the sunlit surface.
3. Decomposition: how dead matter becomes nutrients
The earthworm is the farmer's ‘friend’ because it helps break down complex organic matter and loosens the soil. Decomposers break down complex organic matter into inorganic substances such as carbon dioxide, water and nutrients — decomposition. Dead plant remains (leaves, bark, flowers) and dead animal remains, including faecal matter, form detritus, its raw material. The important steps are fragmentation, leaching, catabolism, humification and mineralisation, and they operate simultaneously on the detritus.
- Fragmentation: detritivores (e.g. earthworm) break detritus into smaller particles.
- Leaching: water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.
- Catabolism: bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
- Humification: accumulation of humus, a dark, amorphous substance, highly resistant to microbial action, decomposing extremely slowly; being colloidal, it is a reservoir of nutrients.
- Mineralisation: humus is further degraded by some microbes, releasing inorganic nutrients.
Decomposition is largely oxygen-requiring. Its rate is controlled by the chemical composition of detritus and by climatic factors. In a given climate it is slower if detritus is rich in lignin and chitin, and quicker if rich in nitrogen and water-soluble substances such as sugars. Temperature and soil moisture are the most important climatic factors, acting through the activity of soil microbes: warm and moist conditions favour decomposition, while low temperature and anaerobiosis inhibit it, so organic matter builds up.
- Detritivores (earthworms, millipedes, woodlice) fragment detritus; decomposers (fungi, bacteria) digest it chemically — catabolism.
- Leaching makes nutrients unavailable (precipitated salts) — it is not uptake.
- Humus decays very slowly and stores nutrients; mineralisation is what releases them.
- Lignin and chitin slow decomposition; nitrogen and sugars speed it up.
- Cold and waterlogged (anaerobic) conditions inhibit decomposition, so organic matter piles up.
- NCERT's Summary says decomposition involves ‘three processes’ (fragmentation, leaching, catabolism); the text lists five steps — write all five, noting that humification and mineralisation occur in the soil.
- Viruses are not decomposers (UPSC 2013).
- Tropical rain forests decompose litter so fast that most nutrients are held in the living plants, not the soil; cleared rain-forest soils are poor (UPSC 2023).
- Peat bogs and permafrost store huge amounts of carbon precisely because decomposition is slowed by waterlogging and cold.
4. Energy flow and the 10 per cent law
Except for the deep-sea hydrothermal ecosystem, the sun is the only source of energy for all ecosystems on Earth. Of incident solar radiation, less than 50 per cent is photosynthetically active radiation (PAR); plants and photosynthetic bacteria capture only 2–10 per cent of the PAR, and this small amount sustains the entire living world. All organisms depend on producers directly or indirectly, so energy flows one way: sun → producers → consumers. Ecosystems are not exempt from the second law of thermodynamics: they need a constant supply of energy to synthesise molecules against the universal tendency toward disorder. On land the major producers are herbaceous and woody plants; in water, phytoplankton, algae and higher plants.
No energy trapped in an organism stays there for ever: it passes to a consumer or the organism dies, and death begins the detritus food chain. Energy decreases at each successive trophic level. Each level has a standing crop — the mass (biomass) or number of living organisms per unit area at a particular time; biomass is more accurately measured as dry weight. The number of trophic levels in the grazing food chain is restricted because transfer follows the 10 per cent law: only about 10 per cent of the energy passes from each level to the next. So nature can have producer, herbivore, primary carnivore and secondary carnivore — four levels — in a grazing food chain.
| Step | Energy (J) | Share of sunlight | Lost before the next level |
|---|---|---|---|
| Sunlight | 1,000,000 | 100% | 99% not fixed as NPP |
| Producers (NPP) | 10,000 | 1% | 9,000 (90%) |
| Primary consumers | 1,000 | 0.1% | 900 |
| Secondary consumers | 100 | 0.01% | 90 |
| Tertiary consumers | 10 | 0.001% | — |
- PAR is under 50% of incident radiation; plants capture 2–10% of PAR — not 2–10% of all sunlight (that is about 1–5%).
- Energy flow is unidirectional — it is not recycled; nutrients are.
- The 10 per cent law limits the grazing chain; the detritus chain is not limited the same way (NCERT's in-text question).
- Dry weight is used for biomass because water content varies.
- Not every ecosystem runs on sunlight: deep-sea hydrothermal ecosystems run on chemical energy (chemosynthesis).
- One-way flow agrees with the first law (energy is converted, not created); the second law explains the heat lost at every step.
- The ten per cent rule is usually traced to Raymond Lindeman's 1942 study of trophic dynamics; real transfer efficiencies range widely, often 5–20%.
- Persistent pesticides such as DDT do the opposite of energy — they become more concentrated at higher levels (biomagnification), because they are stored, not burned.
5. Food chains, food webs and trophic levels
Starting from producers, chains — or rather webs — form as an animal feeds on a plant or on another animal and is in turn food for another. Animals are consumers (heterotrophs): primary consumers eat producers (herbivores: insects, birds and mammals on land, molluscs in water); secondary consumers eat herbivores (primary carnivores); tertiary consumers eat primary carnivores (secondary carnivores). NCERT's simple grazing food chain (GFC): grass (producer) → goat (primary consumer) → man (secondary consumer).
The detritus food chain (DFC) begins with dead organic matter and is made up of decomposers, mainly fungi and bacteria, also called saprotrophs: they secrete digestive enzymes onto dead matter and absorb the simple products. In aquatic ecosystems the GFC is the major conduit of energy; in terrestrial ecosystems a much larger fraction flows through the DFC. The two are linked — some DFC organisms are prey to GFC animals, and omnivores such as cockroaches and crows feed in both — making a food web. An organism's place in the chain by its food is its trophic level: producers first, herbivores second, carnivores third. A trophic level is a functional level, not a species: a sparrow is a primary consumer eating seeds, fruits and peas and a secondary consumer eating insects and worms.
| Grazing food chain (GFC) | Detritus food chain (DFC) | |
|---|---|---|
| Starts with | living producers | dead organic matter (detritus) |
| Main organisms | herbivores, carnivores | decomposers — fungi, bacteria (saprotrophs) — and detritivores |
| Major conduit of energy | in aquatic ecosystems | in terrestrial ecosystems |
| Example | grass → goat → man | dead leaves → earthworm / fungi and bacteria |
| Limited by the 10% law | yes — few levels | not in the same way (NCERT's question) |
- A food chain shows the order in which organisms feed on one another; it does not show how many are eaten, and it links different species, not members of one population (UPSC 2013).
- The chain runs producer first: diatoms → crustaceans → herrings (UPSC 2014).
- On land, more energy flows through the detritus chain; in water, through the grazing chain.
- A trophic level is a role; one species (sparrow, human, crow) can fill two or more levels.
- ‘Secondary consumer’ = ‘primary carnivore’; ‘tertiary consumer’ = ‘secondary carnivore’ — the numbering shifts by one.
- Ocean producers are phytoplankton such as diatoms and cyanobacteria; copepods and foraminifera are consumers (UPSC 2021).
- Removing or adding one link ripples through a web: if dogs introduced into a field eat mice, snakes lose food and decline too (UPSC 1981).
- Top carnivores are the most vulnerable level, depending on the whole pyramid beneath them (UPSC 1992).
6. Ecological pyramids: numbers, biomass and energy
The relationship between trophic levels can be expressed in number, biomass or energy. The base of each pyramid is the producers (first trophic level); the apex is the tertiary or top consumer. NCERT's examples: a pyramid of numbers in a grassland — about 5.8 million producers support 708,000 primary consumers, 354,000 secondary consumers and only 3 top carnivores; a pyramid of biomass falling sharply upward (809, 37, 11, 1.5); an inverted pyramid of biomass in water, where a small standing crop of phytoplankton (4) supports a larger one of zooplankton (21); and an ideal pyramid of energy in which producers convert only 1% of 1,000,000 J of sunlight into NPP (10,000 J), then 1,000, 100 and 10 J.
Any calculation must include all organisms at a level, and a given organism may occupy more than one level. In most ecosystems all three pyramids are upright. Exceptions: count the insects on one big tree, then the small birds eating them and the larger birds eating those, and the pyramid of numbers is inverted at the base; the pyramid of biomass in the sea is generally inverted. The pyramid of energy is always upright and can never be inverted, because some energy is always lost as heat at each step; each bar shows the energy at a level in a given time or annually per unit area. Limitations: pyramids do not account for a species at two or more levels; they assume a simple food chain, which almost never exists, not a food web; and saprophytes get no place although they are vital.
| Pyramid | Usually | Can it invert? | Example of inversion |
|---|---|---|---|
| Numbers | upright | yes | one big tree → many insects → fewer birds |
| Biomass | upright on land | yes | sea: phytoplankton 4 vs zooplankton 21 (standing crop) |
| Energy | upright | never | — |
- Energy pyramids are always upright; numbers and biomass pyramids can invert.
- A tree-dominated ecosystem gives an inverted (or spindle-shaped) pyramid of numbers — exercise 1(b).
- The sea's biomass pyramid inverts because a pyramid shows standing crop at one moment; phytoplankton grow and are eaten so fast that their stock is small though their production is large — not because energy flows backwards.
- NCERT's text says the sea's inversion is because fishes outweigh phytoplankton, while its Figure 12.4(c) shows zooplankton — either consumer level makes the point.
- NCERT labels Figure 12.4(b) in kg m⁻²; 809 kg of dry plant matter per square metre is implausible, and the classic data behind the figure are usually quoted in g m⁻² — remember the shape and ratios, not the unit.
- Figure 12.4(a)'s SC value (354,000) is exactly half the PC value; the classic grassland data are approximately so — fine for the shape.
- Saprophytes have no place in pyramids — one of their three limitations.
- The grassland numbers and the biomass series (809, 37, 11, 1.5) are classic data from American studies quoted by Eugene Odum; the inverted sea biomass is often illustrated with English Channel plankton.
- The pyramid idea goes back to Charles Elton (1927), who described the ‘pyramid of numbers’.
7. Nutrient cycling and ecosystem services (Summary only)
The 2026-27 reprint no longer has sections on these topics; its Summary keeps two sentences, which are all this page treats as NCERT content. Nutrient cycling is the storage and movement of nutrient elements through the components of the ecosystem, so that nutrients are used again and again. It is of two types: gaseous, with the atmosphere or hydrosphere as reservoir (e.g. carbon), and sedimentary, with the Earth's crust as reservoir (e.g. phosphorus). The products of ecosystem processes are ecosystem services — for example, the purification of air and water by forests.
| Type of cycle (NCERT Summary) | Reservoir | Example |
|---|---|---|
| Gaseous | atmosphere or hydrosphere | carbon |
| Sedimentary | Earth's crust | phosphorus |
- Carbon = gaseous cycle; phosphorus = sedimentary cycle — weathering of rocks is the main entry route for phosphorus (UPSC 2021).
- Nutrients cycle; energy does not.
- Exercise 1(e) asks for the major reservoir of carbon, but the reprint no longer teaches it; the answer the book expects is the oceans (dissolved carbon) — by total mass, carbonate rocks and fossil fuels in the crust hold far more, but that store hardly takes part in the short-term cycle.
- Decomposition and mineralisation (section 3) are the steps that return nutrients to the cycle.
- Earlier editions taught ecological succession: a gradual, predictable change in species composition from pioneer species to a climax community; primary succession starts where there was no life (bare rock, new pond), secondary where a community was destroyed (burnt forest, abandoned farm), so secondary is faster. Hydrarch succession runs from water (phytoplankton → submerged, floating, reed-swamp, sedge-meadow plants → forest) and xerarch from dry rock (lichens → mosses → herbs → shrubs → forest); both move towards mesic conditions. Lichens are pioneers on rock (UPSC 2014, on Chapter 11).
- The Millennium Ecosystem Assessment (2005) grouped services as provisioning, regulating, cultural and supporting (nutrient cycling, soil formation, pollination support) — UPSC 2012 tested this.
NCERT's in-text questions — short answers
The chapter asks eight questions in its running text.
Page 207: Discuss the main reason for the low productivity of the ocean.
Page 209: Is the unidirectional flow of energy in keeping with the first law of thermodynamics?
Page 210: How would you classify human beings?
Page 211: Why is measurement of biomass in terms of dry weight more accurate?
Page 211: Is there any such limitation (number of trophic levels) in a detritus food chain?
Page 213: How many trophic levels do human beings function at in a food chain?
Page 213: If you counted the insects feeding on a big tree, then the small birds and the larger birds, what kind of pyramid would you get?
Page 213: The pyramid of biomass in the sea is generally inverted. Isn't that a paradox? How would you explain it?
NCERT exercise — answer outlines
1. Fill in the blanks: (a) Plants are called as ___ because they fix carbon dioxide. (b) In an ecosystem dominated by trees, the pyramid (of numbers) is ___ type. (c) In aquatic ecosystems, the limiting factor for productivity is ___. (d) Common detritivores in our ecosystem are ___. (e) The major reservoir of carbon on earth is ___.
2. Which one of the following has the largest population in a food chain? (a) Producers (b) Primary consumers (c) Secondary consumers (d) Decomposers
3. The second trophic level in a lake is (a) Phytoplankton (b) Zooplankton (c) Benthos (d) Fishes
4. Secondary producers are (a) Herbivores (b) Producers (c) Carnivores (d) None of the above
5. What is the percentage of PAR in the incident solar radiation? (a) 100% (b) 50% (c) 1–5% (d) 2–10%
6. Distinguish between (a) grazing and detritus food chain (b) production and decomposition (c) upright and inverted pyramid (d) food chain and food web (e) litter and detritus (f) primary and secondary productivity.
7. Describe the components of an ecosystem.
8. Define ecological pyramids and describe with examples, pyramids of number and biomass.
9. What is primary productivity? Give a brief description of factors that affect primary productivity.
10. Define decomposition and describe the processes and products of decomposition.
11. Give an account of energy flow in an ecosystem.
What to remember
Numbers
One-liners
- An ecosystem is a functional unit of nature.
- Species composition and stratification make up ecosystem structure.
- Stratification is the vertical layering of species.
- NPP equals GPP minus respiration.
- NPP is the food available to herbivores and decomposers.
- Secondary productivity is the new organic matter made by consumers.
- Oceans produce only about a third of the biosphere's NPP.
- Detritus is the raw material for decomposition.
- Earthworms fragment detritus.
- Humus is colloidal and decomposes very slowly.
- Mineralisation releases inorganic nutrients from humus.
- Lignin and chitin slow down decomposition.
- Warm, moist conditions speed up decomposition.
- PAR is less than half of incident solar radiation.
- Energy flows in one direction through an ecosystem.
- Only about 10% of energy passes to the next trophic level.
- The grazing food chain dominates in aquatic ecosystems.
- The detritus food chain carries more energy on land.
- Decomposers are also called saprotrophs.
- A sparrow can be both a primary and a secondary consumer.
- The pyramid of energy is always upright.
- The pyramid of biomass in the sea is generally inverted.
- Carbon follows a gaseous cycle; phosphorus a sedimentary one.
Traps to avoid
- Matter cycles; energy flows one way.
- NPP is always less than GPP.
- Plants capture 2-10% of PAR, not of all sunlight.
- Leaching makes nutrients unavailable, not available.
- Detritivores fragment; decomposers digest chemically.
- Viruses are not decomposers.
- More energy flows through the detritus chain on land.
- A trophic level is a role, not a species.
- A food chain shows feeding order, not numbers eaten.
- Only the energy pyramid can never be inverted.
- Tree-dominated ecosystems have inverted pyramids of numbers.
- The sea's inverted biomass pyramid reflects standing crop, not production.
- NCERT's Summary lists three decomposition processes; the text lists five steps.
- Saprophytes are left out of ecological pyramids.
How to strategise
Prelims — what UPSC asks
- Concepts and chains: ecosystem = community with its environment (2015); food chain shows feeding order only (2013); diatoms → crustaceans → herrings (2014); ocean producers are cyanobacteria and diatoms (2021); losing phytoplankton hurts food chains and the carbon sink (2012); top carnivores are most delicately balanced (1992); dogs in a mouse–snake–hawk field (1981).
- Decomposition, productivity, cycles: decomposers are fungi and bacteria, not viruses (2013); detritivores — earthworms, millipedes, woodlice (2021); humus (1986); decomposition by bacteria (1979); productivity order mangroves > grasslands > lakes > oceans (2013); phosphorus enters its cycle by weathering of rocks (2021); rain-forest soil is poor because decomposition is fast (2023).
- Key notes: 1979 (decomposition) keys only bacteria, though NCERT names fungi and bacteria equally (UPSC 2013 keys both) — a flawed question; pick bacteria if it reappears in this single-answer form. 1981 keys ‘mice and snakes will decrease’ — the intended reading is that dogs eat mice, so snakes lose food; hawks would eventually fall too, so treat it as a flawed, dated question. 2013 productivity order relies on Beyond NCERT data — NCERT supports only the oceans being lowest.
- Likely next: GPP/NPP/secondary productivity, PAR percentages, the five decomposition steps and their controls, GFC vs DFC on land and water, trophic level as a role, 10% law arithmetic, which pyramids can invert, pyramid limitations, gaseous vs sedimentary cycles, deep-sea hydrothermal ecosystems.
Mains — where it appears and how to answer
- GS3 2022 (the fate of cellulose deposited each year): structure the answer on the decomposition steps — fragmentation, catabolism by cellulases and microbial respiration (CO₂ and water) or anaerobic digestion (methane), humification, mineralisation — then the controls (lignin, temperature, moisture, oxygen) and the carbon-cycle link.
- GS3 environment answers on productivity, wetlands, mangroves or oceans: use NPP and the 10% law for quantitative precision (oceans: ≈70% of area, ≈32% of NPP; four trophic levels) and the pyramid ideas for why top predators are vulnerable.
Self-check
Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.
1. Net primary productivity is:
2. Of the biosphere's annual NPP of about 170 billion tonnes, the oceans contribute:
3. In decomposition, water-soluble inorganic nutrients going down into the soil and precipitating as unavailable salts is called:
4. Decomposition will be slowest for detritus that is:
5. If producers hold 10,000 J and the 10 per cent law applies, a tertiary consumer receives:
6. In a terrestrial ecosystem, most energy flows through:
7. Which pyramid can never be inverted?
8. According to NCERT's Summary, phosphorus follows which type of nutrient cycle?
UPSC questions from this chapter
Open any question to attempt it in the Question bank — your attempt is saved to your progress.