NCERT Solutions for Class 12 Biology Chapter 12: Ecosystem (NCERT 2026–27)
These Class 12 Biology Chapter 12 solutions cover Ecosystem from the NCERT Biology textbook (session 2026–27). The chapter explains the ecosystem as a functional unit of nature and its four working components — productivity, decomposition, energy flow and nutrient cycling. Below you will find every end-of-chapter Exercise question reproduced verbatim and answered in clear, exam-ready prose, along with extra practice, MCQs, assertion–reason questions and FAQs.
An ecosystem is a structural and functional unit of nature where living organisms interact among themselves and with their physical environment. Ecosystems vary in size from a small pond to a large forest or sea, and are broadly grouped into terrestrial (forest, grassland, desert) and aquatic (pond, lake, wetland, river, estuary) types; crop fields and aquaria are man-made examples. Its structure is described by species composition and stratification, while it functions through four linked processes: productivity (input of organic matter), decomposition (breakdown of detritus), energy flow (unidirectional transfer through trophic levels obeying the laws of thermodynamics and the 10 per cent law), and nutrient cycling. The chapter closes with ecological pyramids of number, biomass and energy, and the idea of ecosystem services.
Key Concepts & Definitions
Ecosystem: a functional unit of nature in which biotic (producers, consumers, decomposers) and abiotic (air, water, soil, light, temperature) components interact and exchange energy and matter.
Stratification: the vertical distribution of different species at different levels of an ecosystem (e.g. trees, shrubs, herbs in a forest).
Productivity: the rate of biomass production. Primary production is biomass produced by plants per unit area per unit time; expressed as g m–2 yr–1 or kcal m–2 yr–1. GPP – R = NPP, where R is respiratory loss. Secondary productivity is the rate of formation of new organic matter by consumers.
Decomposition: the breakdown of complex organic detritus into CO2, water and nutrients by decomposers, through fragmentation, leaching, catabolism, humification and mineralisation.
Energy flow: unidirectional movement of energy from the sun → producers → consumers, with only about 10 per cent transferred to each successive trophic level (10 per cent law). Around 2–10 per cent of the photosynthetically active radiation (PAR) is captured by plants; PAR is less than 50 per cent of incident solar radiation.
Ecological pyramids: graphical representations of number, biomass or energy at successive trophic levels. The pyramid of energy is always upright; pyramids of number and biomass may sometimes be inverted.
Standing crop: the mass of living material (biomass) or number of organisms present per unit area at a given time at each trophic level.
NCERT Exercises — Solutions
Questions below are reproduced verbatim from the NCERT textbook; the answers are original and written in CBSE exam-ready style.
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 the productivity is_________.(d) Common detritivores in our ecosystem are_________.(e) The major reservoir of carbon on earth is_________.
ANSWER(a) autotrophs (producers) — they fix atmospheric carbon dioxide into organic matter during photosynthesis.(b) inverted — a single large tree (producer) supports many herbivores and even more parasites, so the producer level is narrow at the base.(c) light (availability of sunlight; productivity falls with depth as light penetration decreases).(d) earthworms (and other detritus feeders such as some insects and millipedes).(e) oceans (which hold about 71 per cent of the Earth’s carbon dissolved in water).
2. Which one of the following has the largest population in a food chain?
(a) Producers(b) Primary consumers(c) Secondary consumers(d) Decomposers
ANSWER(d) Decomposers. Decomposers (bacteria and fungi) act on dead matter from every trophic level and reproduce rapidly, so they are the most numerous organisms in a food chain.
3. The second trophic level in a lake is
(a) Phytoplankton(b) Zooplankton(c) Benthos(d) Fishes
ANSWER(b) Zooplankton. Phytoplankton are the producers (first trophic level); the zooplankton that feed on them are the primary consumers and therefore occupy the second trophic level.
4. Secondary producers are
(a) Herbivores(b) Producers(c) Carnivores(d) None of the above
ANSWER(d) None of the above. Only green plants and other autotrophs are true producers; there is no separate category of “secondary producers,” so the correct choice is “none of the above.”
5. What is the percentage of photosynthetically active radiation (PAR) in the incident solar radiation?
(a) 100%(b) 50 %(c) 1-5%(d) 2-10%
ANSWER(b) 50%. Of the incident solar radiation, less than 50 per cent is photosynthetically active radiation (PAR); plants then capture only about 2–10 per cent of this PAR.
6. Distinguish between
(a) Grazing food chain 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
ANSWER(a) Grazing food chain (GFC) vs Detritus food chain (DFC):
Grazing food chain
Detritus food chain
Begins with living green plants (producers).
Begins with dead organic matter (detritus).
Energy source is directly the sun (via producers).
Energy source is detritus left by the GFC.
Dominant pathway in aquatic ecosystems.
Dominant pathway in terrestrial ecosystems.
e.g. Grass → Goat → Man.
e.g. Dead leaves → Earthworm → Fungi/bacteria.
ANSWER (contd.)(b) Production vs Decomposition: Production is the synthesis of organic matter (biomass) by producers using solar energy during photosynthesis; it builds up complex molecules. Decomposition is the reverse breakdown of complex organic detritus into simple inorganic substances (CO2, water, nutrients) by decomposers.(c) Upright vs Inverted pyramid: In an upright pyramid the value (number/biomass/energy) is greatest at the producer base and decreases towards the apex, e.g. a grassland. In an inverted pyramid the base value is smaller than the higher levels, e.g. the pyramid of biomass in a sea, where a small standing crop of phytoplankton supports a larger biomass of zooplankton/fishes.(d) Food chain vs Food web: A food chain is a single, straight-line sequence of who eats whom (one organism at each level). A food web is the natural network formed by many interconnected food chains, where an organism may feed at several points, giving greater stability.(e) Litter vs Detritus: Litter usually refers to the freshly fallen, undecomposed plant remains (leaves, twigs, bark) lying on the soil surface. Detritus is the broader raw material for decomposition — all dead plant and animal remains including faecal matter that decomposers act upon.(f) Primary vs Secondary productivity: Primary productivity is the rate at which producers fix solar energy and build biomass (measured as GPP and NPP). Secondary productivity is the rate of formation of new organic matter by consumers (heterotrophs).
7. Describe the components of an ecosystem.
ANSWERAn ecosystem has two broad components — abiotic and biotic.1. Abiotic components are the non-living physical and chemical factors: inorganic materials (air, water, soil and dissolved organic/inorganic substances) and climatic factors such as solar radiation, temperature, day-length, moisture and pH. These regulate the rate at which the ecosystem functions.2. Biotic components are the living organisms, grouped by their mode of nutrition: (i) Producers (autotrophs) — green plants, phytoplankton and algae that fix solar energy and synthesise food; (ii) Consumers (heterotrophs) — herbivores (primary consumers), carnivores (secondary and tertiary consumers) and omnivores that depend on producers directly or indirectly; (iii) Decomposers (saprotrophs) — bacteria and fungi that break down dead organic matter and return nutrients to the environment.The interaction of these components gives each ecosystem a characteristic physical structure, defined by its species composition and stratification, and drives the functions of productivity, decomposition, energy flow and nutrient cycling.
8. Define ecological pyramids and describe with examples, pyramids of number and biomass.
ANSWERAn ecological pyramid is a graphical representation of the relationship (in terms of number, biomass or energy) between organisms at the successive trophic levels of a food chain. The base represents the producers (first trophic level) and the apex represents the top consumer; the three types are pyramids of number, biomass and energy.Pyramid of number: shows the number of individuals at each trophic level. In a grassland it is upright — many grasses support fewer herbivores, which support still fewer carnivores. In a tree-dominated ecosystem it is inverted, because one big tree (producer) supports many herbivorous insects and even more parasites.Pyramid of biomass: shows the total dry weight (standing crop) of living matter at each trophic level. In most terrestrial ecosystems it is upright, with a sharp decrease in biomass at higher levels. In a sea it is generally inverted, because a small standing crop of fast-reproducing phytoplankton supports a much larger biomass of zooplankton and fishes at a given time.
9. What is primary productivity? Give brief description of factors that affect primary productivity.
ANSWERPrimary productivity is the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed in weight (g m–2) or energy (kcal m–2); its rate is productivity, expressed as g m–2 yr–1 or kcal m–2 yr–1. It is divided into gross primary productivity (GPP), the total rate of organic matter production in photosynthesis, and net primary productivity (NPP = GPP – R), the biomass left after respiratory losses, available to heterotrophs.Factors affecting primary productivity: (i) the plant species inhabiting the area; (ii) availability of nutrients in the soil or water; (iii) the photosynthetic capacity of the plants; and (iv) environmental factors such as solar radiation/light, temperature, water and moisture. Because these vary, primary productivity differs greatly between ecosystem types (it is, for example, much lower in oceans than on land).
10. Define decomposition and describe the processes and products of decomposition.
ANSWERDecomposition is the process by which decomposers break down complex organic matter (detritus) into simple inorganic substances such as carbon dioxide, water and nutrients. Dead leaves, bark, flowers and animal remains including faecal matter form the detritus that is the raw material for decomposition.Processes (steps):(i) Fragmentation — detritivores (e.g. earthworms) break detritus into smaller particles, increasing surface area.(ii) Leaching — water-soluble inorganic nutrients percolate down into the soil and may get precipitated as unavailable salts.(iii) Catabolism — bacterial and fungal enzymes degrade detritus into simpler inorganic substances.(iv) Humification — accumulation of a dark, amorphous, colloidal substance called humus that is highly resistant to microbial action and acts as a nutrient reservoir.(v) Mineralisation — further degradation of humus by microbes to release inorganic nutrients.Products: carbon dioxide, water and inorganic nutrients (which are released back into the soil for reuse by plants). Decomposition is largely an oxygen-requiring process; it is favoured by warm, moist conditions and by detritus rich in nitrogen and sugars, and slowed by low temperature, anaerobic conditions and detritus rich in lignin and chitin.
11. Give an account of energy flow in an ecosystem.
ANSWERThe sun is the only source of energy for almost all ecosystems (except deep-sea hydrothermal ones). Less than 50 per cent of incident solar radiation is photosynthetically active radiation (PAR), and producers capture only about 2–10 per cent of this PAR to synthesise food. Energy flow is therefore unidirectional: it moves from the sun to producers and then to consumers, and is never reused, in keeping with the laws of thermodynamics.From producers, energy passes along two routes — the grazing food chain (Grass → Goat → Man), important in aquatic ecosystems, and the detritus food chain, which carries a larger fraction of energy in terrestrial ecosystems. These interconnect to form a food web.Organisms occupy trophic levels by their feeding relationships: producers (first), herbivores/primary consumers (second), carnivores/secondary consumers (third) and so on. At each transfer, only about 10 per cent of the energy passes to the next level (the 10 per cent law), the rest being lost mainly as heat during respiration. This loss limits the number of trophic levels and explains why the pyramid of energy is always upright. When organisms die they become detritus, feeding the decomposers and beginning the detritus food chain.
Extra Practice Questions
Short Answer Type Questions
Q1. State the 10 per cent law of energy transfer.
ANSWERAccording to Lindeman’s 10 per cent law, only about 10 per cent of the energy stored at one trophic level is transferred to the next higher trophic level; the remaining 90 per cent is lost mainly as heat during metabolism and respiration. This limits the number of trophic levels in a food chain.
Q2. Why is the pyramid of energy always upright?
ANSWERBecause at every transfer from one trophic level to the next, a large part of the energy is lost as heat. So the energy available always decreases from the producer level upwards, and a higher level can never contain more energy than the level below it; the pyramid can never be inverted.
Q3. Distinguish between standing crop and standing state.
ANSWERStanding crop is the mass of living organisms (biomass) or number of individuals present per unit area at a given time at a trophic level. Standing state refers to the amount of inorganic nutrients (such as nitrogen, phosphorus, calcium) present in the soil or water at a given time.
Q4. Why is measurement of biomass in terms of dry weight more accurate than fresh weight?
ANSWERFresh weight varies with the water content of the organism, which changes constantly. Dry weight removes water and reflects the actual organic matter present, so it gives a more reliable and comparable measure of biomass.
Q5. What are ecosystem services? Give two examples.
ANSWEREcosystem services are the products and benefits obtained from natural ecosystem processes. Examples include purification of air and water by forests, pollination of crops, nutrient cycling, climate regulation and control of floods and soil erosion.
Long Answer Type Questions
Q1. Explain decomposition and list the factors that control its rate.
ANSWERDecomposition is the breakdown of complex organic detritus into simple inorganic substances (CO2, water, nutrients) by decomposers such as bacteria and fungi. It proceeds through fragmentation (by detritivores like earthworms), leaching of soluble nutrients into the soil, catabolism by microbial enzymes, humification (formation of humus) and mineralisation (release of inorganic nutrients from humus). Decomposition is largely oxygen-requiring. Its rate is controlled by (i) the chemical composition of detritus — faster if rich in nitrogen and water-soluble sugars, slower if rich in lignin and chitin; and (ii) climatic factors — warm and moist conditions speed decomposition by favouring soil microbes, while low temperature and anaerobic conditions slow it, causing organic matter to accumulate.
Q2. Describe the structure of a pond ecosystem and how it functions as a self-sustaining unit.
ANSWERA pond is a shallow water body showing all four basic components of an ecosystem. Its abiotic component is the water with dissolved inorganic and organic substances and the rich soil at the bottom; solar input, temperature, day-length and climate regulate its functioning. The autotrophs (producers) are phytoplankton, algae and floating, submerged and marginal plants. The consumers are zooplankton and free-swimming and bottom-dwelling animals. The decomposers are fungi, bacteria and flagellates, abundant at the bottom. The pond functions like any ecosystem: autotrophs convert inorganic material into organic matter using solar energy; heterotrophs consume the autotrophs; and decomposers mineralise dead matter, releasing nutrients for reuse by producers. Energy moves unidirectionally to higher trophic levels and is dissipated as heat, while nutrients are cycled repeatedly — making the pond a fairly self-sustaining unit.
Q3. What are ecological pyramids? Describe the three types and the limitations of ecological pyramids.
ANSWERAn ecological pyramid is a graphical representation of number, biomass or energy at successive trophic levels, with producers at the base and the top consumer at the apex. The pyramid of number shows the number of individuals at each level (upright in a grassland, inverted in a tree-dominated ecosystem). The pyramid of biomass shows the standing crop (dry weight) at each level (upright in most land ecosystems, inverted in a sea). The pyramid of energy shows the energy content at each level and is always upright, because energy is lost as heat at every transfer. Limitations: ecological pyramids assume a simple food chain and cannot accommodate a food web; they do not account for a species belonging to two or more trophic levels at once; and they give no place to saprophytes (decomposers), even though these are vital to the ecosystem.
MCQs
1. The four functional components of an ecosystem are:
For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.
A-R 1. Assertion: Energy flow in an ecosystem is unidirectional.
Reason: Energy lost as heat at each trophic level cannot be reused by the ecosystem.
A-R 2. Assertion: The pyramid of energy is always upright.
Reason: Some energy is always lost as heat when energy flows from one trophic level to the next.
A-R 3. Assertion: The pyramid of biomass in a sea is always upright.
Reason: The biomass of phytoplankton always exceeds that of the fishes that feed on them.
A-R 4. Assertion: Decomposition is slower in cold and waterlogged soils.
Reason: Low temperature and anaerobic conditions inhibit the activity of soil microbes.
A-R 5. Assertion: Measurement of biomass as dry weight is more accurate than fresh weight.
Reason: Fresh weight varies with the changing water content of organisms.
Answer key: 1-(A), 2-(A), 3-(D), 4-(A), 5-(A).
Common Mistakes & Exam Tips
Common mistakes to avoid
Writing the NPP formula incorrectly — it is NPP = GPP – R, not GPP + R.
Saying the pyramid of biomass is always upright — it is inverted in the sea.
Confusing the pyramid of energy (always upright) with pyramids of number/biomass (can be inverted).
Mixing up litter (fresh fallen plant remains) with detritus (all dead matter, including faecal matter).
Stating that PAR is 2–10 per cent — PAR is less than 50 per cent of solar radiation; 2–10 per cent is the fraction of PAR captured by plants.
Forgetting decomposers when describing components or trophic relationships.
How to score full marks in this chapter
Learn the exact NCERT definitions and formulae (GPP – R = NPP, the 10 per cent law) and the sequence of decomposition steps in order: fragmentation → leaching → catabolism → humification → mineralisation. For “distinguish between” questions, answer in two columns. Always give a worked example for each pyramid type (grassland, tree, sea) and remember the two structural features of an ecosystem — species composition and stratification. State that the pyramid of energy is always upright and explain why with thermodynamics.
Frequently Asked Questions
What is Class 12 Biology Chapter 12 Ecosystem about?
Chapter 12, Ecosystem, treats the ecosystem as a functional unit of nature with abiotic and biotic components. It explains its four functions — productivity, decomposition, energy flow and nutrient cycling — along with food chains and webs, trophic levels, the 10 per cent law, ecological pyramids and ecosystem services.
What is the difference between GPP and NPP?
Gross primary productivity (GPP) is the total rate of organic matter produced by producers in photosynthesis. Net primary productivity (NPP) is the biomass left after respiratory losses, given by NPP = GPP – R; it is the energy available to consumers.
Why is the pyramid of energy always upright?
Because energy is lost as heat at every transfer between trophic levels, the energy available always decreases from producers upwards, so a higher level can never hold more energy than the level below it.
Are these Class 12 Biology Chapter 12 solutions free?
Yes. All ClearStudy NCERT Solutions for Class 12 Biology are free and follow the official NCERT textbook for session 2026–27.
