Exam-ready answers to all 11 exercises of Chapter 12, Ecosystem (NCERT Class 12 Biology, 2026-27): fill in the blanks and MCQs on producers, trophic levels and PAR, grazing vs detritus food chains, production and decomposition, upright and inverted pyramids, food chain vs food web, litter and detritus, primary and secondary productivity, components of an ecosystem, pyramids of number and biomass, decomposition and energy flow. All 11 questions are answered, with the key answer highlighted.
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 ______.
Solution
(a) autotrophs (producers)
(b) inverted (partly; a few big trees support many herbivores)
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.
Solution
(a)Grazing food chain (GFC): starts with green plants (producers) and goes to herbivores and carnivores; energy comes from the sun. Detritus food chain (DFC): starts with dead organic matter (detritus) and goes to decomposers (bacteria, fungi) and detritivores; it carries a larger energy flow in terrestrial ecosystems.
(b)Production: the making of organic matter (biomass) by producers through photosynthesis. Decomposition: the breaking down of complex organic matter into inorganic substances (CO₂, water, nutrients) by decomposers.
(c)Upright pyramid: the base (producers) is largest and each higher level is smaller (e.g. pyramid of energy, pyramid of numbers in grassland). Inverted pyramid: the base is smaller than the levels above (e.g. pyramid of biomass in a sea, where a small biomass of phytoplankton supports a larger biomass of fish; pyramid of numbers on a tree).
(d)Food chain: a single linear sequence of who eats whom. Food web: a network of many interconnected food chains in an ecosystem; it is more stable.
(e)Litter: dead plant material (leaves, bark, twigs) lying on the ground. Detritus: all dead plant and animal remains, including litter and faecal matter, that form the raw material for decomposition.
(f)Primary productivity: the rate of production of biomass by plants through photosynthesis (gross and net). Secondary productivity: the rate of formation of new organic matter by consumers.
See the differences explained above for each pair.
An ecosystem has two kinds of components that interact:
Abiotic (non-living): light, temperature, water, soil, air, minerals and inorganic and organic substances.
Biotic (living):producers (green plants, autotrophs), consumers (herbivores, carnivores, omnivores: heterotrophs) and decomposers (bacteria and fungi).
They work together in four functional processes: productivity, decomposition, energy flow and nutrient cycling. Vertical layering of plants (stratification) is also a structural feature.
Abiotic factors and biotic components (producers, consumers, decomposers), linked by productivity, decomposition, energy flow and nutrient cycling.
Define ecological pyramids and describe with examples, pyramids of number and biomass.
Solution
An ecological pyramid is a graphic representation of the relationship between organisms at successive trophic levels in terms of number, biomass or energy, with producers at the base and top carnivores at the apex.
Pyramid of number: shows the number of individuals at each level.
Grassland: many grass plants, fewer herbivores, fewer carnivores: upright.
A single tree supporting many insects which support fewer birds: inverted (or spindle-shaped).
Pyramid of biomass: shows the total dry weight at each level.
Forest or grassland: upright (plant biomass is largest).
Sea: inverted, because the small standing biomass of fast-reproducing phytoplankton supports a larger biomass of zooplankton and fish.
(The pyramid of energy is always upright.)
Diagrams of trophic levels by number, biomass or energy; pyramid of number is upright in grassland and inverted for a tree; pyramid of biomass is upright on land and inverted in the sea.
What is primary productivity? Give brief description of factors that affect primary productivity.
Solution
Primary productivity is the rate at which biomass (organic matter) is produced per unit area over a time period by plants during photosynthesis (g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹). Gross primary productivity (GPP) is the total rate; net primary productivity (NPP) = GPP − respiration losses, which is available to consumers.
Factors affecting it:
the plant species living in the area and their photosynthetic capacity;
Define decomposition and describe the processes and products of decomposition.
Solution
Decomposition is the breakdown of complex organic matter (detritus) into inorganic substances such as CO₂, water and nutrients by decomposers.
Processes:
Fragmentation: detritivores like earthworms break detritus into small pieces.
Leaching: water-soluble nutrients go down into the soil and are precipitated as unavailable salts.
Catabolism: bacterial and fungal enzymes break detritus into simpler inorganic substances.
Humification: formation of humus, a dark, amorphous, colloidal substance that is very resistant to microbial action and decomposes very slowly; it is a reservoir of nutrients.
Mineralisation: humus is slowly degraded by microbes, releasing inorganic nutrients.
Products: CO₂, water, inorganic nutrients and humus. Warm, moist, aerobic conditions and detritus rich in nitrogen and sugars speed up decomposition; lignin and chitin slow it.
The breakdown of detritus into inorganic substances by fragmentation, leaching, catabolism, humification and mineralisation, giving CO₂, water, nutrients and humus.
The sun is the source of energy. Plants capture only 2–10% of the PAR and convert it into chemical energy by photosynthesis.
Energy flows in one direction: from producers to herbivores (primary consumers) to carnivores (secondary and tertiary consumers). It does not return to the sun (unidirectional flow).
At each trophic level, much energy is lost as heat in respiration; only about 10% is passed on to the next level (Lindeman's 10 per cent law).
So the number of trophic levels is limited (usually 4–5), and the pyramid of energy is always upright.
Energy also flows into the detritus food chain from dead organisms and waste, where decomposers use it.
Solar energy fixed by plants flows one way through trophic levels, with about 90% lost as heat at each step (10% law).
Done with the answers?Now revise the whole chapter: Class 12 Biology notes
For NCERT Biology Class 12 (2026-27 edition) - all 13 chapters, CBSE board 2027
Typed concept notes with labelled diagrams for every topic
