Ecosystem (NEET Biology Class 12): Productivity, Decomposition, Energy Flow & Ecological Pyramids
Ecosystem Structure, Productivity & Decomposition
📌 NEET priority: Important. 2–4 questions a year. GPP − R = NPP, the 10 per cent law, and — above all — which ecological pyramid can be inverted and which never can are the three questions this chapter reliably produces.
Structure of an ecosystem
Species composition = identifying and enumerating the plant and animal species of an ecosystem. Stratification = the vertical distribution of species at different levels — trees occupy the top strata, shrubs the second, and herbs and grasses the bottom of a forest.
The ecosystem functions as a unit through four aspects: productivity, decomposition, energy flow and nutrient cycling.
The pond as a model ecosystem: abiotic = water with dissolved inorganic and organic substances + rich bottom soil; autotrophs = phytoplankton, some algae, floating/submerged/marginal plants; consumers = zooplankton, free-swimming and bottom-dwelling forms; decomposers = fungi, bacteria and flagellates, especially abundant at the bottom.
Energy movement is UNIDIRECTIONAL towards higher trophic levels, with dissipation and loss as heat.
Productivity — the equation NEET asks
Term | Definition | Units |
Primary production | Amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis | Weight (g m⁻²) or energy (kcal m⁻²) |
Productivity | The RATE of biomass production | g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹ |
Gross Primary Productivity (GPP) | Rate of production of organic matter during photosynthesis | — |
Net Primary Productivity (NPP) | GPP MINUS respiration losses. This is the biomass AVAILABLE to heterotrophs (herbivores and decomposers) | GPP − R = NPP |
Secondary productivity | Rate of formation of new organic matter by CONSUMERS | — |
The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter. Of this, the oceans produce only 55 billion tons — despite occupying about 70 per cent of the earth's surface.
Decomposition — the five steps
Detritus — dead plant remains (leaves, bark, flowers) and dead animal remains including faecal matter — is the raw material for decomposition.
Step | What happens |
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 a dark coloured amorphous substance called HUMUS — highly resistant to microbial action, decomposes extremely slowly, and being COLLOIDAL serves as a nutrient RESERVOIR |
Mineralisation | Humus is further degraded by microbes, RELEASING inorganic nutrients |
All these steps operate SIMULTANEOUSLY on the detritus — they are not sequential. Decomposition is largely an OXYGEN-REQUIRING process. It is SLOWER if detritus is rich in LIGNIN and CHITIN, and QUICKER if rich in NITROGEN and water-soluble sugars. Warm and moist conditions favour it; low temperature and anaerobiosis inhibit it, causing organic matter to build up.
Energy Flow, Food Chains & Trophic Levels
Energy flow
The sun is the only energy source for all ecosystems — except the deep sea hydro-thermal ecosystem.
Less than 50 per cent of incident solar radiation is photosynthetically active radiation (PAR), and plants capture only 2–10 per cent of the PAR. That small amount sustains the entire living world.
Energy flow is UNIDIRECTIONAL — sun → producers → consumers. Ecosystems obey the Second Law of thermodynamics, needing a constant energy supply to counteract increasing disorder.
Food chains & trophic levels
Trophic level | Who occupies it | Also called |
First | Producers — herbaceous and woody plants on land; phytoplankton, algae and higher plants in water | Autotrophs |
Second | Herbivores — insects, birds and mammals on land; MOLLUSCS in aquatic ecosystems | Primary consumers |
Third | Carnivores feeding on herbivores | Secondary consumers / PRIMARY carnivores |
Fourth | Animals depending on primary carnivores | Tertiary consumers / SECONDARY carnivores |
Food chain | Starts with | Key facts |
Grazing Food Chain (GFC) | Living producers — e.g. Grass → Goat → Man | The MAJOR conduit for energy flow in an AQUATIC ecosystem |
Detritus Food Chain (DFC) | DEAD organic matter | Made of decomposers (mainly fungi and bacteria), also called SAPROTROPHS. They secrete digestive enzymes that break down dead material into simple inorganic materials, then absorb them. In a TERRESTRIAL ecosystem a MUCH LARGER fraction of energy flows through the DFC than through the GFC |
The two chains interconnect — some DFC organisms are prey to GFC animals, and omnivores like cockroaches and crows link them — making a FOOD WEB.
Standing crop = the mass of living material at each trophic level at a particular time, measured as biomass or number per unit area. Biomass is expressed as fresh or dry weight, but DRY weight measurement is more accurate.
The 10 per cent law: only 10 per cent of energy is transferred to each trophic level from the lower one. This is precisely why the number of trophic levels in a grazing food chain is RESTRICTED.
A trophic level is a FUNCTIONAL level, not a species. One species can occupy several simultaneously — a sparrow is a primary consumer when eating seeds and a secondary consumer when eating insects.
Ecological Pyramids & Why This Matters for NEET
Ecological pyramids — which can be inverted
The base of each pyramid represents producers (first trophic level); the apex represents the top-level consumer. Three types are studied: pyramid of number, biomass and energy.
Pyramid type | Usually | Exception — the case NEET asks |
Pyramid of NUMBER | Upright | INVERTED for a single big TREE: one tree supports many insects, which support fewer small birds, then fewer large birds — the producer level is narrowest |
Pyramid of BIOMASS | Upright | INVERTED IN THE SEA — the biomass of FISHES far exceeds that of PHYTOPLANKTON at any one moment |
Pyramid of ENERGY | ALWAYS upright | NEVER inverted — because energy is always LOST AS HEAT at each transfer step. Each bar shows energy at that trophic level per unit area, in a given time or annually |
Limitations of ecological pyramids
They do not account for a species belonging to two or more trophic levels; they assume a simple food chain, which almost never exists in nature, and cannot accommodate a food web; and saprophytes are given no place despite their vital ecosystem role.
Why this matters for NEET
High-value one-liners: GPP − R = NPP; biosphere NPP = 170 billion tons, oceans only 55 billion tons despite 70% of the surface; PAR < 50% of incident radiation, plants capture 2–10% of PAR; 10 per cent law; humus is colloidal and a nutrient reservoir; DFC dominates on land, GFC in water.
Trap: the pyramid of ENERGY can NEVER be inverted, but the pyramid of biomass CAN be (in the sea) and the pyramid of number CAN be (a single tree). This "which one is never inverted" question appears repeatedly — the answer is always energy, because of heat loss at every step.
Second trap: the five decomposition steps operate SIMULTANEOUSLY, not in sequence. Also, decomposition is slower with lignin/chitin-rich detritus and faster with nitrogen-rich detritus — students often assume tougher material simply takes proportionally longer rather than being chemically resistant. And NPP, not GPP, is what heterotrophs actually get.
Test Yourself: MCQs, PYQs & Active Recall
Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one.
Practice Questions
Q1. The vertical distribution of different species occupying different levels is called:
(a) Standing crop
(b) Productivity
(c) Stratification
(d) Species composition
Show answer
Answer: (c) — Stratification is the vertical layering — trees at the top, shrubs second, herbs and grasses at the bottom.
Q2. Net Primary Productivity is calculated as:
(a) GPP + R
(b) R − GPP
(c) GPP − R
(d) GPP × R
Show answer
Answer: (c) — NPP = GPP minus respiration losses, and it represents the biomass available to heterotrophs.
Q3. Secondary productivity is defined as:
(a) Respiration loss
(b) Biomass of decomposers
(c) The rate of formation of new organic matter by consumers
(d) The rate of photosynthesis
Show answer
Answer: (c) — Secondary productivity is the rate of formation of new organic matter by consumers.
Q4. The annual net primary productivity of the whole biosphere is approximately:
(a) 170 billion tons
(b) 17 billion tons
(c) 500 billion tons
(d) 55 billion tons
Show answer
Answer: (a) — The biosphere's annual NPP is about 170 billion tons dry weight, of which the oceans contribute only 55 billion tons.
Q5. Despite covering about 70 per cent of the earth's surface, the oceans produce only:
(a) 170 billion tons
(b) 115 billion tons
(c) 10 billion tons
(d) 55 billion tons
Show answer
Answer: (d) — Ocean productivity is only 55 billion tons of the biosphere's 170 billion tons total.
Q6. The raw material for decomposition is called:
(a) Standing crop
(b) Detritus
(c) Humus
(d) Biomass
Show answer
Answer: (b) — Detritus — dead plant and animal remains including faecal matter — is the raw material for decomposition.
Q7. Breaking detritus into smaller particles by detritivores such as earthworms is called:
(a) Humification
(b) Leaching
(c) Catabolism
(d) Fragmentation
Show answer
Answer: (d) — Fragmentation is the physical breakdown of detritus into smaller particles by detritivores.
Q8. Humus is best described as:
(a) An inorganic salt
(b) A dark amorphous substance highly resistant to microbial action, colloidal and a nutrient reservoir
(c) A type of detritivore
(d) Easily decomposed organic matter
Show answer
Answer: (b) — Humus is dark, amorphous, highly resistant to microbial action, decomposes extremely slowly, and being colloidal serves as a nutrient reservoir.
Q9. Decomposition is SLOWER if the detritus is rich in:
(a) Nitrogen
(b) Water-soluble sugars
(c) Water
(d) Lignin and chitin
Show answer
Answer: (d) — Lignin- and chitin-rich detritus decomposes slowly; nitrogen- and sugar-rich detritus decomposes quickly.
Q10. The only ecosystem NOT dependent on the sun for energy is:
(a) The pond
(b) The deep sea hydro-thermal ecosystem
(c) The desert
(d) The tundra
Show answer
Answer: (b) — The sun is the only energy source for all ecosystems except the deep sea hydro-thermal ecosystem.
Q11. Photosynthetically active radiation (PAR) constitutes what fraction of incident solar radiation?
(a) 100 per cent
(b) About 80 per cent
(c) Less than 50 per cent
(d) About 2 per cent
Show answer
Answer: (c) — Less than 50 per cent of incident solar radiation is PAR, and plants capture only 2-10 per cent of that.
Q12. Plants capture approximately what percentage of PAR?
(a) 20-30 per cent
(b) 50 per cent
(c) 2-10 per cent
(d) 90 per cent
Show answer
Answer: (c) — Plants capture only 2 to 10 per cent of the photosynthetically active radiation.
Q13. In a terrestrial ecosystem, a larger fraction of energy flows through the:
(a) Detritus food chain
(b) Grazing food chain
(c) Neither
(d) Both equally
Show answer
Answer: (a) — In terrestrial ecosystems a much larger fraction of energy flows through the DFC; in aquatic ecosystems the GFC is the major conduit.
Q14. Decomposers that break down dead organic matter are also known as:
(a) Detritivores
(b) Producers
(c) Saprotrophs
(d) Autotrophs
Show answer
Answer: (c) — Decomposers are heterotrophs, mainly fungi and bacteria, also known as saprotrophs.
Q15. The 10 per cent law explains why:
(a) Biomass pyramids are inverted
(b) The number of trophic levels in a grazing food chain is restricted
(c) Oceans are less productive
(d) Decomposition is slow
Show answer
Answer: (b) — Since only 10 per cent of energy transfers to each higher trophic level, the number of trophic levels is restricted.
Q16. Standing crop is most accurately measured as:
(a) Volume
(b) Fresh weight
(c) Number of individuals only
(d) Dry weight
Show answer
Answer: (d) — Standing crop is the biomass or number per unit area; measurement of biomass in terms of dry weight is more accurate.
Q17. The pyramid of biomass in the sea is generally:
(a) Absent
(b) Inverted, because fish biomass exceeds phytoplankton biomass
(c) Rectangular
(d) Upright
Show answer
Answer: (b) — The marine pyramid of biomass is inverted because the biomass of fishes far exceeds that of phytoplankton at any moment.
Q18. Which ecological pyramid can NEVER be inverted?
(a) All can be inverted
(b) Pyramid of energy
(c) Pyramid of number
(d) Pyramid of biomass
Show answer
Answer: (b) — The pyramid of energy is always upright because some energy is always lost as heat at each transfer step.
Q19. A pyramid of number would be inverted in the case of:
(a) A pond
(b) A single big tree supporting many insects
(c) A grassland
(d) An ocean
Show answer
Answer: (b) — One tree supports many insects, fewer small birds and even fewer large birds — so the producer level is narrowest.
Q20. Which of the following is a limitation of ecological pyramids?
(a) They only apply to aquatic systems
(b) They cannot show producers
(c) They are too complex
(d) They assume a simple food chain and cannot accommodate a food web; saprophytes get no place
Show answer
Answer: (d) — Pyramids ignore species at multiple trophic levels, assume a simple food chain rather than a web, and give saprophytes no place.
NEET Previous Year Questions (PYQs)
Real NEET previous-year questions on this chapter, with explanations in our own words.
Q21. Gross Primary Productivity minus respiration losses gives: (NEET PYQ)
(a) Net Primary Productivity
(b) Biomass
(c) Standing crop
(d) Secondary productivity
Show answer
Answer: (a) — GPP − R = NPP, which is the biomass available for consumption by heterotrophs.
Q22. In the decomposition process, the release of inorganic nutrients from humus is called: (NEET PYQ)
(a) Leaching
(b) Catabolism
(c) Fragmentation
(d) Mineralisation
Show answer
Answer: (d) — Mineralisation is the microbial degradation of humus releasing inorganic nutrients.
Q23. Which process causes water-soluble nutrients to go down into the soil horizon and precipitate as unavailable salts? (NEET PYQ)
(a) Fragmentation
(b) Leaching
(c) Humification
(d) Mineralisation
Show answer
Answer: (b) — Leaching moves water-soluble inorganic nutrients down the soil horizon where they precipitate as unavailable salts.
Q24. Which factor INHIBITS decomposition? (NEET PYQ)
(a) Warm and moist conditions
(b) Nitrogen-rich detritus
(c) Low temperature and anaerobiosis
(d) Abundant earthworms
Show answer
Answer: (c) — Low temperature and anaerobiosis inhibit decomposition, resulting in build-up of organic materials.
Q25. The grazing food chain is the major conduit for energy flow in: (NEET PYQ)
(a) Forest ecosystems
(b) Aquatic ecosystems
(c) Desert ecosystems
(d) Terrestrial ecosystems
Show answer
Answer: (b) — In an aquatic ecosystem the GFC is the major conduit; in terrestrial ecosystems the DFC carries more energy.
Q26. A sparrow eating seeds and then insects occupies: (NEET PYQ)
(a) More than one trophic level simultaneously
(b) Only the third trophic level
(c) No trophic level
(d) Only the second trophic level
Show answer
Answer: (a) — A trophic level is a functional level, not a species — a sparrow is a primary consumer eating seeds and a secondary consumer eating insects.
Q27. Detritivores such as earthworms perform which step of decomposition? (NEET PYQ)
(a) Fragmentation
(b) Humification
(c) Mineralisation
(d) Catabolism
Show answer
Answer: (a) — Detritivores break down detritus into smaller particles — the process called fragmentation.
Q28. The steps of decomposition operate: (NEET PYQ)
(a) Only underwater
(b) Simultaneously on the detritus
(c) Strictly in sequence
(d) Only in warm climates
Show answer
Answer: (b) — All the steps in decomposition operate simultaneously on the detritus, not one after another.
Q29. In an aquatic ecosystem, molluscs are typically: (NEET PYQ)
(a) Primary consumers (herbivores)
(b) Producers
(c) Secondary carnivores
(d) Decomposers
Show answer
Answer: (a) — Common herbivores are insects, birds and mammals on land, and molluscs in aquatic ecosystems.
Q30. Each bar in a pyramid of energy indicates: (NEET PYQ)
(a) The decomposition rate
(b) The number of individuals
(c) The amount of energy present at each trophic level per unit area in a given time
(d) The dry biomass only
Show answer
Answer: (c) — Each bar in the energy pyramid indicates the amount of energy at each trophic level in a given time or annually per unit area.
Active Recall Prompt
Write everything you can recall about the Ecosystem chapter, naming each part first: structure (species composition, stratification, the four functional aspects, the pond as a model with each component); productivity (primary production, GPP, NPP with the equation, secondary productivity, and the biosphere/ocean figures); decomposition (detritus, all five steps, what humus is, and what speeds up or slows down the process); energy flow (PAR percentages, why it is unidirectional); food chains and trophic levels (GFC vs DFC, which dominates where, standing crop, the 10 per cent law); and ecological pyramids (all three types, exactly which can be inverted and why energy never can, plus the limitations). Begin each fact with its topic and end it with a full stop.