Ecosystem Class 12: Concepts, Pyramids, Numericals and Board Answers

What Ecosystem is about

Weightage: This chapter is part of Unit X, Ecology and Environment, which carries 10 of the 70 theory marks together with Organisms and Populations and Biodiversity and Conservation.Source: CBSE Biology (044) syllabus 2026-27, course structure, Unit X Ecology and Environment

In real board papers: asked in 8 of the 8 board papers we hold (2023 to 2026), on average 3.1 marks per paper. See the questions

Key concepts

Ecosystem structure and the pond as an example

NCERT section 12.1 Ecosystem – Structure and Function

Structure: interaction of biotic and abiotic components gives each ecosystem a characteristic physical structure.

• Species composition: identification and listing of the plant and animal species of an ecosystem.
• Stratification: vertical distribution of species at different levels; in a forest, trees occupy the top layer, shrubs the second, and herbs and grasses the bottom layers.

Functional aspects: the components work as a unit through productivity, decomposition, energy flow and nutrient cycling (nutrient cycles themselves are not in the syllabus).

A pond: a simple, fairly self-sustaining example:

• Abiotic: water with dissolved inorganic and organic substances, rich soil deposits at the bottom; solar input, temperature cycle, day length and climate regulate its functioning.
• Autotrophs (producers): phytoplankton, some algae, and floating, submerged and marginal plants.
• Consumers: zooplankton, free-swimming and bottom-dwelling forms.
• Decomposers: fungi, bacteria and flagellates, especially abundant at the bottom.

What the pond does (like any ecosystem and the biosphere):
1. Autotrophs convert inorganic into organic material using solar radiant energy.
2. Heterotrophs consume the autotrophs.
3. Dead matter is decomposed and mineralised, releasing nutrients for reuse.
4. Energy moves one way to higher trophic levels and is lost as heat.

Worked example

Identify the producers, consumers and decomposers of a pond, and state why energy flow in the pond is described as unidirectional. (2 marks)

Try it first, then show the solutionSolution

1. Producers: phytoplankton, algae, floating, submerged and marginal plants; consumers: zooplankton, free-swimming and bottom-dwelling animals; decomposers: fungi, bacteria and flagellates at the bottom. (1)
2. Energy goes from the sun to producers to consumers and is lost as heat at each step; it does not return to the producers, so the flow is one-way. (1)

Answer: Producers: phytoplankton and plants; consumers: zooplankton and animals; decomposers: fungi, bacteria, flagellates; energy is lost as heat and never cycles back.

Productivity

NCERT section 12.2 Productivity

A constant input of solar energy is the basic need of every ecosystem.

Primary production: the amount of organic matter (biomass) that plants make by photosynthesis in a unit area during a given period; expressed as weight (g m⁻²) or energy (kcal m⁻²).

Productivity: the rate of biomass production, in g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹, so different ecosystems can be compared.

Gross and net primary productivity:

• GPP: rate of production of organic matter during photosynthesis.
• A large part of GPP is used by plants in respiration (R).
• NPP = GPP − R: the biomass available to heterotrophs (herbivores and decomposers).

Secondary productivity: the rate of formation of new organic matter by consumers.

Factors affecting primary productivity:

• the plant species of the area;
• environmental factors (light, temperature, water);
• availability of nutrients;
• photosynthetic capacity of plants.

So productivity varies among ecosystems.

Global figures (NCERT): annual NPP of the whole biosphere is about 170 billion tons (dry weight). The oceans, despite covering about 70% of the surface, produce only 55 billion tons; the rest (about 115 billion tons) is on land. In aquatic ecosystems, light is the main limiting factor for productivity (it decreases with depth), and nutrients can also limit ocean productivity.

Worked example

A grassland has GPP of 2000 kcal m⁻² yr⁻¹ and plants use 800 kcal m⁻² yr⁻¹ in respiration. Calculate NPP and the percentage of GPP available to herbivores and decomposers. (2 marks)

Try it first, then show the solutionSolution

1. NPP = GPP − R = 2000 − 800 = 1200 kcal m⁻² yr⁻¹. (1)
2. Percentage = 1200 ÷ 2000 × 100 = 60%. Check: respiration uses 800/2000 = 40%, and 60% + 40% = 100%. (1)

Answer: NPP = 1200 kcal m⁻² yr⁻¹; 60% of GPP.

Decomposition

NCERT section 12.3 Decomposition

Decomposition: breakdown of complex organic matter by decomposers into inorganic substances such as carbon dioxide, water and nutrients. The earthworm is the 'farmer's friend' because it helps break down organic matter and loosens the soil.

Detritus: the raw material: dead plant remains (leaves, bark, flowers) and dead animal remains, including faecal matter.

Steps:
1. Fragmentation: detritivores (e.g. earthworm) break detritus into smaller particles.
2. Leaching: water-soluble inorganic nutrients go down into the soil horizon and precipitate as unavailable salts.
3. Catabolism: bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
(These three steps work simultaneously on the detritus.)
4. Humification: accumulation of humus, a dark, amorphous substance, highly resistant to microbial action, decomposed extremely slowly; being colloidal, it is a reservoir of nutrients.
5. Mineralisation: humus is further degraded by some microbes, releasing inorganic nutrients.

Factors controlling the rate:

• Chemical composition of detritus: slower if rich in lignin and chitin; faster if rich in nitrogen and water-soluble substances such as sugars.
• Climate: temperature and soil moisture are the most important, acting through soil microbes. Warm and moist conditions favour decomposition; low temperature and anaerobiosis inhibit it, so organic matter builds up.
• Decomposition is largely an oxygen-requiring process.

Worked example

Two leaf samples are buried in the same moist, warm soil: sample A is rich in lignin; sample B is rich in nitrogen and sugars. Which decomposes faster and why? What would happen if the soil were waterlogged and cold? (3 marks)

Try it first, then show the solutionSolution

1. Sample B decomposes faster: detritus rich in nitrogen and water-soluble substances like sugars is broken down quickly. (1)
2. Sample A is slower because lignin (like chitin) resists breakdown. (1)
3. In waterlogged (anaerobic) and cold soil, microbial activity falls; decomposition is inhibited and organic matter builds up. (1)

Answer: B (nitrogen, sugars) decomposes faster; lignin slows A; cold, anaerobic soil inhibits decomposition.

Energy flow, food chains and trophic levels

NCERT section 12.4 Energy Flow

Source: except for deep-sea hydrothermal ecosystems, the sun is the only source of energy.

• Of the incident solar radiation, less than 50% is photosynthetically active radiation (PAR).
• Plants capture only 2–10% of the PAR, and this small amount sustains the entire living world.
• Energy flow is unidirectional: sun → producers → consumers.
• Ecosystems obey the second law of thermodynamics: they need a constant supply of energy to make the molecules they need and to counter the tendency towards disorder.

Producers: green plants; on land, herbaceous and woody plants; in water, phytoplankton, algae and higher plants.

Consumers (heterotrophs):

• Primary consumers = herbivores (insects, birds, mammals on land; molluscs in water).
• Secondary consumers = primary carnivores (eat herbivores).
• Tertiary consumers = secondary carnivores.

Grazing food chain (GFC): Grass (producer) → Goat (primary consumer) → Man (secondary consumer).

Detritus food chain (DFC): begins with dead organic matter; made up of decomposers (mainly fungi and bacteria), also called saprotrophs. They secrete digestive enzymes that break dead matter into simple inorganic materials, which they absorb.

• In aquatic ecosystems, the GFC is the major conduit of energy.
• In terrestrial ecosystems, a much larger fraction of energy flows through the DFC.

Food web: the DFC and GFC are linked (some DFC organisms are prey of GFC animals; omnivores such as cockroaches and crows); these natural interconnections form a food web.

Trophic level: an organism's place in the food chain based on its source of food: producers 1st, herbivores 2nd, carnivores 3rd, and so on. A trophic level is a functional level, not a species: a sparrow is a primary consumer when eating seeds and fruits and a secondary consumer when eating insects and worms.

Standing crop: the mass of living material at a trophic level at a particular time, measured as biomass or number per unit area. Biomass is better measured as dry weight (more accurate, because water content varies).

10 per cent law: only about 10% of the energy at one trophic level is transferred to the next. This limits the number of trophic levels in a grazing food chain (producer, herbivore, primary carnivore, secondary carnivore).

Worked example

In a grazing food chain, producers fix 20,000 J of energy. Using the 10 per cent law, how much energy reaches (a) the herbivores, (b) the primary carnivores, (c) the secondary carnivores? Why are food chains rarely longer than 4–5 levels? (3 marks)

Try it first, then show the solutionSolution

1. (a) 10% of 20,000 = 2000 J; (b) 10% of 2000 = 200 J; (c) 10% of 200 = 20 J. (1½)
2. Check: 20,000 × (0.1)³ = 20,000 × 0.001 = 20 J for the fourth level. (½)
3. So little energy remains after a few transfers (most is lost as heat) that it cannot support another level. (1)

Answer: (a) 2000 J (b) 200 J (c) 20 J; energy runs out after a few levels.

Ecological pyramids

NCERT section 12.5 Ecological Pyramids

Ecological pyramids show the food or energy relationship between trophic levels as number, biomass or energy. The base is the producers (first trophic level); the apex is the top consumer.

1. Pyramid of number:

• Grassland: upright; NCERT's example: nearly 6 million plants support only 3 top carnivores.
• Tree ecosystem: one big tree supports many insects, which support small birds, which support fewer large birds: the pyramid is inverted at the base (spindle-shaped).

2. Pyramid of biomass:

• Usually upright: biomass drops sharply at higher levels (e.g. on land).
• In the sea it is generally inverted: a small standing crop of phytoplankton supports a large standing crop of zooplankton (and fishes). This is not a paradox: phytoplankton reproduce and are eaten very fast, so their standing crop at any moment is small even though their production over time is large.

3. Pyramid of energy:

• Always upright, never inverted: when energy passes from one trophic level to the next, some is always lost as heat.
• Each bar shows the energy at a trophic level in a given time or annually per unit area.
• NCERT's ideal pyramid notes that primary producers convert only about 1% of the energy in sunlight available to them into NPP.

Rules for calculating: include all organisms at a trophic level; a species can occupy more than one trophic level at the same time.

Limitations of ecological pyramids:
1. They do not account for the same species at two or more trophic levels.
2. They assume a simple food chain, which almost never exists in nature; they cannot show a food web.
3. Saprophytes (decomposers) get no place, although they play a vital role.

Worked example

Why is the pyramid of biomass in the sea inverted, whereas the pyramid of energy can never be inverted? (3 marks)

Try it first, then show the solutionSolution

1. In the sea, the standing crop (biomass at one time) of phytoplankton is small, while zooplankton and fish standing crops are larger: the biomass pyramid is inverted. (1)
2. Phytoplankton multiply and are consumed very rapidly, so a small standing crop can support a larger consumer biomass over time. (1)
3. Energy is measured as flow over time; at each transfer some energy is lost as heat, so each higher level always has less energy: the energy pyramid is always upright. (1)

Answer: Small but fast-renewing phytoplankton standing crop inverts the biomass pyramid; energy is always lost as heat at each step, so the energy pyramid is always upright.

Ecosystem in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Primary Productivity
• Energy Flow and Food Chains
• Ecological Pyramids

Ecosystem: key facts

Net primary productivity

$NPP=GPP-R$

R = energy or biomass used by plants in respiration.

Units of productivity

${\text{g m}}^{-2}{\text{yr}}^{-1}\text{or}{\text{kcal m}}^{-2}{\text{yr}}^{-1}$

Primary production (not rate) is in g m⁻² or kcal m⁻².

Ten per cent law

$E_{n+1}=0.1\times E_n;E_{\text{level }k}=E_1\times (0.1{)}^{k-1}$

Limits the number of trophic levels.

Solar energy capture

$\text{PAR}<50\%\text{of incident radiation};\text{plants capture }2\text{–}10\%\text{of PAR}$

About 1% of sunlight available to producers becomes NPP (NCERT's ideal energy pyramid).

Global NPP

$\text{biosphere}\approx 170;\text{oceans}\approx 55;\text{land}\approx 115\text{(billion tons dry weight per year)}$

Oceans cover about 70% of the surface but give only about a third of NPP.

Decomposition steps

$\text{fragmentation}\to \text{leaching}\to \text{catabolism}\to \text{humification}\to \text{mineralisation}$

First three act together on detritus.

Ecosystem: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the board asks Ecosystem

The question patterns to expect, and the method that works for each.

Tricks to remember Ecosystem

Short memory aids. Say each one aloud once, then use it on a question the same day.

• GPP − R = NPP: 'Gross minus Respiration = Net'

  Think of salary: gross pay minus expenses (respiration) = net pay available for others (herbivores).

  Use it when: Productivity numericals.

• Decomposition: 'Fat Lions Can't Hunt Mice'

  Fragmentation, Leaching, Catabolism, Humification, Mineralisation.

  Use it when: Order of decomposition steps.

• 10% law: 'Drop a zero'

  Each step up the food chain, drop one zero: 10,000 → 1000 → 100 → 10.

  Use it when: Energy at trophic levels.

• GFC in water, DFC on land

  Aquatic: grazing food chain is the main conduit. Terrestrial: more energy flows through the detritus food chain.

  Use it when: Comparing food chains.

• Energy pyramid: 'Never Upside down'

  Energy is always lost as heat, so the energy pyramid is always upright; numbers (tree) and biomass (sea) can be inverted.

  Use it when: Pyramid questions.

• Slow rot: 'Lignin and Chitin are Lazy'

  Lignin- and chitin-rich detritus decomposes slowly; nitrogen and sugars speed it up.

  Use it when: Factors affecting decomposition.

Ecosystem: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section D style)

Read the situation, then answer every part. In the board paper this is a 4-mark question split into 1 + 2 + 1 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Ecosystem

The slips examiners see most often, and what to write instead.

• ✗Mistake: Writing NPP = GPP + R

  Wrong working

  GPP = 1500, R = 500, so NPP = 2000.

  Right working

  NPP = GPP − R = 1500 − 500 = 1000.

  ✓Do this: Respiration is a loss; NPP is always less than GPP.

  Usually costs: 1 mark.

• ✗Mistake: Saying the pyramid of energy can be inverted

  ✓Do this: It is always upright because energy is lost as heat at each transfer; only number and biomass pyramids can be inverted.

  Usually costs: 1 mark.

• ✗Mistake: Mixing up leaching and catabolism

  ✓Do this: Leaching: water-soluble nutrients move down and precipitate as unavailable salts. Catabolism: bacterial and fungal enzymes break detritus into simpler inorganic substances.

  Usually costs: 1 mark.

• ✗Mistake: Saying the GFC carries most energy on land

  ✓Do this: On land, a much larger fraction of energy flows through the detritus food chain; in water the GFC is the main conduit.

  Usually costs: 1 mark.

• ✗Mistake: Treating a trophic level as a species

  ✓Do this: A trophic level is a functional level; one species (sparrow, humans) can occupy more than one level.

  Usually costs: ½ to 1 mark.

• ✗Mistake: Answering about nutrient cycles or succession

  ✓Do this: Carbon and phosphorus cycles, ecological succession and ecosystem services are not in the 2026-27 syllabus.

  Usually costs: Time and marks spent on unasked content.

NCERT exercise guide: Ecosystem

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Ecosystem: previous year questions

• Asked in 8 of the 8 board papers we hold (2023 to 2026), on average 3.1 marks per paper; 17 questions in all, counting OR alternatives. In 4 of 4 sample papers.

Counted from the real papers on our previous year papers page, not estimated. Average marks are over every board paper we hold, with an OR pair counted once. Chapter tags are ours.

Solved board questions from Ecosystem

Real questions from recent papers. Try each one first, then open our step-by-step solution with marks per step.

Every Ecosystem question, by year

Practice questions on Ecosystem

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   Which step of decomposition makes nutrients unavailable by precipitating them as salts?
   (a) Fragmentation (b) Leaching (c) Catabolism (d) Mineralisation

   Hint

   Water carries them down.

   Answer

   (b) Leaching

2. Q2 · 1 mark · Assertion-reasonBoard level

   Assertion (A): The pyramid of energy is always upright.
   Reason (R): Some energy is lost as heat at each transfer between trophic levels.
   (a) Both A and R are true and R is the correct explanation of A.
   (b) Both A and R are true but R is not the correct explanation of A.
   (c) A is true but R is false.
   (d) A is false but R is true.

   Hint

   Second law of thermodynamics.

   Answer

   (a)

3. Q3 · 1 mark · MCQHarder board level

   A sparrow eating seeds and later eating insects occupies:
   (a) only the second trophic level (b) only the third trophic level (c) the second and third trophic levels (d) the first and second trophic levels

   Hint

   Trophic level is functional.

   Answer

   (c) the second and third trophic levels

4. Q4 · 2 marks · Very short answerBoard level

   Define stratification and give an example. (2 marks)

   Hint

   Vertical layers.

   Answer

   Stratification is the vertical distribution of different species at different levels in an ecosystem (1). In a forest, trees occupy the top layer, shrubs the second, and herbs and grasses the bottom layers (1).

5. Q5 · 2 marks · Very short answerHarder board level

   Distinguish between litter and detritus. (2 marks)

   Hint

   Detritus includes animal remains.

   Answer

   Litter refers to dead plant parts (leaves, twigs, bark, flowers) lying on the soil surface (1). Detritus is all dead organic matter, plant remains as well as dead animal remains and faecal matter, which is the raw material for decomposition (1).

6. Q6 · 2 marks · Very short answerHOTS / competency

   Give two reasons why a terrestrial ecosystem sends more energy through the detritus food chain than an aquatic one. (2 marks)

   Hint

   What do land plants leave behind?

   Answer

   Land plants such as trees produce large amounts of tough tissue (wood, leaves) that herbivores eat little of, so much NPP becomes litter and detritus (1). In water, small phytoplankton are eaten rapidly by zooplankton, so most energy passes through the grazing chain; on land, most NPP reaches decomposers instead (1).

7. Q7 · 3 marks · Short answerBoard level

   Describe the components of an ecosystem. (3 marks)

   Hint

   Abiotic and three biotic groups.

   Answer

   Abiotic: air, water, soil, dissolved substances, sunlight and climate (1). Biotic: producers (green plants, phytoplankton) that make food from inorganic materials using sunlight; consumers (herbivores and carnivores) that depend on producers (1). Decomposers (fungi, bacteria, flagellates) break dead matter into inorganic nutrients for reuse (1).

8. Q8 · 3 marks · Short answerHarder board level

   An ecosystem has NPP of 6000 kcal m⁻² yr⁻¹, which is 75% of GPP. Calculate GPP and R, and the energy available to primary carnivores using the 10 per cent law. (3 marks)

   Hint

   NPP = 0.75 × GPP.

   Answer

   GPP = 6000 ÷ 0.75 = 8000 kcal m⁻² yr⁻¹ (1). R = 8000 − 6000 = 2000 kcal m⁻² yr⁻¹; check 2000/8000 = 25% (1). Herbivores get 600; primary carnivores get 60 kcal m⁻² yr⁻¹ (1).

9. Q9 · 5 marks · Long answerHarder board level

   (a) Give an account of energy flow in an ecosystem, including PAR and the second law of thermodynamics. (3)
   (b) Explain why the number of trophic levels in a grazing food chain is limited. (2) (5 marks)

   Hint

   Sun → producers → consumers.

   Answer

   (a) The sun is the energy source of all ecosystems except deep-sea hydrothermal ones; less than 50% of incident radiation is PAR, and plants capture only 2–10% of PAR (1). Energy flows one way: from the sun to producers, then to herbivores and carnivores, and to decomposers through detritus (1). Ecosystems obey the second law of thermodynamics: they need a constant energy input to build molecules and counter disorder, and energy is lost as heat at each step (1). (b) Only about 10% of energy passes from one level to the next (1). After a few transfers the energy left is too little to support another level, so chains have only about 4–5 levels (1).

10. Q10 · 3 marks · Short answerHOTS / competency

    Suppose a lake receives warm wastewater and a large algal bloom develops, but the lake bottom becomes oxygen-poor and dead plant matter piles up. Explain the pile-up of organic matter using factors that control decomposition. (3 marks)

    Hint

    Oxygen-requiring process.

    Answer

    The bloom produces a large amount of dead organic matter (detritus) that sinks (1). Decomposition is largely oxygen-requiring; the oxygen-poor (anaerobic) bottom inhibits microbial breakdown (1). So detritus is added faster than it is decomposed and builds up, as NCERT notes for anaerobic conditions (1).

11. Q11 · 2 marks · Very short answerHarder board level

    Explain why the pyramid of numbers is upright in a grassland but spindle-shaped for a single large tree. (2 marks)

    Hint

    Size of producers.

    Answer

    In a grassland, producers are many small grass plants, so their number far exceeds herbivores and carnivores (about 6 million plants support 3 top carnivores): upright (1). A single big tree (one producer) supports thousands of insects, which support fewer birds, so the base is narrow and the next bar wide: spindle-shaped (1).

12. Q12 · 2 marks · Very short answerHarder board level

    Why is humus important for soil fertility even though it decomposes very slowly? (2 marks)

    Hint

    Colloidal reservoir.

    Answer

    Humus is colloidal and holds nutrients, acting as a reservoir of nutrients in the soil (1). Its slow mineralisation by microbes releases inorganic nutrients gradually, supplying plants over a long period (1).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What is stratification?Show answer

   Vertical distribution of species at different levels, e.g. trees at the top, shrubs next, herbs and grasses at the bottom of a forest.

2. 2.Define primary productivity.Show answer

   The rate of biomass or organic matter production per unit area by plants during photosynthesis.

3. 3.What is secondary productivity?Show answer

   The rate of formation of new organic matter by consumers.

4. 4.What is the annual NPP of the biosphere?Show answer

   About 170 billion tons (dry weight) of organic matter.

5. 5.What is detritus?Show answer

   Dead plant remains (leaves, bark, flowers) and dead animal remains, including faecal matter.

6. 6.What are the properties of humus?Show answer

   Dark, amorphous, highly resistant to microbial action, decomposes very slowly, colloidal and a reservoir of nutrients.

7. 7.What fraction of incident solar radiation is PAR?Show answer

   Less than 50%.

8. 8.What percentage of PAR do plants capture?Show answer

   2–10%.

9. 9.What are saprotrophs?Show answer

   Decomposers (mainly fungi and bacteria) that obtain energy and nutrients by degrading dead organic matter.

10. 10.What is standing crop?Show answer

    The mass of living material at a trophic level at a particular time, measured as biomass or number per unit area.

11. 11.Which is the only kind of ecosystem not powered by the sun?Show answer

    The deep-sea hydrothermal ecosystem.

12. 12.Give one limitation of ecological pyramids.Show answer

    They do not account for a species at more than one trophic level, assume simple food chains (not webs), and leave out saprophytes (any one).

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you describe the components of a pond ecosystem?
• ☐Can you define GPP, NPP and secondary productivity and calculate NPP?
• ☐Can you explain why oceans have low productivity?
• ☐Can you list the five steps of decomposition and the factors that speed or slow it?
• ☐Can you compare grazing and detritus food chains, and food chains and food webs?
• ☐Can you apply the 10 per cent law to find energy at the fourth trophic level?
• ☐Can you draw pyramids of number for a grassland and a tree, and biomass for the sea?
• ☐Can you explain why the pyramid of energy is always upright, and list three limitations of pyramids?

Ecosystem: frequently asked questions