Ecosystem ISC Class 12 Biology: Notes and Numericals

What Ecosystem is about

Weightage: Ecosystem is part of the unit Ecology and Environment, which carries 15 of the 70 marks in the ISC Biology Paper 1 (Theory), shared with 2 other chapters. Questions on it can appear in any section of the paper.Source: CISCE ISC Biology syllabus, examination year 2027, distribution of marks

Key concepts

Structure of an ecosystem and the pond ecosystem

Abiotic components (brief): light, temperature, water, soil, air and inorganic nutrients (C, N, P, K…) and organic matter. Biotic components: producers (autotrophs: green plants, phytoplankton), consumers (herbivores, carnivores, omnivores) and decomposers (bacteria, fungi, saprotrophs). Species composition and stratification (vertical layering: trees, shrubs, herbs, grasses in a forest) describe structure. Pond ecosystem: a shallow water body showing all components. Abiotic: water with dissolved inorganic and organic substances, and the rich soil deposit at the bottom; solar input, cycle of temperature, day length and other climatic conditions regulate its function. Autotrophs: phytoplankton, some algae, floating, submerged and marginal plants found at the edges. Consumers: zooplankton, free-swimming and bottom-dwelling forms. Decomposers: fungi, bacteria and flagellates, especially abundant at the bottom. Functions it shows: conversion of inorganic into organic material by autotrophs using sunlight; consumption of autotrophs by heterotrophs; decomposition and mineralisation of dead matter to release minerals for reuse; unidirectional movement of energy towards higher trophic levels and its dissipation and loss as heat to the environment.

Worked example

Name the producers, consumers and decomposers in a pond, and state two functions of the pond ecosystem.

Try it first, then show the solutionSolution

1. Producers: phytoplankton, algae, submerged and floating plants. Consumers: zooplankton, insects, fish. Decomposers: bacteria, fungi and flagellates at the bottom.
2. Functions: conversion of inorganic to organic matter (productivity); decomposition and mineral recycling (also one-way energy flow).

Answer: As above

Productivity: GPP, NPP and secondary productivity

Primary production: 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). Productivity: the rate of biomass production, expressed in g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹. Gross primary productivity (GPP): the rate of production of organic matter during photosynthesis. A considerable amount is used by plants in respiration (R). Net primary productivity (NPP) = GPP − R: the biomass available for consumption by heterotrophs (herbivores and decomposers). Secondary productivity: the rate of formation of new organic matter by consumers. Primary productivity depends on the plant species, environmental factors, availability of nutrients and photosynthetic capacity of plants, so it varies between ecosystems. The annual net primary productivity of the whole biosphere is about 170 billion tonnes (dry weight) of organic matter; oceans, despite covering about 70% of the surface, contribute only about 55 billion tonnes.

Worked example

A grassland has a GPP of 30 000 kcal m⁻² yr⁻¹, and 35% of it is used in plant respiration. Find the NPP.

Try it first, then show the solutionSolution

1. R = 0.35 × 30 000 = 10 500 kcal m⁻² yr⁻¹
2. NPP = GPP − R = 30 000 − 10 500 = 19 500 kcal m⁻² yr⁻¹

Answer: 19 500 kcal m⁻² yr⁻¹

Decomposition

Decomposers break down complex organic matter into inorganic substances like CO₂, water and nutrients; this is decomposition. Detritus (raw material): dead plant remains (leaves, bark, flowers) and dead remains of animals, including faecal matter. Steps: (1) Fragmentation: detritivores (e.g. earthworm, centipedes, millipedes) break down detritus into smaller particles. (2) Leaching: water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts. (3) Catabolism: bacterial and fungal enzymes degrade detritus into simpler inorganic substances. (Fragmentation, leaching and catabolism operate simultaneously.) (4) Humification: accumulation of a dark-coloured amorphous substance, humus, highly resistant to microbial action, decomposing extremely slowly; being colloidal, it is a reservoir of nutrients. (5) Mineralisation: humus is further degraded by microbes, releasing inorganic nutrients. Factors affecting the rate: chemical composition of detritus (slow if rich in lignin and chitin; fast if rich in nitrogen and water-soluble substances like sugars) and climatic factors: warm and moist conditions favour decomposition; low temperature and anaerobiosis inhibit it, causing build-up of organic material. Importance: releases nutrients for producers (nutrient cycling), cleans the environment of dead matter, forms humus and improves soil fertility.

Worked example

In cold, waterlogged bogs, dead plant matter piles up as peat instead of decomposing. Explain.

Try it first, then show the solutionSolution

1. Most decomposers are aerobic; waterlogging cuts off oxygen (anaerobiosis), so catabolism by bacteria and fungi is very slow.
2. Low temperature further reduces microbial activity, so detritus accumulates faster than it is broken down.

Answer: Anaerobic, cold conditions inhibit decomposers

Energy flow, PAR and the 10% law

Except for deep-sea hydrothermal ecosystems, the Sun is the only source of energy for all ecosystems. Of the incident solar radiation, less than 50% is photosynthetically active radiation (PAR) (the 400-700 nm range used by plants). Plants capture only 2-10% of PAR, and this small amount sustains the entire living world. Energy flow is unidirectional: from the Sun to producers, then to consumers; energy is not recycled but lost as heat at each level (second law of thermodynamics). Ecosystems need a constant input of energy to synthesise molecules and counter the universal tendency towards disorder. Trophic levels: producers (first), primary consumers (herbivores, second), secondary consumers (carnivores, third), tertiary consumers (top carnivores, fourth). Each trophic level has a certain mass of living material at a particular time: the standing crop, measured as biomass or number in a unit area. Standing state: the amount of nutrients (C, N, P, Ca…) present in the soil at any given time. Lindeman's 10% law: only about 10% of the energy at one trophic level is transferred to the next; the rest is used in respiration and lost as heat. So the number of trophic levels is limited (usually 4-5).

Worked example

Producers in a grassland fix 20 000 kJ of energy. How much reaches (a) primary consumers, (b) secondary consumers, (c) tertiary consumers, using the 10% law?

Try it first, then show the solutionSolution

1. (a) 10% of 20 000 = 2000 kJ.
2. (b) 10% of 2000 = 200 kJ; (c) 10% of 200 = 20 kJ.

Answer: 2000 kJ, 200 kJ, 20 kJ

Food chains and food webs

Grazing food chain (GFC): starts with producers: Grass → Goat → Man; Phytoplankton → Zooplankton → Small fish → Large fish. Detritus food chain (DFC): begins with dead organic matter, made up of decomposers (heterotrophic organisms, mainly fungi and bacteria, called saprotrophs) that secrete digestive enzymes to break down detritus and absorb the products. In an aquatic ecosystem the GFC is the major conduit of energy flow; in a terrestrial ecosystem a much larger fraction of energy flows through the detritus food chain. The two chains are connected: some organisms of the DFC are prey to GFC animals, and some animals (cockroaches, crows) are omnivores. Food web: natural interconnection of food chains; it gives stability because alternative food sources exist. Organisms occupy a place in the natural surroundings or community according to their feeding relationship: their trophic level. An organism may occupy more than one trophic level in different food chains (e.g. sparrow eating seeds and insects).

Worked example

Why does a larger fraction of energy flow through the detritus food chain than through the grazing food chain in a forest?

Try it first, then show the solutionSolution

1. Herbivores eat only a small part of forest plant biomass; most leaves, wood and roots die and fall as litter.
2. This detritus is used by decomposers and detritivores, so most of the NPP enters the detritus food chain.

Answer: Most plant biomass becomes detritus

Ecological pyramids

The relationship between trophic levels expressed in terms of number, biomass or energy gives an ecological pyramid; the base is formed by producers and the apex by top carnivores. Three types: pyramid of number, pyramid of biomass, pyramid of energy. In most ecosystems all three are upright: producers are more in number and biomass than herbivores, and so on. Exceptions: inverted pyramid of number: a single tree supports many insects (and many birds) (tree → insects is inverted). Inverted pyramid of biomass in the sea: the biomass of fish far exceeds that of phytoplankton at any moment, because phytoplankton reproduce very rapidly (small standing crop, high turnover). Pyramid of energy is always upright and can never be inverted, because some energy is always lost as heat at each step (10% law). Limitations of pyramids: they do not take into account the same species belonging to two or more trophic levels; they assume a simple food chain, which rarely exists in nature (they do not accommodate a food web); saprotrophs (decomposers) are not given any place, though they play a vital role.

Worked example

Why can the pyramid of biomass be inverted in a sea while the pyramid of energy cannot?

Try it first, then show the solutionSolution

1. Phytoplankton have a small standing crop but multiply very rapidly; at any moment the fish biomass they support is larger, giving an inverted biomass pyramid.
2. Energy is measured as flow over time; at each transfer about 90% is lost as heat, so each level always receives less energy than the one below: the energy pyramid is always upright.

Answer: High turnover of phytoplankton; energy loss at each level

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

• Decomposition
• Energy Flow and Food Chains
• Ecological Pyramids

Ecosystem: key facts

Net primary productivity

$NPP=GPP-R$

R = respiration losses of producers.

10% law

$E_{n+1}\approx 0.1\times E_n$

After k transfers: $E=E_1\times 0.1^k$.

Ecological efficiency

$\text{efficiency}=\frac{\text{energy at level }n+1}{\text{energy at level }n}\times 100$

About 10% on average.

PAR capture

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

PAR: 400-700 nm.

Decomposition steps

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

First three occur simultaneously.

Biosphere NPP

$\approx 170\text{billion tonnes (dry weight) per year};\text{oceans}\approx 55$

Oceans are less productive per area.

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.

• Net = Gross minus Respiration

  NPP = GPP − R ('net pay = gross pay minus expenses').

  Use it when: Productivity numericals.

• Funny Leaves Can Hide Minerals

  Fragmentation, Leaching, Catabolism, Humification, Mineralisation.

  Use it when: Decomposition steps.

• Energy never stands on its head

  Pyramid of energy is always upright.

  Use it when: Pyramid questions.

• Tree with insects: number upside down; sea: biomass upside down

  Number inverted on land (tree), biomass inverted in the sea.

  Use it when: Inverted pyramids.

• PAR = Photosynthetically Active Radiation

  Plants capture 2-10% of PAR.

  Use it when: Definitions.

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 5-mark question split into 1 + 1 + 1 + 2 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: Reversing the GPP-NPP relation

  CISCE examiners' comments, 2024 and 2025: candidates wrote GPP = NPP − R instead of NPP = GPP − R, and for a forest with GPP 20 000 kcal and 40% respiration they used an incorrect formula, made calculation errors or confused GPP and NPP.

  Wrong working

  NPP = 20 000 × 0.4 = 8000

  Right working

  NPP = 20 000 − 0.4 × 20 000 = 12 000 kcal m⁻² yr⁻¹

  ✓Do this: Subtract respiration from GPP; check that NPP < GPP.

  Usually costs: 1 mark.

• ✗Mistake: Repeating the question instead of explaining decomposition

  CISCE examiners' comments, 2024: candidates repeated the question's phrase about breakdown instead of explaining its importance, ignored the climatic differences between forest and desert, and named later stages (catabolism, leaching) instead of fragmentation for earthworms and centipedes.

  Wrong working

  Breakdown is important because organic matter is broken down.

  Right working

  It releases nutrients for producers, forms humus and clears dead matter; earthworms perform fragmentation.

  ✓Do this: Answer 'why' with a consequence for the ecosystem.

  Usually costs: 1 mark each.

• ✗Mistake: Misreading pyramid assertions and inventing expansions

  CISCE examiners' comments, 2025: in an assertion-reason item on pond biomass pyramids, candidates overlooked the word 'decrease' and confused options; they gave 'green plants' or even top levels instead of producers as having most energy; invented expansions of PAR such as 'photosynthetic aided respiration'; and gave only the first name or surname of the Father of Indian Ecology.

  Wrong working

  PAR = photosynthetically active region.

  Right working

  PAR = photosynthetically active radiation; producers (first trophic level) have the most energy; Ramdeo Misra.

  ✓Do this: Read every word of assertion-reason items; learn exact expansions and full names.

  Usually costs: 1 mark each.

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

  After learning inverted pyramids of number and biomass, it is easy to assume energy pyramids can invert too.

  Wrong working

  In the sea, the pyramid of energy is inverted.

  Right working

  Only number (tree) or biomass (sea) pyramids invert; energy pyramids are always upright.

  ✓Do this: Link 'energy' to the 10% law.

  Usually costs: 1 mark.

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 pyramid is always upright?
   (a) number
   (b) biomass
   (c) energy
   (d) all of these

   Hint

   10% law.

   Answer

   (c)

2. Q2 · 1 mark · MCQBoard level

   The first step of decomposition is:
   (a) leaching
   (b) fragmentation
   (c) catabolism
   (d) humification

   Hint

   Detritivores.

   Answer

   (b)

3. Q3 · 1 mark · Very short answerBoard level

   Define secondary productivity.

   Hint

   Consumers.

   Answer

   The rate of formation of new organic matter by consumers.

4. Q4 · 1 mark · Very short answerBoard level

   Which food chain carries more energy in a terrestrial ecosystem?

   Hint

   Dead matter.

   Answer

   The detritus food chain.

5. Q5 · 2 marks · Short answerHarder board level

   If secondary consumers in an ecosystem receive 150 J, how much energy did the producers fix (10% law)?

   Hint

   Two transfers.

   Answer

   Primary consumers 1500 J; producers 15 000 J.

6. Q6 · 2 marks · Short answerHarder board level

   Why do warm and humid conditions favour decomposition while anaerobic conditions inhibit it?

   Hint

   Microbial activity.

   Answer

   Decomposers are most active at warm temperatures with moisture; most decomposers are aerobic, so low oxygen (waterlogging) slows catabolism and organic matter accumulates (e.g. peat).

7. Q7 · 2 marks · Short answerBoard level

   Give an example of a grazing food chain and a detritus food chain, and state how they are linked.

   Hint

   Earthworm and bird.

   Answer

   GFC: grass → grasshopper → frog → snake. DFC: dead leaves → earthworm → bacteria and fungi. Linked because animals of the DFC (earthworms) are eaten by GFC animals (birds, frogs).

8. Q8 · 2 marks · Short answerBoard level

   Give three limitations of ecological pyramids.

   Hint

   Webs, decomposers.

   Answer

   They do not account for a species belonging to two or more trophic levels; they assume simple food chains, not food webs; they give no place to saprotrophs (decomposers).

9. Q9 · 2 marks · Short answerHarder board level

   A lake's producers have GPP of 12 000 kcal m⁻² yr⁻¹ and NPP of 9000. What percentage of GPP is used in respiration?

   Hint

   R = GPP − NPP.

   Answer

   R = 3000; 3000/12 000 × 100 = 25%.

10. Q10 · 2 marks · Short answerHarder board level

    Explain why the number of trophic levels in an ecosystem is limited.

    Hint

    Energy loss.

    Answer

    Only about 10% of energy passes to the next level; the rest is lost as heat. After 4-5 transfers too little energy remains to support another level of organisms.

11. Q11 · 5 marks · Long answerHarder board level

    (a) Describe the five steps of decomposition. [3]
    (b) How do chemical composition of detritus and climate affect its rate? [2]

    Hint

    Lignin, nitrogen.

    Answer

    (a) Fragmentation by detritivores; leaching of soluble nutrients; catabolism by bacterial and fungal enzymes; humification producing humus; mineralisation of humus releasing nutrients. (b) Slow if detritus is rich in lignin and chitin, fast if rich in nitrogen and water-soluble substances like sugars; warm and moist climate speeds it, low temperature and anaerobiosis slow it.

12. Q12 · 2 marks · Short answerHOTS / competency

    Draw a pyramid of number for a single tree supporting 2000 caterpillars which are eaten by 20 birds, and explain its shape.

    Hint

    Spindle shape.

    Answer

    Narrow base (1 tree), very wide middle (2000 caterpillars), narrower top (20 birds): partly inverted (spindle-shaped) because one large producer supports many small herbivores.

13. Q13 · 2 marks · Short answerHOTS / competency

    A farmer removes all crop residues and burns them each year. Predict the long-term effect on soil fertility and explain using decomposition.

    Hint

    Humus and mineralisation.

    Answer

    Less detritus enters the soil, so less humus forms and fewer nutrients are mineralised; soil organic matter, water-holding capacity and fertility decline, and soil organisms decrease.

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.Define ecosystem.Show answer

   A functional unit of nature where living organisms interact among themselves and with the physical environment.

2. 2.NPP formula?Show answer

   NPP = GPP − R.

3. 3.What is humus?Show answer

   A dark amorphous substance formed by humification, resistant to microbial action, rich in nutrients.

4. 4.What fraction of PAR do plants capture?Show answer

   2-10%.

5. 5.State Lindeman's law.Show answer

   Only about 10% of energy at one trophic level passes to the next.

6. 6.Example of inverted pyramid of number?Show answer

   A tree with many insects feeding on it.

7. 7.Annual NPP of the biosphere?Show answer

   About 170 billion tonnes dry weight of organic matter.

8. 8.What is standing state?Show answer

   The amount of nutrients in the soil at a given time.

Quick revision checklist

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

• ☐Can you describe the components and functions of a pond?
• ☐Can you calculate NPP from GPP and % respiration?
• ☐Can you list the five steps of decomposition in order?
• ☐Can you explain why forest soil has more humus than desert soil?
• ☐Can you apply the 10% law across several levels?
• ☐Can you define PAR, standing crop and standing state?
• ☐Can you distinguish GFC and DFC with examples?
• ☐Can you draw upright and inverted pyramids and explain them?

Ecosystem: frequently asked questions