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Ecosystem

NCERT Class 12 · Biology Based on NCERT Class 12 Biology textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter defines an ecosystem as a functional unit of nature where living organisms interact with each other and the physical environment. It exists in the syllabus to explain how energy flows, nutrients cycle, and matter is recycled through biotic and abiotic components. The one big idea a student must walk away with is that ecosystems are self-sustaining units driven by solar energy, with unidirectional energy flow and cyclic nutrient movement, and that productivity, decomposition, and ecological pyramids quantify these processes.
Key Concepts (25)
Ecosystem
A functional unit of nature where living organisms interact among themselves and with the surrounding physical environment.
e.g. A pond, a forest, or a crop field.
Stratification
Vertical distribution of different species occupying different levels in an ecosystem.
e.g. In a forest, trees occupy the top strata, shrubs the second, and herbs and grasses the bottom layers.
Primary productivity
The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis.
e.g. Measured in gm⁻² or kcal m⁻².
Gross primary productivity (GPP)
The rate of production of organic matter during photosynthesis.
e.g. Total energy captured by plants before respiration losses.
Net primary productivity (NPP)
Gross primary productivity minus respiration losses (R); the available biomass for consumption by heterotrophs.
e.g. GPP – R = NPP.
Secondary productivity
The rate of formation of new organic matter by consumers.
e.g. Growth of a herbivore after eating plants.
Decomposition
The process by which decomposers break down complex organic matter into inorganic substances like carbon dioxide, water, and nutrients.
e.g. Fungi and bacteria breaking down dead leaves.
Detritus
Dead plant remains such as leaves, bark, flowers, and dead remains of animals, including fecal matter, which is the raw material for decomposition.
e.g. Fallen leaves on the forest floor.
Fragmentation
The process by which detritivores break down detritus into smaller particles.
e.g. Earthworms breaking down leaf litter.
Leaching
The process by which water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.
e.g. Rain carrying minerals deeper into the soil.
Catabolism
The process by which bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
e.g. Enzymes breaking down cellulose into glucose.
Humification
The process that leads to accumulation of a dark coloured amorphous substance called humus, which is highly resistant to microbial action.
e.g. Formation of dark, nutrient-rich soil layer.
Mineralisation
The process by which humus is further degraded by some microbes, releasing inorganic nutrients.
e.g. Release of nitrates and phosphates into the soil.
Photosynthetically active radiation (PAR)
The portion of incident solar radiation (less than 50%) that is used by plants for photosynthesis.
e.g. Plants capture only 2-10% of PAR.
Food chain
A sequence of organisms through which energy is transferred, starting from producers to consumers.
e.g. Grass → Goat → Man.
Grazing food chain (GFC)
A food chain that begins with living plants as the primary energy source.
e.g. Phytoplankton → Zooplankton → Fish.
Detritus food chain (DFC)
A food chain that begins with dead organic matter (detritus) and is made up of decomposers.
e.g. Dead leaves → Fungi → Bacteria → Earthworm.
Food web
Natural interconnection of food chains in an ecosystem.
e.g. A single plant eaten by multiple herbivores, which are eaten by multiple carnivores.
Trophic level
A specific place in the food chain occupied by an organism based on its source of nutrition or food.
e.g. Producers belong to the first trophic level, herbivores to the second, carnivores to the third.
Standing crop
The mass of living material (biomass) or the number of organisms in a unit area at a particular time at each trophic level.
e.g. Biomass of trees in a forest measured in kg per hectare.
10 per cent law
Only 10 per cent of the energy is transferred to each trophic level from the lower trophic level.
e.g. If producers have 1000 kcal, only 100 kcal reaches primary consumers.
Ecological pyramids
Graphical representations of the relationship between organisms at different trophic levels, expressed in terms of number, biomass, or energy.
e.g. Pyramid of number, pyramid of biomass, pyramid of energy.
Pyramid of energy
An ecological pyramid that is always upright, showing the amount of energy at each trophic level, with energy decreasing at each step due to heat loss.
e.g. Producers have the most energy; top carnivores have the least.
Detritivores
Organisms that break down detritus into smaller particles (e.g., earthworm).
e.g. Earthworms, millipedes.
Saprotrophs
Heterotrophic organisms, mainly fungi and bacteria, that meet their energy and nutrient requirements by degrading dead organic matter or detritus.
e.g. Mushrooms growing on dead wood.
Formulas & Equations (2)
GPP – R = NPP
Reads as: Gross primary productivity minus respiration losses equals net primary productivity
Variables: GPP = rate of production of organic matter during photosynthesis (gm⁻² yr⁻¹ or kcal m⁻² yr⁻¹); R = respiration losses (same units); NPP = available biomass for heterotrophs (same units)
Use when: Calculating the net biomass available for consumers in an ecosystem or comparing productivity of different ecosystems
⚠ Students forget to subtract respiration losses and directly equate GPP with NPP, or they confuse GPP with NPP in exam answers.
10 per cent law
Reads as: Only ten percent of energy is transferred from one trophic level to the next
Variables: Energy at trophic level n = 10% of energy at trophic level n-1
Use when: Explaining why the number of trophic levels in a grazing food chain is limited, or calculating energy available at successive trophic levels
⚠ Students think 10% of energy is lost as heat, but the law states 10% is transferred, not lost; the rest (90%) is used for respiration, growth, reproduction, or lost as heat.
How It Works
Decomposition
  1. Step 1 — Fragmentation: Detritivores (e.g., earthworms) break down detritus into smaller particles.
  2. Step 2 — Leaching: Water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.
  3. Step 3 — Catabolism: Bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
  4. Step 4 — Humification: Accumulation of a dark coloured amorphous substance called humus, which is highly resistant to microbial action and undergoes decomposition at an extremely slow rate.
  5. Step 5 — Mineralisation: Humus is further degraded by some microbes, releasing inorganic nutrients.
💡 All steps (fragmentation, leaching, catabolism) operate simultaneously on the detritus, not sequentially. Humification and mineralisation occur during decomposition in the soil. Decomposition is largely an oxygen-requiring process.
Energy flow through trophic levels
  1. Step 1 — Solar energy (PAR) is captured by autotrophs (producers) during photosynthesis; plants capture only 2-10% of PAR.
  2. Step 2 — Energy is transferred from producers to primary consumers (herbivores) when they eat plants.
  3. Step 3 — Energy is transferred from primary consumers to secondary consumers (carnivores) when they eat herbivores.
  4. Step 4 — Energy is transferred from secondary consumers to tertiary consumers (top carnivores).
  5. Step 5 — At each trophic level, energy is lost as heat due to respiration and other metabolic activities; only 10% of energy is transferred to the next level (10 per cent law).
  6. Step 6 — Dead organisms and waste become detritus, entering the detritus food chain, where decomposers break them down.
💡 Energy flow is unidirectional from sun to producers to consumers; it cannot be recycled. Ecosystems obey the first and second laws of thermodynamics.
Formation of ecological pyramids
  1. Step 1 — Identify the trophic levels in the ecosystem: producers (first), primary consumers (second), secondary consumers (third), tertiary consumers (fourth).
  2. Step 2 — Measure or estimate the number, biomass, or energy at each trophic level.
  3. Step 3 — Draw a pyramid with the broadest base representing producers and the narrowest apex representing top consumers.
  4. Step 4 — For pyramid of number: count individuals at each level. For pyramid of biomass: measure dry weight of organisms. For pyramid of energy: measure energy content (kcal) per unit area per year.
  5. Step 5 — Note exceptions: pyramid of biomass in sea is inverted; pyramid of energy is always upright.
💡 Pyramid of energy is always upright and can never be inverted because energy is always lost as heat at each step. Pyramid of number and biomass can be inverted in some ecosystems (e.g., tree supporting many insects).
Exam Traps — Don't Make These Mistakes
⚠ Confusing GPP and NPP
✗ Gross primary productivity is the biomass available for consumers.
→ GPP is the total organic matter produced during photosynthesis, but a considerable amount is used by plants in respiration. The biomass available for consumers is NPP (GPP – R).
✓ Gross primary productivity is the rate of production of organic matter during photosynthesis. Net primary productivity (GPP – R) is the available biomass for consumption by heterotrophs.
⚠ Thinking energy flow is cyclic
✗ Energy flows in a cycle in an ecosystem, similar to nutrients.
→ Energy flow is unidirectional from sun to producers to consumers and is lost as heat at each step; it cannot be recycled. Only nutrients cycle.
✓ Energy flow is unidirectional. Solar energy is captured by producers, transferred to consumers, and dissipated as heat. It does not cycle back.
⚠ Misapplying the 10% law to detritus food chain
✗ The 10% law applies to both grazing and detritus food chains equally.
→ The 10% law is specifically mentioned for the grazing food chain. The detritus food chain involves decomposers and dead matter, and energy transfer efficiency may differ; the chapter does not state the same limitation for DFC.
✓ The 10 per cent law restricts the number of trophic levels in the grazing food chain. In nature, it is possible to have so many levels – producer, herbivore, primary carnivore, secondary carnivore in the grazing food chain.
⚠ Assuming all ecological pyramids are upright
✗ All ecological pyramids are upright with producers at the base and consumers at the top.
→ Pyramids of number and biomass can be inverted. For example, the pyramid of biomass in the sea is inverted because the biomass of fishes far exceeds that of phytoplankton. Pyramid of energy is always upright.
✓ Pyramid of energy is always upright. However, pyramid of number and pyramid of biomass can be inverted in certain ecosystems, such as the pyramid of biomass in aquatic ecosystems.
⚠ Confusing detritivores with decomposers
✗ Detritivores and decomposers are the same organisms.
→ Detritivores (e.g., earthworms) break down detritus into smaller particles (fragmentation), while decomposers (fungi and bacteria) perform catabolism by secreting enzymes to degrade detritus into simpler inorganic substances.
✓ Detritivores, such as earthworms, carry out fragmentation of detritus. Decomposers, mainly fungi and bacteria, carry out catabolism by secreting digestive enzymes.
⚠ Ignoring that a species can occupy multiple trophic levels
✗ Each species occupies only one trophic level in an ecosystem.
→ A given species may occupy more than one trophic level simultaneously. For example, a sparrow is a primary consumer when it eats seeds and a secondary consumer when it eats insects.
✓ A given species may occupy more than one trophic level in the same ecosystem at the same time. Trophic level represents a functional level, not a species as such.
Compare & Contrast
Source of energy
Grazing food chain (GFC)
Detritus food chain (DFC)
GFC begins with living plants (producers) as the primary energy source, while DFC begins with dead organic matter (detritus).
Biomass available
Gross primary productivity (GPP)
Net primary productivity (NPP)
GPP is the total organic matter produced during photosynthesis, whereas NPP is GPP minus respiration losses (R) and represents the biomass available for consumption by heterotrophs.
Shape of pyramid
Pyramid of biomass in a terrestrial ecosystem
Pyramid of biomass in an aquatic ecosystem
In terrestrial ecosystems, the pyramid of biomass is typically upright (producers have more biomass than consumers). In aquatic ecosystems (sea), it is often inverted because the biomass of fishes far exceeds that of phytoplankton.
Invertibility
Pyramid of number
Pyramid of energy
Pyramid of number can be inverted (e.g., a single tree supporting many insects), but pyramid of energy is always upright and can never be inverted because energy is always lost as heat at each trophic level.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 Very Short Answer (1 mark), 1 Short Answer (3 marks), 1 Long Answer (5 marks) — total approximately 11 marks
Most asked: Energy flow and the 10% law · Ecological pyramids (types, shapes, exceptions) · Productivity (GPP, NPP, factors affecting) · Decomposition (steps and factors affecting rate) · Food chains and food webs · Trophic levels
Don't skip: Draw and explain ecological pyramids with examples (5 marks); Explain energy flow through different trophic levels with the 10% law (3-5 marks); Describe the process of decomposition and factors affecting its rate (3 marks); Distinguish between GPP and NPP (2-3 marks)
One-Line Revision (read aloud before the exam)
  1. An ecosystem is a functional unit of nature where living organisms interact among themselves and with the physical environment.
  2. Stratification is the vertical distribution of different species occupying different levels, e.g., trees, shrubs, and herbs in a forest.
  3. Gross primary productivity (GPP) is the rate of production of organic matter during photosynthesis.
  4. Net primary productivity (NPP) = GPP – respiration losses (R); it is the biomass available for heterotrophs.
  5. Secondary productivity is the rate of formation of new organic matter by consumers.
  6. Decomposition involves fragmentation, leaching, catabolism, humification, and mineralisation.
  7. Detritivores like earthworms carry out fragmentation; decomposers like fungi and bacteria carry out catabolism.
  8. Humus is a dark coloured amorphous substance that is colloidal and serves as a reservoir of nutrients.
  9. Decomposition rate is slower if detritus is rich in lignin and chitin, and faster if rich in nitrogen and sugars.
  10. Warm and moist environments favour decomposition; low temperature and anaerobiosis inhibit it.
  11. Only 2-10% of photosynthetically active radiation (PAR) is captured by plants.
  12. Energy flow is unidirectional from sun to producers to consumers and is lost as heat.
  13. Only 10% of energy is transferred to each trophic level from the lower trophic level (10 per cent law).
  14. The grazing food chain begins with living plants; the detritus food chain begins with dead organic matter.
  15. A food web is the natural interconnection of food chains in an ecosystem.
  16. Trophic level is a functional level, not a species; a sparrow can be a primary or secondary consumer.
  17. Standing crop is the mass of living material (biomass) at a trophic level at a given time.
  18. Ecological pyramids can be of number, biomass, or energy.
  19. Pyramid of energy is always upright; pyramid of biomass in the sea is inverted.
  20. Pyramid of number can be inverted, e.g., a tree supporting many insects.
  21. Saprotrophs (fungi and bacteria) are decomposers that secrete digestive enzymes.
  22. The annual net primary productivity of the whole biosphere is approximately 170 billion tons of organic matter.
  23. Oceans occupy about 70% of the surface but contribute only 55 billion tons to NPP due to low productivity.

MCQs — 10 Questions with Answers

Q1. Primary productivity of an ecosystem is primarily limited by which factor?

  • A. Temperature only
  • B. Availability of solar radiation, nutrients, and photosynthetic capacity of plants ✓
  • C. Number of herbivores present
  • D. Soil pH and moisture alone

Answer: B — Primary productivity depends on multiple factors including solar energy, nutrient availability, and plant photosynthetic efficiency; temperature and moisture affect these but are not sole limiters.

Q2. If GPP of an ecosystem is 100 kcal m⁻² yr⁻¹ and respiration loss is 30 kcal m⁻² yr⁻¹, what is the NPP?

  • A. 30 kcal m⁻² yr⁻¹
  • B. 70 kcal m⁻² yr⁻¹ ✓
  • C. 130 kcal m⁻² yr⁻¹
  • D. 100 kcal m⁻² yr⁻¹

Answer: B — NPP = GPP − R = 100 − 30 = 70 kcal m⁻² yr⁻¹; this is the biomass available to heterotrophs.

Q3. Which of the following statements about decomposition is correct?

  • A. Decomposition occurs only in the absence of oxygen
  • B. Fragmentation by detritivores is the only step in decomposition
  • C. All five steps — fragmentation, leaching, catabolism, humification, and mineralisation — occur simultaneously on detritus ✓
  • D. Humification releases inorganic nutrients immediately for plant uptake

Answer: C — All decomposition steps operate simultaneously; decomposition is mostly aerobic; fragmentation is only the first physical breakdown step; mineralisation (not humification) releases nutrients.

Q4. In a warm, moist soil environment, decomposition is faster than in a cold, dry environment because:

  • A. Microbes are completely absent in cold soils
  • B. Warm and moist conditions increase microbial metabolic activity and enzymatic action ✓
  • C. Low temperature speeds up enzyme kinetics
  • D. Dry soils contain more oxygen for faster aerobic respiration

Answer: B — Warm temperature increases enzyme activity and microbial growth rate; moist soil provides optimal water for metabolic processes; cold and anaerobic conditions inhibit decomposition.

Q5. Which assertion and reason pair is correct regarding ocean productivity?

  • A. Assertion: Oceans have high productivity. Reason: They cover 70% of Earth's surface.
  • B. Assertion: Oceans have low productivity (55 billion tons/year). Reason: Most ocean water lacks sufficient nutrients in photic zone. ✓
  • C. Assertion: Ocean productivity exceeds land productivity. Reason: Water supports unlimited plant growth.
  • D. Assertion: Ocean productivity = land productivity. Reason: Both receive equal solar radiation.

Answer: B — Ocean productivity is only 55 billion tons/year despite 70% coverage due to nutrient limitation in most ocean areas and light penetration only in shallow photic zone.

Q6. Detritus rich in nitrogen and water-soluble sugars decomposes faster than detritus rich in lignin and chitin because:

  • A. Microbes lack enzymes to break down nitrogen
  • B. Lignin and chitin are complex, resistant polymers that require slower microbial degradation; nitrogen and sugars are readily available nutrients that speed catabolism ✓
  • C. Nitrogen inhibits bacterial enzyme activity
  • D. Only sugars can undergo decomposition

Answer: B — Nitrogen compounds and sugars are easily metabolised by decomposer microbes, accelerating catabolism; lignin and chitin are structurally complex and resistant to microbial enzymes.

Q7. The process of leaching in decomposition is best described as:

  • A. Breakdown of complex organic matter by enzymes
  • B. Movement of water-soluble inorganic nutrients down soil horizon, where they precipitate as unavailable salts ✓
  • C. Accumulation of dark colloidal humus
  • D. Physical fragmentation of detritus by earthworms

Answer: B — Leaching is specifically the downward movement of water-soluble nutrients through soil layers; it is distinct from catabolism (enzyme action), humification, and fragmentation.

Q8. Humification differs from mineralisation in that humification (i) produces humus and (ii) is _____, while mineralisation (i) breaks down humus and (ii) releases _____.

  • A. slow; fast inorganic nutrients ✓
  • B. fast; slow inorganic nutrients
  • C. slow; slow inorganic nutrients
  • D. fast; fast inorganic nutrients

Answer: A — Humification is a slow process forming resistant humus; mineralisation is the further microbial breakdown of humus to release inorganic nutrients for plant reuse.

Q9. A pond ecosystem is ideal for studying ecosystem functions because (i) it is a self-sustaining unit with all components, (ii) it exhibits low complexity, and (iii) it demonstrates _____.

  • A. only energy flow, not nutrient cycling
  • B. only abiotic components, not biotic ones
  • C. productivity, decomposition, energy flow, and nutrient cycling simultaneously ✓
  • D. only secondary productivity, not primary

Answer: C — A pond ecosystem demonstrates all four ecosystem functions: primary production (phytoplankton, aquatic plants), decomposition (bottom microbes), energy transfer through food chains, and nutrient cycling.

Q10. Anaerobic conditions in soil (low oxygen) inhibit decomposition because:

  • A. Detritus is unavailable to microbes
  • B. Most decomposer bacteria and fungi require oxygen for aerobic respiration and enzyme activity; anaerobiosis severely reduces their metabolic rate ✓
  • C. Water content increases decomposition rate indefinitely
  • D. Plants release inhibitory chemicals in anaerobic soils

Answer: B — Decomposition is largely an oxygen-requiring (aerobic) process; lack of oxygen limits microbial respiration and enzyme function, slowing organic matter breakdown and causing humus accumulation.

Flashcards

Define primary productivity and state its units.

Primary productivity is the rate of biomass production per unit area per unit time, expressed in gm⁻² yr⁻¹ or kcal m⁻² yr⁻¹.

What is the relationship between GPP, NPP, and respiration?

NPP = GPP − R, where R is respiration loss; NPP is the biomass available to heterotrophs.

Name the five steps of decomposition in correct order.

Fragmentation, leaching, catabolism, humification, and mineralisation occur simultaneously on detritus.

What is detritus and give one example.

Detritus is dead organic matter including fallen leaves, bark, flowers, and animal remains; it is the raw material for decomposition.

Define humification and state one property of humus.

Humification is the accumulation of dark, amorphous, colloidal humus that resists microbial action and serves as a nutrient reservoir.

Which two climatic factors most strongly control decomposition rate?

Temperature and soil moisture are the most important factors; warm, moist environments favour decomposition while low temperature and anaerobiosis inhibit it.

Why is decomposition described as oxygen-requiring?

Most decomposition is aerobic respiration by bacteria and fungi, requiring oxygen; anaerobic conditions slow or prevent the process.

What is the role of detritivores like earthworms in decomposition?

Detritivores fragment detritus into smaller particles through physical breakdown, increasing surface area for microbial decomposition.

State why ocean productivity is only 55 billion tons despite covering 70% of Earth.

Low nutrient availability in most ocean waters and limited light penetration below photic zone restrict primary productivity.

How does detritus rich in lignin and chitin differ from detritus rich in nitrogen?

Lignin and chitin-rich detritus decomposes slowly; nitrogen and sugar-rich detritus decomposes quickly due to easier microbial degradation.

Important Board Questions

Define net primary productivity (NPP) and explain how it differs from gross primary productivity (GPP). [2 marks]

State GPP = total organic matter produced; NPP = GPP − respiration losses; give one sentence on why NPP is available to heterotrophs but GPP is not.

Describe the process of decomposition in an ecosystem by explaining the five sequential steps and how climatic factors influence the rate of decomposition. [5 marks]

Name steps in order: fragmentation (detritivores), leaching (nutrient movement), catabolism (enzyme degradation), humification (humus formation), mineralisation (nutrient release). Then explain how warm+moist soils speed decomposition vs. cold+anaerobic conditions slowing it; mention role of microbial enzyme activity.

Using a pond as an example, explain how an ecosystem functions as a unit by addressing productivity, decomposition, energy flow, and nutrient cycling. Also discuss why ocean productivity is disproportionately low despite covering 70% of Earth's surface. [6 marks]

In pond: producers (phytoplankton, plants) → consumers (zooplankton) → decomposers (bacteria, fungi); show NPP available to consumers; explain decomposition cycle; state energy flows one-way with heat loss. For oceans: nutrient limitation in most waters + light only in photic zone + vast deep zones with no production = low overall productivity despite large area; global NPP ~170 billion tons, oceans only ~55 billion tons.

Assertion-Reason Questions for Ecosystem

Q1. Assertion (A):

Gross Primary Productivity (GPP) is always greater than Net Primary Productivity (NPP) in any ecosystem.

Reason (R):

A portion of GPP is consumed by plants during respiration, and this respiration loss must be subtracted to obtain NPP.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both assertion and reason are true, and the reason correctly explains why GPP > NPP. The relationship NPP = GPP – R demonstrates that respiration loss is the cause of the difference between GPP and NPP.

Q2. Assertion (A):

Humus serves as a reservoir of nutrients in soil.

Reason (R):

Humus is immediately available for plant uptake because of its high solubility in water.

Answer: C — A is true but R is false — The assertion is true—humus does act as a nutrient reservoir. However, the reason is false because humus is colloidal and resistant to microbial action, making it slowly available, not immediately available due to water solubility. Nutrient release occurs through mineralisation.

Q3. Assertion (A):

In a pond ecosystem, phytoplankton represent the trophic level that captures solar energy and converts it into organic matter.

Reason (R):

Phytoplankton are autotrophic organisms that perform photosynthesis, making them primary producers in the aquatic food chain.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both assertion and reason are true, and the reason correctly explains why phytoplankton capture energy. Phytoplankton's photosynthetic capacity is the basis for all primary production in aquatic ecosystems.

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Case-Based Questions (CBQ) for Ecosystem

Case Study:

A research team studied a 10-hectare paddy field in Punjab to assess its ecosystem productivity. They measured that the rice plants' Gross Primary Productivity was 8000 kcal m⁻² yr⁻¹. After accounting for plant respiration of 3200 kcal m⁻² yr⁻¹, they calculated the NPP available to consumers. During the growing season, 35% of this NPP was consumed by herbivorous insects and 25% by soil decomposers. The remaining biomass was harvested as grain by farmers. The soil temperature averaged 28°C with adequate moisture throughout the season.

Q1. What is the Net Primary Productivity of the rice paddy field?

  • A. 8000 kcal m⁻² yr⁻¹
  • B. 4800 kcal m⁻² yr⁻¹ ✓
  • C. 3200 kcal m⁻² yr⁻¹
  • D. 11200 kcal m⁻² yr⁻¹

Answer: B

Q2. If herbivorous insects consumed 35% of the NPP, what mass of insect biomass (in kcal m⁻² yr⁻¹) was produced?

  • A. 2800 kcal m⁻² yr⁻¹
  • B. 2400 kcal m⁻² yr⁻¹
  • C. 1680 kcal m⁻² yr⁻¹ ✓
  • D. 1200 kcal m⁻² yr⁻¹

Answer: C

Q3. Why did decomposition occur relatively quickly in this paddy field soil?

  • A. High temperature and moisture, combined with readily degradable plant material
  • B. Low lignin content and high nitrogen in the soil from fertilizers
  • C. Abundant earthworms and detritivores in the paddy ecosystem
  • D. Both A and B are correct ✓

Answer: D

Q4. What percentage of the original NPP was available for harvest as grain?

  • A. 60%
  • B. 40% ✓
  • C. 35%
  • D. 25%

Answer: B

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Previous Year Questions (PYQ) for Ecosystem

Real CBSE board exam questions from past papers — free to attempt.

CBSE 2026 · Set 57/1/1 · 1 mark

In an ecosystem, different species occupy different levels and vertical distribution of species is found. This is called _________.

  • Stratification
  • Layering
  • Fragmentation
  • Population

CBSE 2026 · Set 57/1/1 · 1 mark

Assertion (A): In terrestrial ecosystem much larger fraction of energy flows through grazing food chain. Reason (R): Grazing food chain may be connected to Detritus food chain at some levels.

  • Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A).
  • Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation for Assertion (A).
  • Assertion (A) is true, but Reason (R) is false.
  • Assertion (A) is false, but Reason (R) is true.

CBSE 2026 · Set 57/1/1 · 2 marks

(a) Given below is a pyramid found in an ecosystem, where each bar represents the standing crop available in the trophic level. (i) Identify the kind of pyramid and with the help of an example explain the conditions where this kind of pyramid is possible in nature. (1 mark) (ii) Write any two limitations of ecological pyramids. (1 mark)

CBSE 2026 · Set 57/1/2 · 1 mark

Which of the following is not a component of detritus food chain?

  • Dead Leaves
  • Bacteria
  • Fungi
  • Zooplankton

CBSE 2026 · Set 57/1/2 · 2 marks

(a) Given below is a pyramid found in an ecosystem, where each bar represents the standing crop available in the trophic level. (i) Identify the kind of pyramid and with the help of an example explain the conditions where this kind of pyramid is possible in nature. (1 mark) (ii) Write any two limitations of ecological pyramids. (1 mark) OR (b) In an ecosystem there was loss of biodiversity due to some project in that area. (i) How will biodiversity be affected? (1 mark) (ii) List two major causes of loss of biodiversity. (1 mark)

CBSE 2026 · Set 57/1/2 · 4 marks

The diagram below shows the distribution of two barnacle species, Chthamalus and Balanus on a rocky sea shore. When Balanus is experimentally removed, Chthamalus expands its range in lower intertidal zone.

(a) Identify and define the ecological phenomenon demonstrated by this observation. (1 mark) OR (a) State the principle that explains the elimination of one species. (1 mark)

CBSE 2026 · Set 57/1/3 · 2 marks

What is primary productivity of an ecosystem and how is it expressed? Explain the equation given below: NPP = GPP – R

CBSE 2026 · Set 57/2/1 · 1 mark

Observe the contents 1, 2, 3 and 4 of soil samples I, II and III shown in the graph. If the temperature and moisture of all soil samples are identical, which soil sample(s) will show faster decomposition ?

  • Soil Sample I
  • Soil Sample II
  • Soil Sample III
  • Both Soil Samples II and III

CBSE 2026 · Set 57/2/1 · 4 marks

Below is shown a pond that represents a self-sustaining natural ecosystem with both biotic and abiotic components interacting in a balanced food web.

(a) Mention two abiotic components of a pond ecosystem.

CBSE 2026 · Set 57/2/2 · 1 mark

Observe the contents 1, 2, 3 and 4 of soil samples I, II and III shown in the graph. If the temperature and moisture of all soil samples are identical, which soil sample(s) will show faster decomposition?

  • Soil Sample I
  • Soil Sample II
  • Soil Sample III
  • Both Soil Samples II and III

CBSE 2026 · Set 57/2/2 · 4 marks

Below is shown a pond that represents a self-sustaining natural ecosystem with both biotic and abiotic components interacting in a balanced food web.

Below is shown a pond that represents a self-sustaining natural ecosystem with both biotic and abiotic components interacting in a balanced food web. Mention two abiotic components of a pond ecosystem.

CBSE 2026 · Set 57(B) Biology · 2 marks

(a) (i) Which organisms capture energy that sustains the entire living world? What is the source of this energy? (ii) How do ecosystems comply with the Second Law of Thermodynamics? OR (b) "Alien species are highly invasive and are a threat to indigenous species." Substantiate this statement with the help of any two examples.

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