Class 12 Biology Chapter 14 Ecosystem Notes
14.1 Ecosystem – Structure and Function
An ecosystem is a functional unit of nature where living organisms (biotic components) interact with the physical environment (abiotic components).
Fill in the Blanks (Concepts Explained)
Plants are autotrophs because they fix CO₂ using sunlight.
In a tree-dominated ecosystem, pyramid of numbers is spindle-shaped.
In aquatic ecosystems, sunlight is the major limiting factor for productivity.
Earthworms, bacteria and fungi are common detritivores.
The largest reservoir of carbon on Earth is oceans.
Trophic Levels (Quick Facts)
Largest population in a food chain → Decomposers
Second trophic level in a lake → Zooplankton
Secondary producers → None (only plants are producers)
Photosynthetically Active Radiation (PAR)
PAR = 50% of total incident solar radiation
Only 2–10% of PAR is actually used in photosynthesis
14.2 Productivity
Primary Productivity
Rate of biomass or energy production by plants
Depends on:
Sunlight
Temperature
Water
Nutrients
Plant species
Secondary Productivity
Rate of biomass formation by consumers
Always less than primary productivity
14.3 Decomposition
Definition
Decomposition is the breakdown of dead organic matter into simpler inorganic substances.
Steps of Decomposition:
Fragmentation – Detritivores break detritus
Leaching – Soluble nutrients seep into soil
Catabolism – Enzymatic breakdown by microbes
Humification – Formation of humus
Mineralisation – Release of inorganic nutrients
Products:
CO₂
Water
Mineral nutrients
Humus
14.4 Energy Flow in an Ecosystem
Energy flow is unidirectional
Sun → Producers → Consumers → Decomposers
10% Law:
Only 10% energy is transferred to next trophic level
90% is lost as heat
14.5 Ecological Pyramids
Ecological Pyramid
A graphical representation of numbers, biomass or energy at each trophic level.
Types:
Pyramid of Numbers
Upright (grassland)
Spindle-shaped (forest)
Pyramid of Biomass
Upright (forest, grassland)
Inverted (pond ecosystem)
Pyramid of Energy
Always upright
14.6 Ecological Succession
Ecological succession is the gradual and orderly change in species composition over time.
Types:
Primary succession – On bare rock
Secondary succession – On disturbed land
14.7 Nutrient Cycling
Biogeochemical Cycles
Movement of nutrients between biotic and abiotic components.
Sedimentary Cycle
Nutrients do not enter atmosphere easily
Example: Phosphorus cycle
Reservoir: Earth’s crust and rocks
Carbon Cycle
Carbon enters ecosystem as CO₂
Fixed by plants during photosynthesis
Released by:
Respiration
Decomposition
Burning of fossil fuels
14.8 Ecosystem Services
Ecosystem services are benefits humans get from nature.
Major Ecosystem Services:
Oxygen production
Pollination
Nutrient cycling
Climate regulation
Soil formation
Waste decomposition
Important Distinctions (Exam Favourite)
Grazing vs Detritus Food Chain
Grazing: Starts from plants
Detritus: Starts from dead organic matter
Food Chain vs Food Web
Food chain → single pathway
Food web → interconnected chains (more stable)
Litter vs Detritus
Litter: Dead matter on soil surface
Detritus: Dead matter below soil
Upright vs Inverted Pyramid
Upright: Producers > Consumers
Inverted: Producers < Consumers (pond)
14.9 Summary (Exam Ready ✨)
Ecosystem has biotic and abiotic components
Productivity depends on sunlight, water and nutrients
Decomposition recycles nutrients
Energy flow follows 10% law
Ecological pyramids show trophic structure
Nutrient cycles maintain ecosystem balance
Ecosystem services are essential for human survival
✍️ Top 10 Short Question–Answers
Q1. What is an ecosystem?
Answer: An ecosystem is a functional unit where biotic and abiotic components interact.
Q2. Why are plants called autotrophs?
Answer: Because they fix CO₂ using sunlight.
Q3. What is PAR?
Answer: Photosynthetically Active Radiation; about 50% of total solar radiation.
Q4. What is primary productivity?
Answer: Rate of biomass or energy production by plants.
Q5. What is secondary productivity?
Answer: Rate of biomass formation by consumers.
Q6. What is decomposition?
Answer: Breakdown of dead organic matter into simpler inorganic substances.
Q7. State the 10% law of energy flow.
Answer: Only 10% energy is transferred to the next trophic level.
Q8. Which ecological pyramid is always upright?
Answer: Pyramid of energy.
Q9. What is ecological succession?
Answer: Gradual and orderly change in species composition over time.
Q10. Name one ecosystem service.
Answer: Oxygen production.
📝 Long Answer Questions
Q1. Explain productivity in an ecosystem.
Answer:
Productivity refers to the rate of biomass production in an ecosystem.
Primary productivity:
Carried out by plants
Depends on sunlight, temperature, water and nutrients
Secondary productivity:
Carried out by consumers
Always less than primary productivity
Q2. Describe the process of decomposition.
Answer:
Decomposition breaks down dead organic matter.
Steps:
Fragmentation: Detritivores break detritus
Leaching: Soluble nutrients enter soil
Catabolism: Microbial enzymatic breakdown
Humification: Formation of humus
Mineralisation: Release of inorganic nutrients
Products: CO₂, water, minerals and humus.
Q3. Explain ecological pyramids.
Answer:
Ecological pyramids represent trophic levels graphically.
Types:
Pyramid of numbers: Upright or spindle-shaped
Pyramid of biomass: Upright or inverted
Pyramid of energy: Always upright
They show structure of an ecosystem.
📝 PYQs (Previous Year Questions)
🔹 1️⃣ Very Short Answer Questions (1 Mark)
What is an ecosystem?
Why are plants called autotrophs?
What does PAR stand for?
Name the largest reservoir of carbon on Earth.
What is primary productivity?
What is secondary productivity?
Define decomposition.
State the 10% law of energy flow.
Which ecological pyramid is always upright?
Name any one ecosystem service.
🔹 2️⃣ Short Answer Questions (2–3 Marks)
What are biotic and abiotic components of an ecosystem?
Explain primary productivity and mention factors affecting it.
Define secondary productivity. Why is it always less than primary productivity?
Describe the steps of decomposition.
What is Photosynthetically Active Radiation (PAR)?
Explain the unidirectional flow of energy in an ecosystem.
What are ecological pyramids? Name their types.
Differentiate between grazing and detritus food chain.
What is ecological succession? Name its two types.
Write any two ecosystem services.
🔹 3️⃣ Long Answer Questions (3–5 Marks)
Describe the structure and function of an ecosystem.
Explain productivity in an ecosystem.
Describe the process of decomposition in detail.
Explain energy flow in an ecosystem with the 10% law.
Describe different types of ecological pyramids with examples.
Explain nutrient cycling with reference to carbon cycle.
Describe ecological succession.
🔹 4️⃣ Difference-Based PYQs (Very Important)
Primary productivity vs Secondary productivity
Grazing food chain vs Detritus food chain
Food chain vs Food web
Upright pyramid vs Inverted pyramid
Primary succession vs Secondary succession
🔹 5️⃣ Assertion–Reason / Concept-Based PYQs
Pyramid of energy is always upright. Justify.
Only a small fraction of solar radiation is used in photosynthesis. Explain.
Decomposers occupy the lowest trophic level. Explain.
Food webs are more stable than food chains. Justify.
Ecosystem services are essential for human survival. Explain.
🔹 6️⃣ Diagram / Graph-Based PYQs
Draw and label different ecological pyramids.
Diagram showing steps of decomposition.
Flow diagram of energy flow in an ecosystem.
Diagrammatic representation of carbon cycle.
❓ FAQs (5)
FAQ 1. Why is sunlight a limiting factor in aquatic ecosystems?
Because light penetration decreases with depth.
FAQ 2. Why is pyramid of energy always upright?
Because energy decreases at each trophic level.
FAQ 3. What is the largest reservoir of carbon?
Oceans.
FAQ 4. What is the difference between food chain and food web?
Food chain is single pathway; food web is interconnected.
FAQ 5. Why are ecosystem services important?
They support life by providing oxygen, nutrients and climate regulation.
| Chapter No. | Chapter Name | Visit |
|---|---|---|
| 1 | Reproduction in Organisms | Visit |
| 2 | Sexual Reproduction in Flowering Plants | Visit |
| 3 | Human Reproduction | Visit |
| 4 | Reproductive Health | Visit |
| 5 | Principles of Inheritance and Variation | Visit |
| 6 | Molecular Basis of Inheritance | Visit |
| 7 | Evolution | Visit |
| 8 | Human Health and Disease | Visit |
| 9 | Strategies for Enhancement in Food Production | Visit |
| 10 | Microbes in Human Welfare | Visit |
| 11 | Biotechnology: Principles and Processes | Visit |
| 12 | Biotechnology and its Applications | Visit |
| 13 | Organisms and Populations | Visit |
| 14 | Ecosystem | Visit |
| 15 | Biodiversity and Conservation | Visit |
| 16 | Environmental Issues | Visit |
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