CBSE Class 12 Biology Study Module – Ecosystem (NCERT Based)

Unit V: Ecology and Environment | Chapter 14: Ecosystem

CBSE Class 12 Biology Chapter 14 – Ecosystem | NCERT Study Module & Revision Notes

Course & Examination Details

• Course: CBSE Class 12 Biology
• Unit: Unit V – Ecology and Environment
• Chapter: Chapter 14 – Ecosystem
• Board: Central Board of Secondary Education (CBSE)
• Syllabus Base: Strictly as per NCERT Textbook
• Exam Relevance: CBSE Class 12 Board Examinations, School Assessments, NEET (Conceptual Foundation)

Study Module with Revision Notes

(Prepared strictly according to NCERT content for conceptual clarity and board-exam readiness)

Introduction to Ecosystem

An ecosystem is a functional unit of nature where living organisms interact among themselves and with the physical environment. It includes biotic components such as plants, animals, and microorganisms, and abiotic components such as air, water, soil, light, and nutrients. These components are interlinked through energy flow and nutrient cycling. Understanding ecosystems helps explain how energy moves through living systems, how matter is recycled, and how ecological balance is maintained.

The ecosystem concept forms the core of ecology and is essential for understanding environmental sustainability and conservation.

1. Ecosystem: Structure and Function

1.1 Structure of an Ecosystem

The structure of an ecosystem refers to the organization of its biotic and abiotic components.

Abiotic Components

• Inorganic substances: carbon dioxide, oxygen, nitrogen, phosphorus
• Organic compounds: carbohydrates, proteins, lipids
• Climatic factors: temperature, light, rainfall, humidity

Biotic Components

• Producers: Autotrophic organisms like green plants and algae that synthesize food through photosynthesis
• Consumers: Heterotrophic organisms that depend on producers for food
  • Primary consumers (herbivores)
  • Secondary and tertiary consumers (carnivores)
• Decomposers: Microorganisms like bacteria and fungi that break down organic matter

1.2 Functions of an Ecosystem

The major functional aspects of an ecosystem include:

• Productivity
• Decomposition
• Energy flow
• Nutrient cycling

These functions maintain ecosystem stability and continuity.

2. Productivity

2.1 Primary Productivity

Primary productivity is the rate at which producers synthesize organic matter from inorganic substances using sunlight.

Types of Primary Productivity

• Gross Primary Productivity (GPP): Total rate of photosynthesis
• Net Primary Productivity (NPP): GPP minus respiration losses

NPP = GPP – Respiration

NPP represents the biomass available to consumers and is expressed as mass per unit area per unit time.

2.2 Secondary Productivity

Secondary productivity refers to the rate of formation of new biomass by consumers. It depends on the amount of primary productivity available in the ecosystem.

3. Decomposition

Decomposition is the process by which decomposers break down dead organic matter into simpler inorganic substances.

3.1 Steps of Decomposition

1. Fragmentation: Breakdown of detritus into smaller particles by detritivores
2. Leaching: Soluble nutrients are washed into the soil
3. Catabolism: Enzymatic degradation by bacteria and fungi
4. Humification: Formation of humus
5. Mineralisation: Release of inorganic nutrients

3.2 Factors Affecting Decomposition

• Chemical composition of detritus
• Climate (temperature and moisture)
• Presence of decomposers

Warm and moist conditions favor rapid decomposition.

4. Energy Flow in Ecosystems

Energy flow in an ecosystem is unidirectional and follows the laws of thermodynamics.

4.1 Flow of Energy

• Energy enters the ecosystem through sunlight
• Producers convert solar energy into chemical energy
• Energy is transferred to consumers through food chains

Only about 10% of energy is transferred from one trophic level to the next (Ten Percent Law).

4.2 Food Chain and Food Web

• Food Chain: Linear sequence of organisms through which energy flows
• Food Web: Network of interconnected food chains

Food webs provide stability to ecosystems.

5. Ecological Pyramids

Ecological pyramids represent the trophic structure of ecosystems.

5.1 Types of Ecological Pyramids

Pyramid of Number

• Represents number of organisms at each trophic level
• May be upright or inverted

Pyramid of Biomass

• Represents biomass at each trophic level
• Upright in terrestrial ecosystems, inverted in aquatic ecosystems

Pyramid of Energy

• Always upright
• Shows energy flow per unit area per unit time

6. Nutrient Cycles

Nutrient cycles refer to the movement of nutrients through biotic and abiotic components of ecosystems.

6.1 Types of Nutrient Cycles

• Gaseous cycles: Carbon, nitrogen
• Sedimentary cycles: Phosphorus

6.2 Carbon Cycle

• Carbon moves through atmosphere, biosphere, hydrosphere, and lithosphere
• Photosynthesis fixes atmospheric CO₂
• Respiration and decomposition release CO₂

6.3 Nitrogen Cycle

• Nitrogen fixation (biological and atmospheric)
• Nitrification
• Assimilation
• Ammonification
• Denitrification

Nitrogen is essential for proteins and nucleic acids.

6.4 Phosphorus Cycle

• Involves movement of phosphorus through rocks, soil, water, and organisms
• Lacks a gaseous phase
• Essential for ATP, nucleic acids, and membranes

7. Ecological Succession

Ecological succession is the gradual and orderly change in species composition in an area over time.

7.1 Types of Succession

Primary Succession

• Occurs on bare rocks or newly formed land
• Slow process

Secondary Succession

• Occurs in areas where vegetation existed previously
• Faster than primary succession

7.2 Stages of Succession

1. Nudation
2. Invasion
3. Competition and coaction
4. Reaction
5. Stabilization (Climax community)

7.3 Climax Community

The climax community is stable, self-perpetuating, and in equilibrium with the environment.

8. Ecosystem Services

Ecosystem services are benefits humans obtain from ecosystems.

8.1 Types of Ecosystem Services

Provisioning Services

• Food, fuel, fiber, fresh water

Regulating Services

• Climate regulation
• Pollination
• Flood control

Cultural Services

• Recreation
• Aesthetic value
• Spiritual benefits

Supporting Services

• Nutrient cycling
• Soil formation

8.2 Economic Value of Ecosystem Services

Ecosystem services are invaluable for human survival and economic stability. Their degradation can lead to ecological imbalance and loss of biodiversity.

Revision Highlights (Quick Notes)

• Ecosystem = biotic + abiotic components
• Energy flow is unidirectional
• Only 10% energy passes to next trophic level
• Decomposition recycles nutrients
• Pyramid of energy is always upright
• Succession leads to climax community
• Ecosystem services support human life

CBSE Board Exam Focus Tips

• Practice labeled diagrams of nutrient cycles
• Write clear differences between GPP and NPP
• Use flow charts for succession stages
• Revise definitions and key terms from NCERT

Conclusion

Chapter 14 Ecosystem explains how living organisms and physical components interact to sustain life on Earth. Understanding ecosystem structure, energy flow, nutrient cycles, and ecosystem services is essential for environmental conservation and sustainable development. These NCERT-aligned revision notes are designed to help students score well in CBSE Class 12 board examinations through conceptual clarity and structured learning.