Complete  notes on Ecosystem and Food Chain  for BPSC and Other Competitive Exams in 2025

Complete  notes on Ecosystem and Food Chain  for BPSC and Other Competitive Exams in 2025

General Science- 22 (Environment)

Ecosystem and Food Chain

1. Ecosystem – Definition and Components

Definition:
An ecosystem is a functional unit of nature where living organisms interact with each other and with the physical environment, exchanging energy and matter.

Main Components:

1. Abiotic (Non-living): Sunlight, temperature, water, air, soil, minerals, etc.
2. Biotic (Living):
   • Producers (Autotrophs): Green plants, algae, phytoplankton
   • Consumers (Heterotrophs):
     • Primary (herbivores) – deer, rabbit, zooplankton
     • Secondary (carnivores) – frogs, small fishes
     • Tertiary (top carnivores) – tiger, lion, eagle
   • Decomposers (Saprotrophs): Bacteria, fungi

2. Types of Ecosystems

Natural:

• Terrestrial: Forest, grassland, desert
• Aquatic: Freshwater (pond, lake, river), Marine (ocean, estuary, coral reef)

Artificial: Created and maintained by humans – agricultural fields, urban parks, aquariums.

3. Functions of Ecosystem

• Energy flow (unidirectional – from sun → producers → consumers → decomposers)
• Nutrient cycling (biogeochemical cycles – carbon, nitrogen, phosphorus, water)
• Regulation of climate & gases (O₂, CO₂ balance)
• Pollution control & waste decomposition
• Biodiversity support

4. Food Chains

Definition: Sequence of organisms showing the transfer of food and energy from one trophic level to another.

Types:

1. Grazing Food Chain (GFC):
   • Starts with producers (plants)
   • Example: Grass → Grasshopper → Frog → Snake → Eagle
2. Detritus Food Chain (DFC):
   • Starts with dead organic matter
   • Example: Dead leaves → Earthworms → Birds → Hawk

5. Trophic Levels

• Trophic Level 1: Producers
• Trophic Level 2: Primary consumers (herbivores)
• Trophic Level 3: Secondary consumers
• Trophic Level 4: Tertiary consumers/top predators
• Trophic Level 5: Quaternary consumers (rare – apex predators)

Rule:

• 10% Law of Energy Flow (Lindeman’s Law): Only ~10% of the energy is transferred from one trophic level to the next; rest lost as heat.

6. Food Web

• Interconnected food chains in an ecosystem.
• Increases stability of the ecosystem because organisms have alternative food sources.

7. Ecological Pyramids

Types:

1. Pyramid of Number – Number of organisms at each level
2. Pyramid of Biomass – Total biomass at each level
3. Pyramid of Energy – Always upright (energy decreases with each level)

8. Energy Flow in Ecosystem

• Primary Source: Sun (except deep-sea hydrothermal ecosystems)
• Pathway: Solar energy → photosynthesis → chemical energy in biomass → consumption & decomposition
• Unidirectional (no recycling of energy, only matter is recycled)

9. Ecological Efficiency

• Gross Primary Productivity (GPP): Total energy captured by producers
• Net Primary Productivity (NPP): GPP – energy used for respiration by producers (available to consumers)
• Secondary Productivity: Energy stored in consumers

10. Human Impact on Ecosystems

• Deforestation → loss of biodiversity, carbon imbalance
• Pollution → bioaccumulation & biomagnification
• Overfishing & hunting → trophic cascade effects
• Invasive species introduction
• Climate change → coral bleaching, species migration

11. Key Environmental Laws & Concepts (Prelims Favourites)

• Lindeman’s 10% Law – Energy transfer efficiency
• Biological Magnification – Increase in pollutant concentration at higher trophic levels
• Law of Tolerance (Shelford) – Species survival depends on environmental limits
• Keystone Species – Disproportionate role in ecosystem stability (e.g., Tiger, Sea Otter)
• Ecotone – Transition zone between ecosystems (high biodiversity – edge effect)

12. Exam-Focused Examples

• Largest Ecosystem: World’s Oceans
• Smallest Ecosystem: Pond
• Example of Inverted Pyramid of Biomass: Aquatic ecosystems (phytoplankton biomass less than zooplankton)
• Example of Detritus Food Chain: Mangrove forest floor

MCQs for Practice

Q1. Which pyramid is always upright in any ecosystem?
A) Pyramid of Number
B) Pyramid of Biomass
C) Pyramid of Energy
D) All of the above

Q2. Which law governs the transfer of energy between trophic levels?
A) Hardy-Weinberg Law
B) Lindeman’s Law
C) Le Chatelier’s Principle
D) Gause’s Law

Q3. In a grassland ecosystem, the primary consumers are:
A) Grass
B) Rabbit
C) Tiger
D) Fungi

Q4. The term ecotone refers to:
A) Transition area between two ecosystems
B) Species at the top of the food chain
C) Energy loss between trophic levels
D) Pollutant accumulation in organisms

Mains Answer Writing Tips

For questions like “Discuss the role of food chains in maintaining ecosystem stability”, you should:

1. Begin with definition & example
2. Explain energy flow & nutrient cycling
3. Highlight interdependence & food webs
4. Mention threats (human activities)
5. Suggest conservation strategies