CBSE Class 12 Biology – Ecosystem
NCERT-aligned chapter resource | Biology Code 044
1. Ecosystem Structure
An ecosystem contains biotic components—producers, consumers and decomposers—and abiotic components such as light, water, temperature and minerals. Energy enters mainly through photosynthesis and flows through trophic levels.
2. Productivity
Gross primary productivity is the total rate of photosynthetic production. Net primary productivity is the biomass available after accounting for plant respiration: NPP = GPP − R.
3. Decomposition
Decomposition involves fragmentation, leaching, catabolism, humification and mineralisation. Detritivores and decomposers play important roles in returning nutrients to the environment.
4. Energy Flow
Energy flow through an ecosystem is unidirectional. Only a fraction of energy at one trophic level is transferred to the next, while much is dissipated as heat.
5. Ecological Pyramids
Pyramids may represent number, biomass or energy. The pyramid of energy is always upright because energy decreases at successive trophic levels.
6. Food Chains and Food Webs
A food chain represents a feeding pathway; a food web links multiple feeding relationships. Grazing and detritus food chains illustrate different routes of energy transfer.
7. Ecological Succession
Succession is a directional change in community composition over time. Primary succession begins where life was previously absent or soil is not developed; secondary succession begins where a community has been disturbed but soil remains.
8. Nutrient Cycling
Carbon and phosphorus cycles illustrate movement of elements between organisms and the physical environment. Unlike energy, nutrients are recycled.
Worked Calculation
If GPP of an ecosystem is 10,000 units and plant respiration accounts for 4,000 units, NPP = 10,000 − 4,000 = 6,000 units.
Chapter-Specific MCQs
- Energy flow is: (A) unidirectional (B) cyclic like nutrients (C) absent in producers (D) random. Answer: A.
- NPP equals: (A) GPP + R (B) GPP − R (C) R − GPP (D) GPP×R. Answer: B.
- Which pyramid is always upright? (A) Energy (B) Some biomass pyramids (C) Some number pyramids (D) None. Answer: A.
Competency Questions
- Explain why nutrients cycle but energy does not.
- Calculate NPP from supplied GPP and respiration data.
- Interpret a food web and identify possible trophic pathways.