Chapter 14 — Ecosystem — covers energy flow, food chains, nutrient cycling, ecological pyramids, and succession. Carries 6-8 marks. Focus on energy flow diagrams and nutrient cycles.
Key Concepts
Components of Ecosystem
| Component | Description | Examples |
|---|---|---|
| Producers (Autotrophs) | Make food via photosynthesis | Plants, algae, cyanobacteria |
| Primary consumers (Herbivores) | Eat producers | Deer, grasshopper, rabbit |
| Secondary consumers (Carnivores) | Eat herbivores | Frog, small fish, snake |
| Tertiary consumers (Top carnivores) | Eat secondary consumers | Lion, eagle, shark |
| Decomposers (Saprotrophs) | Break down dead matter | Bacteria, fungi |
Energy Flow
10% Law (Lindeman, 1942): Only 10% of energy is transferred from one trophic level to the next. 90% is lost as heat (respiration).
Energy flow is unidirectional (cannot flow backwards in food chain)
Sun → Producers (1% of sunlight captured) → Herbivores (10%) → Carnivores (10%) → Top carnivores (10%)
Energy flow is unidirectional (cannot flow backwards in food chain)
Sun → Producers (1% of sunlight captured) → Herbivores (10%) → Carnivores (10%) → Top carnivores (10%)
Food Chain Types
- Grazing food chain (GFC): Starts with green plants → herbivore → carnivore (most ecosystems)
- Detritus food chain (DFC): Starts with dead organic matter → decomposers → detritivores (dominant in forests — 80% energy flows through DFC!)
Ecological Pyramids
| Pyramid | Usually | Exception |
|---|---|---|
| Pyramid of Number | Upright (grassland) | Inverted in tree ecosystem (1 tree supports many insects) |
| Pyramid of Biomass | Upright (terrestrial) | Inverted in aquatic (phytoplankton biomass < zooplankton) |
| Pyramid of Energy | Always upright | No exceptions! (10% law always applies) |
Pyramid of energy is ALWAYS upright — this is a universally true statement and a favourite exam question.
Nutrient Cycling
Carbon Cycle
- CO₂ fixed by photosynthesis → enters food chain
- Returned by respiration, combustion, decomposition
- Fossil fuels: ancient organic matter → burning releases CO₂ → global warming
- Oceans are the largest reservoir of carbon
Phosphorus Cycle
- No gaseous phase — purely sedimentary cycle
- Rocks → weathering → soil → plants → animals → decomposition → soil
- Phosphorus is often a limiting nutrient in ecosystems
Ecological Succession
| Type | Starts On | Example |
|---|---|---|
| Primary Succession | Bare/lifeless area | Newly formed rock, lava, sand dune |
| Secondary Succession | Previously colonised but disturbed area | Abandoned farmland, burnt forest |
Primary succession on rock (Xerosere):
Lichens (pioneers) → Mosses → Herbs → Shrubs → Trees (climax community)
Primary succession in water (Hydrosere):
Phytoplankton → Submerged → Free-floating → Reed-swamp → Marsh → Forest (climax)
Lichens (pioneers) → Mosses → Herbs → Shrubs → Trees (climax community)
Primary succession in water (Hydrosere):
Phytoplankton → Submerged → Free-floating → Reed-swamp → Marsh → Forest (climax)
Ecosystem Services
According to Robert Costanza (1997): ecosystem services worth $33 trillion/year
- O₂ production, CO₂ absorption, nutrient cycling, pollination, water purification, soil formation
Quick Revision Points
- 10% Law: only 10% energy transferred per trophic level (Lindeman)
- Energy flow: unidirectional; Nutrient cycling: cyclic
- Pyramid of energy: ALWAYS upright
- GFC: plants → herbivore → carnivore; DFC: dead matter → decomposers (80% in forests)
- Carbon cycle: photosynthesis fixes, respiration/combustion releases
- Phosphorus: sedimentary cycle (no gaseous phase)
- Primary succession: bare area → lichens first; Secondary: disturbed area → faster
- Climax community: final stable community in succession
Chapter Navigation
Previous: Organisms and Populations Class 12 Notes
Next: Biodiversity and Conservation Class 12 Notes
Related Chapters in Class 12 Biology
- Organisms and Populations Class 12 Notes
- Biodiversity and Conservation Class 12 Notes
- Environmental Issues Class 12 Notes
Practice What You Learned
Test yourself with our NEET Mock Test Set 1 to see how well you’ve mastered the concepts.