Ecosystem
A community of living organisms (biotic) interacting with each other and their non-living environment (abiotic) as a functional unit. Term coined by Arthur Tansley (1935).
A. Components of an Ecosystem
Biotic & Abiotic Components
| Component | Type | Examples |
|---|---|---|
| Producers (Autotrophs) | Biotic | Plants, algae, cyanobacteria — make their own food via photosynthesis; base of all food chains |
| Primary Consumers | Biotic | Herbivores that eat producers directly (grasshoppers, rabbits, caterpillars, deer) |
| Secondary Consumers | Biotic | Carnivores that eat primary consumers (frogs, small fish, foxes) |
| Tertiary Consumers | Biotic | Top predators (eagles, sharks, lions, crocodiles) |
| Decomposers | Biotic | Fungi and bacteria — break down dead organic matter, returning nutrients to the soil |
| Abiotic Factors | Abiotic | Sunlight, temperature, water, soil type, pH, wind, humidity, minerals |
⚡ MCQ Tip Biotic = living. Abiotic = non-living. Decomposers = bacteria AND fungi. Producers are always at the base of food chains. Tansley (1935) coined "ecosystem".
B. Food Chains, Food Webs & Energy Flow
Energy Flow & the 10% Rule
- Food chain: A linear sequence showing who eats whom → Grass → Grasshopper → Frog → Snake → Eagle
- Food web: A complex, interconnected network of multiple food chains in an ecosystem
- Energy flows in ONE direction only — from producers (sun) through consumers
- 10% Rule (Lindeman's efficiency): Only 10% of energy is transferred to the next trophic level; 90% is lost as heat, movement, and metabolic processes
- Trophic level: The position an organism occupies in the food chain
- Arrows in a food chain point in the direction of energy flow (from prey to predator)
Energy Transfer Rule
Energy at next level = 10% of previous level
If producers have 10,000 kJ → primary consumers get 1,000 kJ → secondary get 100 kJ → tertiary get 10 kJ
C. Biogeochemical Cycles
Four Major Cycles
| Cycle | Key Points |
|---|---|
| Carbon Cycle | CO₂ absorbed by plants (photosynthesis); returned to atmosphere by respiration, decomposition, and combustion of fossil fuels |
| Nitrogen Cycle | N₂ (gas) fixed by nitrogen-fixing bacteria (Rhizobium) → ammonia → nitrites → nitrates → absorbed by plants → returned by denitrifying bacteria |
| Water Cycle | Evaporation (water → vapour) → Condensation (clouds) → Precipitation (rain/snow) → Runoff/infiltration → back to oceans |
| Oxygen Cycle | O₂ released by photosynthesis; consumed by aerobic respiration and combustion; closely linked to carbon cycle |
D. Ecological Terms
Key Definitions
| Term | Definition |
|---|---|
| Habitat | The specific physical place where an organism naturally lives (e.g. a pond, oak tree, desert burrow) |
| Niche | The role and position of an organism in its ecosystem — what it eats, when it's active, where it shelters, how it affects others |
| Population | All individuals of the same species living in the same area at the same time |
| Community | All populations of different species living and interacting in the same area |
| Biome | A large geographical region defined by its characteristic climate and organisms (e.g. tropical rainforest, desert, tundra, grassland) |
| Biodiversity | The variety of life in an area — includes species diversity, genetic diversity, and ecosystem diversity |
⚡ Hierarchy Individual → Population → Community → Ecosystem → Biome → Biosphere
Quick MCQ Revision
| Fact | Answer |
|---|---|
| Term "ecosystem" coined by | Arthur Tansley (1935) |
| Energy transfer efficiency | 10% — only 10% passes to next trophic level; 90% lost as heat |
| Decomposers are | Bacteria and fungi — break down dead organic matter |
| Abiotic factors | Non-living: sunlight, temperature, water, soil, pH, wind |
| Biotic factors | Living: producers, consumers, decomposers |
| Trophic level | Position of an organism in a food chain |
| Population vs Community | Population = same species; Community = all species in an area |
| Nitrogen fixation | Rhizobium bacteria convert N₂ gas → ammonia in soil |