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📗 Nayab Subba syllabus: Paper I, General Knowledge, Unit 1.9.1 · Open the full Nayab Subba course
An ecosystem (पारिस्थितिक प्रणाली) is a community of living things together with their non-living environment, all interacting as one unit. The term was coined by A. G. Tansley in 1935.
1. Components of an ecosystem
| Component | Parts | Examples |
|---|---|---|
| Abiotic (non-living) | Sunlight, temperature, water, air, soil, minerals | Rainfall, humus |
| Biotic – producers | Green plants and algae that make food by photosynthesis | Grass, trees, phytoplankton |
| Biotic – consumers | Animals that eat others | Primary (herbivores), secondary and tertiary (carnivores), omnivores |
| Biotic – decomposers | Break down dead matter and return nutrients to soil | Bacteria and fungi |
2. Types of ecosystem
- Natural – terrestrial: forest, grassland, desert, mountain (alpine) ecosystems.
- Natural – aquatic: freshwater (ponds, lakes, rivers, wetlands) and marine (sea, ocean).
- Artificial (man-made): crop fields, gardens, aquariums, fish ponds, cities.
- Nepal has many ecosystems in a short distance, from tropical forests in the Tarai to alpine meadows and snow in the Himalaya. Wetlands such as Koshi Tappu and Rara are Ramsar sites.
3. Food chain, food web and trophic levels
- A food chain shows who eats whom: grass → deer → tiger.
- Many interconnected food chains form a food web.
- Each step in a food chain is a trophic level: producers (level 1), herbivores (2), carnivores (3 and above).
- Grazing food chain starts with green plants; detritus food chain starts with dead organic matter.
4. Energy flow
- The source of energy for almost all ecosystems is the sun.
- Energy flows in one direction (unidirectional): sun → producers → consumers; it is lost as heat at each step.
- Ten percent law (Raymond Lindeman, 1942): only about 10% of energy passes from one trophic level to the next. That is why food chains are short (usually 3–5 levels).
- Ecological pyramids show number, biomass or energy at each level; the pyramid of energy is always upright.
5. Biogeochemical (nutrient) cycles
Unlike energy, nutrients move in cycles between living things and the environment.
| Cycle | Main points |
|---|---|
| Water (hydrological) cycle | Evaporation, transpiration, condensation (clouds), precipitation (rain, snow), runoff and infiltration |
| Carbon cycle | Plants take CO₂ by photosynthesis; CO₂ returns by respiration, decomposition and burning fossil fuels; oceans store carbon |
| Nitrogen cycle | Air is about 78% nitrogen, but plants cannot use it directly. Nitrogen fixation by bacteria (*Rhizobium* in legume root nodules) and lightning → nitrification (ammonia → nitrites → nitrates) → used by plants → denitrification returns N₂ to air |
| Oxygen cycle | Released by photosynthesis; used in respiration and burning |
| Phosphorus cycle | A sedimentary cycle (no gaseous stage); phosphorus moves from rocks to soil, plants and animals and back to sediments |
- Gaseous cycles: carbon, nitrogen, oxygen. Sedimentary cycles: phosphorus, sulphur.
- Human activities (deforestation, burning fossil fuels, fertiliser overuse) disturb these cycles and cause global warming and eutrophication.
6. Importance of a balanced ecosystem
- Provides food, water, clean air, medicine and raw materials (ecosystem services).
- Controls floods, soil erosion and climate.
- Supports biodiversity, tourism and livelihoods, especially in Nepal’s forests and wetlands.
7. Types of ecosystems
| Type | Examples | Example in Nepal |
|---|---|---|
| Forest | Tropical, temperate and coniferous forests | Sal forests of Chitwan |
| Grassland | Savanna, meadows | Grasslands of Shuklaphanta |
| Desert and cold desert | Hot and cold dry lands | Upper Mustang |
| Freshwater | Rivers, lakes, wetlands | Rara Lake, Koshi Tappu |
| Marine | Seas and oceans | None (Nepal is landlocked) |
8. Threats to ecosystems
- Deforestation and forest fires.
- Pollution of rivers and lakes.
- Invasive species such as water hyacinth in Phewa Lake.
- Climate change shifting plant and animal ranges upward.
9. How this topic is asked in the exam
Objective questions ask the parts of an ecosystem, the types of ecosystems and examples from Nepal. Subjective questions ask about the importance of ecosystem conservation.
📌 Key points to remember
- Ecosystem = living community + non-living environment; term by Tansley (1935).
- Components: abiotic, producers, consumers, decomposers (bacteria and fungi).
- Food chain → food web; each step is a trophic level.
- Energy flows one way; only about 10% passes to the next level (Lindeman’s 10% law); pyramid of energy is always upright.
- Nutrients cycle: water, carbon, nitrogen, oxygen (gaseous) and phosphorus (sedimentary).
- Rhizobium fixes nitrogen in legume root nodules; air is about 78% nitrogen.
📝 Quiz
Test yourself: Ecosystem
50 questions check what you read about Components of an ecosystem, Energy flow and Importance of a balanced ecosystem. Each attempt picks 25 at random.
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