Ecosystem
'An ecosystem is a COMMUNITY of organisms interacting with each other and with their PHYSICAL ENVIRONMENT — a self-sustaining unit of nature.'
1. Chapter Overview
This chapter explores the STRUCTURE and FUNCTION of ecosystems. Topics include: ECOSYSTEM COMPONENTS (abiotic and biotic), FOOD CHAINS and FOOD WEBS (grazing and detritus food chains), ECOLOGICAL PYRAMIDS (number, biomass, energy), ENERGY FLOW (the 10% law — Lindemann's trophic efficiency), PRODUCTIVITY (primary and secondary), DECOMPOSITION (steps, factors affecting), and NUTRIENT CYCLING (carbon cycle, nitrogen cycle, phosphorus cycle).
2. Ecosystem Structure
Components
- Abiotic (Non-living) : Climate, sunlight, temperature, water, soil, nutrients, pH.
- Biotic (Living) : PRODUCERS (autotrophs — plants, algae, cyanobacteria), CONSUMERS (heterotrophs — herbivores, carnivores, omnivores, decomposers), DECOMPOSERS (bacteria, fungi — break down dead organic matter).
Standing Crop
- The BIOMASS (total weight of living matter) present in an ecosystem at a given time.
- 'Standing crop is a MOMENTARY MEASURE — it changes with seasons, disturbances, and succession.'
3. Food Chains and Food Webs
Grazing Food Chain
- Starts with GREEN PLANTS → Herbivores → Carnivores.
- Example: Grass → Grasshopper → Frog → Snake → Hawk.
- 'The basis of MOST terrestrial ecosystems — solar energy captured by plants flows through herbivores to carnivores.'
Detritus Food Chain
- Starts with DEAD ORGANIC MATTER (detritus) → Decomposers → Detritivores → Predators.
- Example: Dead leaves → Fungi → Earthworm → Bird.
- 'In some ecosystems (mangroves, deep sea), the detritus food chain is the MAJOR energy pathway.'
Food Web
- 'A food web is a NETWORK of INTERCONNECTED food chains — more REALISTIC than a simple linear food chain.'
- Most organisms feed at MULTIPLE trophic levels — creating a complex web.
4. Ecological Pyramids
| Pyramid | What it Represents | Limitations |
|---|---|---|
| Pyramid of NUMBER | Number of individuals at each trophic level | Does NOT ACCOUNT for size of organisms (one tree = millions of insects) |
| Pyramid of BIOMASS | DRY WEIGHT of living matter at each trophic level | Seasonal variation. Does NOT account for rate of production |
| Pyramid of ENERGY | Rate of ENERGY FLOW (productivity) at each trophic level | ALWAYS UPRIGHT — energy DECREASES at each level |
| Inverted pyramids | Possible for NUMBER and BIOMASS (e.g., one tree supports many insects). IMPOSSIBLE for ENERGY | Energy is ALWAYS LOST — cannot be inverted |
Key Rule
- 'The pyramid of ENERGY is ALWAYS UPRIGHT — energy is LOST at each trophic transfer. It can NEVER be inverted.'
5. Energy Flow and the 10% Law
The 10% Law (Lindemann, 1942)
- 'Only about 10% of the energy at one trophic level is TRANSFERRED to the next trophic level.'
- Where does the other 90% go?: Respiration (heat loss), undigested matter (faeces), not consumed, excretion.
- Consequence: SHORTER food chains mean MORE energy available at the TOP.
- Sun → Grass (1000 J) → Cow (100 J) → Human (10 J). 'Humans get MORE energy eating plants DIRECTLY than eating herbivores.'
Why are Food Chains Short?
- 'Energy LOSS limits the number of trophic levels. Typically: 4-5 levels maximum. Beyond that, there is NOT ENOUGH energy to support another level.'
6. Productivity
Primary Productivity
- Gross Primary Productivity (GPP) : Total RATE of PHOTOSYNTHESIS by producers.
- Net Primary Productivity (NPP) : GPP − Respiration of plants. 'The energy AVAILABLE to consumers.'
- NPP = GPP − R.
- 'NPP is the RATE at which new biomass is PRODUCED by plants — it is the ENERGY BASE for the entire ecosystem.'
Secondary Productivity
- Rate at which CONSUMERS CONVERT ingested food into their OWN new biomass.
| Ecosystem Type | NPP (g/m²/yr) | Characteristics |
|---|---|---|
| Tropical rainforest | 2000-2500 | VERY HIGH — year-round warmth and rain |
| Coral reef | 1500-2500 | HIGH productivity in nutrient-poor waters |
| Temperate forest | 1000-1500 | MODERATE — seasonal |
| Desert | < 200 | VERY LOW — limited by water |
| Open ocean | 125 | LOW — nutrient-poor (but LARGEST total due to area) |
7. Decomposition
Steps
- Fragmentation: Detritivores BREAK DOWN dead matter into smaller pieces.
- Leaching: Water-soluble nutrients are DISSOLVED and percolate into the soil.
- Catabolism: Bacterial and fungal ENZYMES degrade organic matter into simpler compounds.
- Humification: Formation of HUMUS (dark, amorphous, resistant organic matter).
- Mineralisation: Release of INORGANIC NUTRIENTS (N, P, K) back into the soil.
Factors Affecting Decomposition
- Temperature: WARMER → faster decomposition (up to an optimum).
- Moisture: ADEQUATE moisture → faster. Too DRY or WATERLOGGED → slower.
- Chemical composition: HIGH lignin → SLOWER decomposition. HIGH nitrogen → FASTER.
- 'Decomposition is SLOWEST in COLD, DRY environments (tundra) and FASTEST in WARM, MOIST environments (tropical rainforest).'
8. Nutrient Cycling
Carbon Cycle
- Key processes: PHOTOSYNTHESIS (CO₂ → organic C). RESPIRATION (organic C → CO₂). DECOMPOSITION. COMBUSTION (forest fires, fossil fuels).
- Human impact: Burning FOSSIL FUELS releases HUGE amounts of CO₂ → CLIMATE CHANGE.
Nitrogen Cycle
- Steps: NITROGEN FIXATION (N₂ → NH₃ — by Rhizobium bacteria, lightning, Haber process). NITRIFICATION (NH₃ → NO₂⁻ → NO₃⁻ by Nitrosomonas, Nitrobacter). ASSIMILATION (plants take up NO₃⁻). AMMONIFICATION (decomposers release NH₃ from dead organisms). DENITRIFICATION (NO₃⁻ → N₂ by Pseudomonas — returns N₂ to atmosphere).
Phosphorus Cycle
- 'The phosphorus cycle is SLOW and has NO ATMOSPHERIC component — phosphorus moves mainly through ROCK, SOIL, and LIVING ORGANISMS.'
- Key: Phosphate is RELEASED by weathering of rocks → absorbed by plants → consumed by animals → returned to soil by decomposition.
9. Common Mistakes
- Pyramid of energy is ALWAYS upright: Unlike pyramids of number and biomass, the energy pyramid can NEVER be inverted — energy always decreases as you go up trophic levels.
- GPP vs NPP: GPP = TOTAL photosynthesis. NPP = GPP − R. NPP is what is AVAILABLE to consumers.
- Decomposition vs mineralisation: Decomposition includes ALL steps (fragmentation → mineralisation). Mineralisation is just the FINAL STEP — release of inorganic nutrients.
- Nitrogen fixation vs nitrification: Fixation = N₂ → NH₃ (by Rhizobium, Azotobacter). Nitrification = NH₃ → NO₂⁻ → NO₃⁻ (by Nitrosomonas, Nitrobacter). They are DIFFERENT processes.
10. CBSE Exam Focus
- Ecosystem components — abiotic and biotic, producers, consumers, decomposers
- Food chains (grazing and detritus) and food webs
- Ecological pyramids — number, biomass, energy — characteristics and limitations
- Energy flow — 10% law, Lindemann's efficiency
- Productivity — GPP, NPP, secondary productivity
- Decomposition — steps, factors affecting
- Nutrient cycles — carbon cycle, nitrogen cycle (steps, bacteria involved), phosphorus cycle
11. Self-Test
Q1: What is the 10% law of energy transfer? What happens to the other 90%? A1: Only about 10% of energy is TRANSFERRED from one trophic level to the next. The remaining 90% is LOST as HEAT (respiration), in FAECES, as UNDIGESTED matter, and through EXCRETION.
Q2: Differentiate between GPP and NPP. A2: GPP (Gross Primary Productivity) = Total rate of PHOTOSYNTHESIS by producers. NPP (Net Primary Productivity) = GPP − RESPIRATION of producers. NPP is the energy stored as BIOMASS — available to consumers.
Q3: Why is the pyramid of energy always upright? A3: Energy is ALWAYS LOST at each trophic transfer (mainly as heat from respiration). Therefore, ENERGY DECREASES as you go UP the trophic levels — the base (producers) always has the MOST energy.
Q4: Name TWO bacteria involved in the nitrogen cycle and state their roles. A4: (1) Rhizobium — NITROGEN FIXATION (N₂ → NH₃) in root nodules of legumes. (2) Nitrosomonas — NITRIFICATION (NH₃ → NO₂⁻). (3) Nitrobacter — further NITRIFICATION (NO₂⁻ → NO₃⁻).
Q5: What are the differences between the grazing and detritus food chains? A5: Grazing food chain starts with LIVING plants → herbivores → carnivores. Detritus food chain starts with DEAD organic matter → decomposers → detritivores → predators. In some ecosystems (like mangroves), the detritus chain is the MAJOR energy pathway.
12. Conclusion
Ecosystems are the FUNCTIONAL UNITS of nature:
- ENERGY FLOW: 'One-way — from SUN to producers to consumers to decomposers. Energy is LOST at each step.'
- NUTRIENT CYCLING: 'CYCLICAL — nutrients are RECYCLED within the ecosystem through decomposition and uptake.'
- 'The health of an ecosystem depends on the BALANCE between energy flow and nutrient cycling — disrupt either, and the entire system is affected.'
