Chapter Overview & Weightage
Ecosystem covers structure and function of ecosystems, productivity (GPP, NPP), decomposition, energy flow, ecological pyramids, nutrient cycling, and ecological succession. Energy flow and ecological pyramids are NEET favourites.
This chapter carries 3-4% weightage in NEET with 2-3 questions. The 10% law of energy transfer, types of ecological pyramids, and ecological succession are the most tested topics.
Key Concepts You Must Know
Tier 1 (Core)
- Ecosystem components: abiotic (sunlight, water, temperature, soil) + biotic (producers, consumers, decomposers)
- Food chain: grazing food chain (starts with plants) vs detritus food chain (starts with dead organic matter)
- Energy flow: unidirectional, 10% law (Lindeman’s law) — only 10% of energy transfers to the next trophic level
- Ecological pyramids: of number, biomass, energy — always upright for energy
- Productivity: GPP (total photosynthesis), NPP = GPP - Respiration
Tier 2 (Frequently tested)
- Decomposition: fragmentation → leaching → catabolism → humification → mineralisation
- Nutrient cycling: carbon cycle, phosphorus cycle (sedimentary, no gaseous phase)
- Ecological succession: primary (bare rock → pioneer species) vs secondary (disturbed area with soil)
- Sere: entire sequence of communities from pioneer to climax community
Tier 3 (Occasionally tested)
- Standing crop vs standing state
- Net Ecosystem Productivity (NEP)
- Inverted pyramid of biomass in aquatic ecosystems (phytoplankton < zooplankton)
Important Formulas
Where:
- GPP = Gross Primary Productivity (total photosynthesis)
- NPP = Net Primary Productivity (energy available to consumers)
- = Respiration of producers
10% Law (Lindeman): Only ~10% of energy at one trophic level is transferred to the next.
If producers have 1000 kcal, then:
- Primary consumers: ~100 kcal
- Secondary consumers: ~10 kcal
- Tertiary consumers: ~1 kcal
| Pyramid Type | Grassland | Aquatic (Pond) | Forest |
|---|---|---|---|
| Number | Upright | Upright | Inverted (parasites on tree) |
| Biomass | Upright | Inverted (phytoplankton less mass than zooplankton at any instant) | Upright |
| Energy | Always upright | Always upright | Always upright |
Key rule: Pyramid of energy is NEVER inverted in any ecosystem.
The pyramid of energy is always upright — this is the most tested fact about ecological pyramids. Energy dissipates as heat at each level, so there’s always less energy at higher trophic levels. Biomass pyramid can be inverted (aquatic), number pyramid can be inverted (parasites on trees), but energy pyramid — never.
Solved Previous Year Questions
PYQ 1 — NEET 2024
Problem: According to Lindeman’s 10% law, if the energy at the producer level is 10,000 J, the energy available at the tertiary consumer level is:
(A) 100 J (B) 10 J (C) 1000 J (D) 1 J
Solution:
Applying 10% at each transfer:
- Producers: 10,000 J
- Primary consumers: 1,000 J (10%)
- Secondary consumers: 100 J (10% of 1,000)
- Tertiary consumers: 10 J (10% of 100)
Answer: (B) 10 J
PYQ 2 — NEET 2023
Problem: In which ecosystem is the pyramid of biomass inverted?
(A) Grassland (B) Forest (C) Aquatic (pond) (D) Desert
Solution:
In an aquatic (pond) ecosystem, the pyramid of biomass is inverted. At any given moment, the biomass of phytoplankton (producers) is less than the biomass of zooplankton and fish (consumers) because phytoplankton reproduce and turn over very rapidly despite having low standing biomass.
Answer: (C) Aquatic (pond)
PYQ 3 — NEET 2022
Problem: The
pioneer species in primary succession on a bare rock is:
(A) Grasses (B) Lichens (C) Mosses (D) Shrubs
Solution:
Lichens (crustose lichens specifically) are the first colonisers of bare rock (xerosere/lithosere). They secrete acids that weathering the rock surface, creating a thin layer of soil. The succession sequence is: lichen → moss → herbs/grasses → shrubs → trees (climax community).
Answer: (B) Lichens
Difficulty Distribution
| Difficulty | % of Questions | What to Expect |
|---|---|---|
| Easy | 45% | 10% law calculation, pyramid identification |
| Medium | 40% | Succession stages, nutrient cycling, GPP/NPP relationship |
| Hard | 15% | Inverted pyramids, decomposition steps |
Expert Strategy
Day 1: Energy flow — understand food chains, food webs, and the 10% law. Practice calculating energy at different trophic levels. Know that energy flow is always unidirectional.
Day 2: Ecological pyramids — make a table comparing number, biomass, and energy pyramids across grassland, forest, and aquatic ecosystems. Know which ones can be inverted and why.
Day 3: Nutrient cycling and succession. For the carbon and phosphorus cycles, know the key reservoirs and processes. For succession, know the complete xerosere sequence and the difference between primary and secondary succession.
For succession: primary = starting from scratch (bare rock, new volcanic island). Secondary = starting from existing soil after disturbance (after fire, flood, deforestation). Secondary succession is faster because soil and seed bank already exist.
Common Traps
Trap 1 — Pyramid of energy is ALWAYS upright. Never inverted, in any ecosystem. Number and biomass pyramids can be inverted in certain ecosystems, but energy pyramid cannot — because energy is lost as heat at each level.
Trap 2 — NPP is NOT the same as GPP. NPP = GPP - Respiration. GPP is the total carbon fixed by photosynthesis. NPP is what’s actually available for consumers to eat. NEET asks for this formula directly.
Trap 3 — Energy flow is unidirectional; nutrient cycling is cyclic. Energy enters the ecosystem as sunlight, flows through trophic levels, and exits as heat — no recycling. Nutrients (C, N, P) are recycled between biotic and abiotic components. Don’t say “energy is recycled.”
Trap 4 — Detritus food chain is NOT less important. In many ecosystems (especially forests), the detritus food chain handles MORE energy than the grazing food chain. A large fraction of net primary productivity goes to decomposers, not herbivores.