UPSC Prelims 2021 Q28: Environment & Ecology – Ecosystems & Ecology Concepts
- (a) 1, 2 and 4 only
- (b) 2, 3, 4 and 5 only
- (c) 1, 3 and 5 only
- (d) 1, 2, 3, 4 and 5
Answer: (c) 1, 3 and 5 only
Explanation
Detritivores: Organisms that Feed on Dead Matter
| Organism | Feeding Style | Detritivore? |
|---|---|---|
| 1. Earthworms | Feed on decaying organic matter in soil | YES - Classic soil detritivores. |
| 2. Jellyfish | Predators (nematocysts sting plankton and small fish) | NO |
| 3. Millipedes | Feed on decaying leaves and wood | YES - Litter detritivores. |
| 4. Seahorses | Predators (suck small crustaceans) | NO |
| 5. Woodlice | Feed on rotting wood and leaf litter | YES - Terrestrial isopod detritivores. |
Detritivores: Earthworms (1) + Millipedes (3) + Woodlice (5). Answer: (c).
Answer: (c) 1, 3 and 5 only.
Why each option is right or wrong
- (a) Incorrect. Jellyfish and seahorses are predators, not detritivores.
- (b) Incorrect. Jellyfish and seahorses are predators.
- (c) CORRECT. Earthworms, millipedes and woodlice all feed on dead/decaying organic matter (detritus), breaking it down for decomposers. Jellyfish and seahorses are active predators.
- (d) Incorrect. Jellyfish (2) and seahorses (4) are predators, not detritivores.
Key point
Detritivores = feed on dead organic matter (detritus). Earthworms, millipedes, woodlice, dung beetles, termites. They FRAGMENT organic matter; decomposers (bacteria, fungi) then mineralise it. Jellyfish + seahorses = active predators.
Remember
Detritivores feed on dead organic matter (detritus). Earthworms (1), Millipedes (3) and Woodlice (5) are detritivores. Jellyfish (2) and Seahorses (4) are active predators. Correct answer: (c) 1, 3 and 5 only.
Common mistakes
- Confusing detritivores with decomposers. Detritivores physically fragment (earthworms, millipedes); decomposers chemically break down (bacteria, fungi).
- Assuming all invertebrates are detritivores. Jellyfish and seahorses are ACTIVE PREDATORS.
- Not knowing woodlice (pill bugs) are terrestrial crustaceans - the only land-adapted crustaceans besides some crabs.
Exam tip
Ecosystem decomposition chain: Detritivores (fragment) - Decomposers (mineralise) - Nutrients returned to soil. Detritivores include earthworms (Darwin's 'plough of nature'), millipedes, woodlice, dung beetles, termites. Central to soil formation and carbon cycling.
Sources
- NCERT Class XII Biology - Chapter 14: Ecosystem
- NCERT Class XI Biology - Animal Kingdom