UPSC Prelims 2021 Q28: Detritivores and Decomposers — Explained
Which of the following are detritivores?
1. Earthworms
2. Jellyfish
3. Millipedes
4. Seahorses
5. Woodlice
Select the correct answer using the code given below.
- 1, 2 and 4 only
- 2, 3, 4 and 5 only
- 1, 3 and 5 only
- 1, 2, 3, 4 and 5
Answer: (c) 1, 3 and 5 only
Statement-wise explanation
- 1: correct — Earthworms swallow dead leaves, decaying plant matter and soil rich in organic debris. National Geographic gives them as a standard example of a detritivore. In NCERT's account of decomposition, earthworms carry out fragmentation: they break detritus into smaller pieces so microbes can act on it.
- 2: incorrect — Jellyfish are carnivores. Stinging cells called nematocysts on their tentacles catch and paralyse live prey such as copepods, fish larvae and other zooplankton. They are predators, not feeders on dead organic matter.
- 3: correct — Millipedes live in leaf litter and mainly eat decaying plant material. National Geographic names them alongside earthworms and termites as detritivores. Do not confuse them with centipedes, which hunt other soil animals.
- 4: incorrect — Seahorses are ambush predators. They suck tiny live crustaceans, mainly copepods, amphipods and mysid shrimp, into their snout. NOAA sources describe them as taking live, moving prey, so they are carnivores.
- 5: correct — Woodlice are land-dwelling crustaceans (terrestrial isopods). They feed on dead leaves, rotting wood and other decaying organic matter. Ecologists class them as macro-decomposers in leaf-litter decomposition and nutrient cycling.
How to eliminate
Sort the animals by where they live. A detritivore here is a soil or leaf-litter animal that eats dead organic matter. Earthworms are the textbook detritivore (NCERT Class XII, Ecosystem), so statement 1 is almost certainly right. That rules out the option that leaves out 1. Jellyfish and seahorses are marine animals known for catching live prey: jellyfish sting it, seahorses suck it in. If you remove either of them, the only option left is the one with 1, 3 and 5. You do not need to know much about woodlice or millipedes. Knowing the two marine predators is enough. The option with all five is a trap for anyone who thinks every small, humble animal must be a detritivore. Fact to remember: detritivores in the leaf-litter trio are earthworms, millipedes and woodlice. Jellyfish and seahorses are carnivores.
Know more
Detritus is dead organic matter: fallen leaves, dead roots, carcasses and faeces. Two groups break it down.
- Detritivores ingest detritus and digest it internally. They break it physically into smaller pieces.
- Decomposers (saprotrophs), mainly bacteria and fungi, secrete enzymes outside their bodies. They absorb the simple compounds that result.
NCERT (Class XII Biology, Ecosystem) lists the steps of decomposition:
| Step | What happens | Main agent |
|---|---|---|
| Fragmentation | Detritus broken into small particles | Detritivores (e.g., earthworm) |
| Leaching | Water-soluble nutrients sink into soil as salts | Water |
| Catabolism | Enzymes convert detritus to simple inorganic substances | Bacteria, fungi |
| Humification | Dark, amorphous humus forms; very slow to decompose | Microbes |
| Mineralisation | Humus degraded, inorganic nutrients released | Microbes |
Decomposition is faster in warm, moist, oxygen-rich conditions and when detritus is rich in nitrogen and sugars. It is slower when detritus is high in lignin and chitin.
Common confusions
| Animal | Feeding type |
|---|---|
| Earthworm, millipede, woodlouse, termite | Detritivore |
| Dung beetle | Detritivore (eats dung) |
| Vulture, hyena (partly) | Scavenger (eats carcasses) |
| Centipede | Predator in the same leaf litter |
| Jellyfish, seahorse, sea anemone | Carnivore |
| Sea cucumber, fiddler crab | Marine detritivore |
The last row is why 'marine = not a detritivore' is a shortcut, not a rule. Many seabed animals feed on detritus. Detritivores start the detritus food chain, which carries much more energy than the grazing food chain in forests and soils. Their work also underpins vermicomposting, which uses earthworms to turn organic waste into manure.
NCERT Concepts to Revise
- Vermicomposting: Vermicomposting puts a detritivore's feeding habit to practical use, which helps you judge which listed animals eat dead organic matter.