What serves as an energy source for decomposers?

The correct answer is Option (3) → Dead biomass or detritus

Decomposers, such as bacteria and fungi, break down dead organic matter. When any organism dies, its body becomes detritus, or dead biomass. Detritus is the raw material for decomposition and the energy source for decomposers.

Option (1) Fresh biomass — Incorrect. Fresh biomass is living material. It is a food source for herbivores and other consumers, not for decomposers, which feed on dead matter.

Option (2) Standing crop — Incorrect. The standing crop is the mass of living material present at a trophic level at a given time. Decomposers do not draw their energy from living organisms.

Option (3) Dead biomass or detritus — Correct. Dead plant and animal remains, including fecal matter, form detritus. Decomposers break it down and use the energy stored in it.

Option (4) Primary consumers — Incorrect. Primary consumers are living herbivores. They are eaten by secondary consumers (carnivores); decomposers act only after an organism dies.

NCERT Extract:
Energy Flow – “… The important point to note is that the amount of energy decreases at successive trophic levels. When any organism dies it is converted to detritus or dead biomass that serves as an energy source for decomposers. Organisms at each trophic level depend on those at the lower trophic level for their energy demands. Each trophic level has a certain mass of living material at a particular time called as the standing crop.”