In which of the following ecosystems, detritus food chain (DFC) is th…

The correct answer is Option (3) → Forest Ecosystem

Energy flows through two kinds of food chain. The grazing food chain (GFC) starts with producers, which are eaten by herbivores. The detritus food chain (DFC) starts with dead organic matter, which is broken down by decomposers such as fungi and bacteria.

Option (1) Pond Ecosystem — Incorrect. A pond is an aquatic ecosystem. Its phytoplankton are eaten fast by zooplankton, so most energy moves through the GFC.

Option (2) River Ecosystem — Incorrect. A river is also aquatic, and the GFC carries most of its energy.

Option (3) Forest Ecosystem — Correct. A forest is a terrestrial ecosystem. Most of the leaves, wood and bark are never eaten by herbivores; they fall and are broken down as detritus.

Option (4) Marine Ecosystem — Incorrect. The sea is the largest aquatic ecosystem, and there too the GFC is the main path of energy.

NCERT Extract:
Energy Flow – “In an aquatic ecosystem, GFC is the major conduit for energy flow. As against this, in a terrestrial ecosystem, a much larger fraction of energy flows through the detritus food chain than through the GFC. Detritus food chain may be connected with the grazing food chain at some levels: some of the organisms of DFC are prey to the GFC animals, and in a natural ecosystem, some animals like cockroaches, crows, etc., are omnivores.”