Which of the following series is true about energy flow in an ecosystem?

Energy flow through an ecosystem is one-directional. In most ecosystems producers capture solar energy through photosynthesis (chemosynthesis serves this role in a few specialised ecosystems, such as those around deep-sea hydrothermal vents), converting it into chemical (food) energy, and this energy then passes stepwise to consumers at successive trophic levels — with only a fraction (roughly the ten percent law's average) transferred onward at each step, the rest being lost to respiration and heat or left as matter that is never consumed or assimilated. Decomposers act on dead organic matter and waste from every trophic level, extracting and using the remaining chemical energy for their own metabolism (which is likewise eventually dissipated) while recycling the locked-up nutrients back into the ecosystem for producers to reuse. In the standard trophic-sequence representation used in such questions, decomposers therefore mark the final, not an intermediate, link of the energy pathway.

Applying this to the given sequences: in a typical, photosynthesis-driven ecosystem the chain must begin with producers, since they are the primary entry point for energy into the food chain, and — in this standard representation — it must end with decomposers, since they act on the dead matter left behind by every other level. Only the sequence Producers → Consumers → Decomposers satisfies both conditions.

• Producers → Decomposers → Consumers: places decomposers second, right after producers — but decomposers depend on dead organic matter and waste, which exists only once something further along the chain has died or been fed upon; nothing has reached that stage yet at this position, so decomposers have nothing to act on here.

• Decomposers → Consumers → Producers: wrongly starts with decomposers and ends with producers, reversing the true starting point of the flow.

• Consumers → Producers → Decomposers: places consumers first — but consumers obtain energy only by feeding on something that already holds it, and at the very start of the flow nothing has yet captured any energy for them to feed on.

Hence, the series that correctly describes energy flow in an ecosystem is Producers → Consumers → Decomposers.