List the main trophic levels in a typical ecosystem, starting with producers.

• A. Producers, primary consumers, secondary consumers, tertiary consumers, and decomposers/detritivores as a separate group.
• B. Producers, secondary consumers, primary consumers, decomposers, tertiary consumers.
• C. Producers, primary consumers, decomposers, secondary consumers, tertiary consumers.
• D. Producers, decomposers, primary consumers, secondary consumers, tertiary consumers.

Answer: (A)

Understanding how energy moves through an ecosystem helps explain why the sequence starts with producers and ends with decomposers as a separate group. Producers are autotrophs that capture light energy and build organic matter. Primary consumers are herbivores that eat those producers. Secondary consumers are animals that eat primary consumers, and tertiary consumers are predators that eat secondary consumers. Decomposers and detritivores—like bacteria, fungi, and some invertebrates—don’t rely on the next higher living trophic level for energy. Instead, they break down dead matter and waste, recycling nutrients back into the environment. Because they act on detritus rather than feeding through a single line of living organisms, they’re best shown as a separate group from the linear producer–consumer sequence. Other arrangements lump decomposers into the chain or reorder the levels, which doesn’t reflect how energy flow and nutrient cycling actually operate.