Nutrient Cycling in Ecosystems

Steps:

• Ecosystems involve energy flow (unidirectional) and nutrient cycling (recycling matter like carbon).
• Decomposers break down dead organisms, releasing nutrients back into the soil for producers.
• Evaluate options: Check direction of energy vs. matter transfer.
• Select the option showing matter return to producers via decomposers.

Why D is correct:

• Decomposers mineralize organic matter, releasing simple carbohydrates and nutrients that producers absorb to rebuild biomass, per the nutrient cycle definition.

Why the others are wrong:

• A: Energy flows from producers to herbivores to carnivores, not starting at herbivores.
• B: Proteins flow from producers to consumers, not reverse; no cycling to producers.
• C: Carbohydrates flow from producers to herbivores as energy transfer, not cycling.

Final answer: D