Q1. [3] medium exam-ready
Explain why the number of individuals at successive trophic levels in an ecosystem keeps decreasing. In a food chain with four trophic levels, if producers have 10,000 units of energy, how much energy is available at the fourth trophic level? Show your working.
Generated by claude-sonnet-4-6 · 2026-06-26 00:59 · grounding rag
Model Answer
At each trophic level, only 10% of the energy is transferred to the next level. The rest is lost as heat, used in digestion, and for life processes. Therefore, energy decreases rapidly at each successive trophic level, supporting fewer individuals.
Working:
- Producers (T1) = 10,000 units
- Primary consumers (T2) = 10% of 10,000 = 1,000 units
- Secondary consumers (T3) = 10% of 1,000 = 100 units
- Tertiary consumers (T4) = 10% of 100 = 10 units
∴ Energy available at the fourth trophic level = 10 units.
Source: Chapter 13, Section 13.1.1 (Food Chains and Webs)
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Explanation
- The examiner expects the 10% law to be stated clearly — this is the key concept.
- The calculation must show all three steps (T2, T3, T4) with working, not just the final answer.
- "Loss as heat/digestion/life processes" is the reason — one line is enough; don't over-explain.
- Always state the final answer explicitly. Skipping the working loses marks even if the answer is correct.