Q1. [4] deep exam-ready
Read the following and answer the questions that follow:
In a grassland ecosystem, the following food chain exists:
Grass → Rabbit → Fox → Lion
Assume that the grass (producers) fixes 1,00,000 units of energy from sunlight. A wildlife biologist observing this ecosystem notes that at higher trophic levels, there are far fewer individual organisms. He also notes that when a disease wiped out most of the fox population one year, rabbit numbers exploded and the grass was severely overgrazed.
(i) Using the 10% law, calculate the amount of energy available to the lion (4th trophic level). Show your calculation. [2]
(ii) Why are there far fewer individual lions than rabbits in this ecosystem? [1]
(iii) Based on the biologist's observation about foxes, explain what this tells us about the interdependence of organisms in a food chain. [1]
Generated by claude-sonnet-4-6 · 2026-06-26 01:00 · grounding rag
Model Answer
(i) Energy available to the Lion (4th trophic level):
Using the 10% law:
- Grass (1st trophic level) = 1,00,000 units
- Rabbit (2nd trophic level) = 10% of 1,00,000 = 10,000 units
- Fox (3rd trophic level) = 10% of 10,000 = 1,000 units
- Lion (4th trophic level) = 10% of 1,000 = 100 units
∴ Energy available to the lion = 100 units
(ii) Since only 10% of energy is transferred at each trophic level, very little energy remains by the time it reaches the lion (4th trophic level). Less available energy can support fewer individuals, so the number of organisms decreases at higher trophic levels.
(iii) When foxes were wiped out, rabbit numbers exploded and grass was severely overgrazed. This shows that every organism in a food chain is interdependent — removal of one organism at any trophic level disturbs the balance of the entire ecosystem.
Source: Chapter 13, Section 13.1.1 — Food Chains and Webs
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Explanation
- (i) Always show each step of the 10% law calculation clearly — examiners award marks for working, not just the final answer.
- (ii) Link the reason directly to energy loss: less energy = fewer individuals. This is the concept of the energy pyramid.
- (iii) The biologist's observation is a classic example of a trophic cascade — use it to explain interdependence and balance in an ecosystem. Stick to the passage; don't over-elaborate.