A food chain has four trophic levels: grass → rabbit → fox → eagle. Suggest why there are far fewer eagles than rabbits in this food chain, and explain why food chains rarely have more than five trophic levels.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Fewer eagles than rabbits because energy is lost at each trophic level – only about 10 % of the energy from the rabbit is transferred to the fox and even less to the eagle. Energy is lost through respiration, movement, heat loss and excretion, so the amount available to support the next level is reduced. Consequently, fewer eagles can be sustained.
Food chains rarely have more than five trophic levels because by the fifth level the energy remaining is so low that there is not enough to support another level or a large population of organisms.
Food chains rarely have more than five trophic levels because by the fifth level the energy remaining is so low that there is not enough to support another level or a large population of organisms.
Examiner tips
- Use the 10 % rule to justify the drop in numbers
- Explain energy loss mechanisms (respiration, heat, excretion)
- Show why the fifth level is the practical limit
Common mistakes
- Claiming eagles are fewer because they are larger, not energy
- Forgetting to mention the 10 % transfer efficiency
- Not linking energy loss to the inability to sustain more levels
Mark scheme (4 marks)
- Energy is lost at each trophic level as it passes from rabbit to fox to eagle
- Energy is lost through processes such as respiration, movement, or keeping warm (heat loss), or through excretion / material that cannot be digested
- Because less energy is available, fewer eagles can be supported / there is not enough energy to sustain a large population of eagles
- By the fifth trophic level so little energy remains that there would not be enough to sustain another level / population of organisms
Key terms in this question
Related
- All Cambridge International IGCSE Biology (0610) revision notes →
- How to answer a "Suggest and Explain" question →
- Decode the mark scheme abbreviations →
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