A pond ecosystem contains the following organisms: microscopic algae, water fleas, small fish, and pike. Describe what is meant by a food chain and explain why the number of pike that can be supported in the pond is much smaller than the number of water fleas.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In a freshwater pond, microscopic algae are eaten by water fleas. Water fleas are eaten by small fish, and small fish are eaten by pike.
Model answer (4 marks)
A food chain is a linear sequence that shows which organisms eat which, illustrating the transfer of energy and biomass from one trophic level to the next.
Only about 10 % of the energy or biomass in one trophic level is passed on to the next level. The rest is lost as heat through respiration and as waste products such as faeces, urine and excretion.
Because so little energy reaches the highest trophic level, the number of pike that can be supported is far smaller than the number of water fleas; there is simply insufficient energy or biomass to sustain a large population of pike.
Only about 10 % of the energy or biomass in one trophic level is passed on to the next level. The rest is lost as heat through respiration and as waste products such as faeces, urine and excretion.
Because so little energy reaches the highest trophic level, the number of pike that can be supported is far smaller than the number of water fleas; there is simply insufficient energy or biomass to sustain a large population of pike.
Examiner tips
- Define a food chain and state the 10 % transfer rule; explain energy loss mechanisms; link these to population size at the top level.
Common mistakes
- Confusing food chain with food web; giving the 10 % figure as a percentage of the original source rather than per transfer; ignoring energy loss by respiration and waste.
Mark scheme (4 marks)
- A food chain shows the feeding relationships between organisms / shows the transfer of biomass from one organism to the next
- Only around 10% of the energy / biomass is transferred from one trophic level to the next
- Energy / biomass is lost at each trophic level through respiration (releasing heat) and in waste products such as faeces / urine / excretion
- Because so little energy / biomass reaches the highest trophic level (pike), fewer organisms can be supported / there is insufficient energy / biomass to support a large population of pike
Key terms in this question
Related
- All Pearson Edexcel International GCSE Biology (4BI1) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →