A freshwater lake ecosystem contains the following food chain: aquatic plants → water fleas → small fish → pike. Scientists measured the energy content at each trophic level. The aquatic plants contained 200,000 kJ of energy per unit area. Only 10% of energy is transferred from one trophic level to the next. Explain why only a small percentage of energy is transferred between trophic levels in this food chain, and state two reasons why energy is lost at each stage.

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 3.2 Energy flow and ecosystems · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

In a freshwater lake ecosystem, energy enters through photosynthesis in aquatic plants and passes along the food chain: aquatic plants → water fleas → small fish → pike. Only around 10% of the energy stored in one trophic level is passed on to the next.

Model answer (5 marks)

Only a small percentage of the energy is transferred because most of the energy that enters a trophic level is used by the organisms for their own metabolic processes.

1. During respiration the organisms convert stored energy into ATP, and the excess energy is released as heat to the surroundings.
2. Not all of the organism’s biomass is eaten; a portion of the prey is not consumed or is only partially digested.
3. The remaining biomass is lost as excretion (faeces, urine) and other waste products.

Therefore, only the energy that is stored in the biomass of the organisms that are actually eaten is available to the next trophic level.

Examiner tips

  • Use the phrase ‘used in respiration’ and ‘released as heat’ to hit the key points; mention both unconsumed biomass and excretion; keep the answer concise and structured with bullet points or short sentences.
  • Remember the 10% rule is a consequence of the above losses – state it as a conclusion to show understanding.

Common mistakes

  • Writing that energy is ‘used up’ without specifying respiration and heat; "lost" is too vague.
  • Forgetting to mention that not all biomass is eaten or that excretion also removes energy.

Mark scheme (5 marks)

  1. Much of the energy is used in respiration by organisms at each trophic level
  2. Energy is lost as heat to the surroundings during respiration
  3. Not all of the organism is eaten / some biomass is not consumed
  4. Energy is lost in excretion / faeces / urine
  5. Therefore only the energy stored in the biomass of the organism is available to the next trophic level

Key terms in this question

trophic level · food chain

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