A grassland ecosystem contains the following organisms: grass, grasshoppers, frogs, and hawks. Only 10% of the energy stored in one trophic level is available to the next trophic level. Explain why energy is lost between trophic levels in this food chain, and describe how this affects the number of organisms that can be supported at each level.

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 grassland food chain: grass → grasshoppers → frogs → hawks. Grass absorbs light energy and produces biomass through photosynthesis. At each subsequent trophic level, less energy is available.

Model answer (5 marks)

1. Energy is lost because organisms use it for life processes such as respiration.
2. Energy is also lost as heat to the surroundings during metabolic processes.
3. Not all of the organism’s biomass is consumed – some material is not eaten and is lost in faeces or indigestible parts.
4. Because only 10 % of the energy is transferred, fewer organisms can be supported at each successive trophic level.
5. This means the food chain cannot support many trophic levels; the pyramids of biomass or numbers are wider at the base and narrow upwards.

Examiner tips

  • Use the exact phrases from the mark scheme – e.g. ‘respiration’, ‘heat’, ‘faeces’. Show the 10 % rule and link it to the narrowing of the pyramid. Keep each point short and to the point to fit the 5‑mark limit.

Common mistakes

  • Forgetting to mention heat loss or faeces. Using vague terms like ‘energy waste’ instead of specific processes. Not linking the 10 % transfer efficiency to the number of organisms at each level.

Mark scheme (5 marks)

  1. Energy is lost because organisms use energy for life processes such as respiration
  2. Energy is also lost as heat (to the surroundings) from metabolic processes
  3. Not all of the organism's biomass is consumed / some material is not eaten and is lost in faeces or indigestible parts
  4. Because only 10% of energy is transferred, fewer organisms can be supported at each successive trophic level
  5. This means the food chain cannot support many trophic levels / pyramids of biomass or numbers are wider at the base

Key terms in this question

trophic level · food chain

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