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Biology · Topic

Ecology, food chains and energy transfer

Energy is lost through respiration and movement, so chains are short — say what happens, not just "less energy".

2 original questions in the bank, worth 7 marks · above the two-per-topic floor

Drill this topic All ecology, food chains and energy transfer questions

The questions, with the examiner reasoning

Explain3 marksIGCSE

Explain why a food chain rarely contains more than five trophic levels.

Show the mark scheme and the examiner note

Answer: Only about a tenth of the energy transfers at each level; the rest is lost in respiration, movement and undigested material, so too little energy remains to support another level.

  1. Energy is lost at each transfer
  2. Losses: respiration, movement, heat and egestion (undigested material)
  3. So each level receives around 10%, and the energy runs out

Examiner technique: Name the losses rather than saying "energy is lost". The named losses are what the mark scheme looks for, and it is the same three every year.

Compare4 marksBoth boards

A farmer ploughs a field and leaves the bare soil exposed for three weeks before planting. Compare the soil conditions on the ploughed field with those under the undisturbed grassland beside it, and explain why the ploughed field loses more topsoil.

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Answer: Ploughed soil is loose, has little plant cover and dries out; the grassland has roots binding particles and a litter layer. Loose bare soil is lifted by wind and washed by rain, so both wind and water erosion remove more topsoil.

  1. Ploughed soil is loose with no plant cover; grassland soil is bound by roots (1 mark)
  2. Bare soil dries and its particles are easily moved; litter and roots hold moisture and particles (1 mark)
  3. Names a mechanism: wind lifting loose particles, or rain washing them downhill (1 mark)
  4. Links the mechanism to the removal of the nutrient-rich topsoil (1 mark)

Examiner technique: In compare questions, keep the two sides in the same sentence. A structure of "Field: ... Grassland: ..." repeated beats a long paragraph about one and a short afterthought about the other.