1. 1
   A grassland food chain is studied: grass → grasshopper → shrew → barn owl. Ecologists measure the energy stored in the tissues of each organism over one year.

   (a)

   Which trophic level does the grasshopper occupy?

   [1 mark]

   • AProducer
   • BPrimary consumer
   • CSecondary consumer
   • DTertiary consumer

   (b)

   The ecologists find that only a small proportion of the energy stored in the grasshoppers' tissues is eventually stored in the shrews' tissues. What is the main reason for this loss of energy between trophic levels?

   [1 mark]

   • AMost of the energy is used in respiration and lost as heat
   • BMost of the energy is lost as light during photosynthesis
   • CMost of the energy is absorbed directly from the soil
   • DMost of the energy is converted into oxygen

   (c)

   Explain why this food chain is unlikely to be extended by a fifth trophic level feeding on barn owls.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A pond ecosystem contains algae, water fleas and small fish. Algae are eaten by water fleas, which are eaten by small fish. Scientists draw a pyramid of biomass for this food chain.

   (a)

   What shape would this pyramid usually have?

   [1 mark]

   • AThe same width at every level
   • BNarrow at the base, widening towards the top
   • CWide at the base, narrowing towards the top
   • DWidest in the middle trophic level

   (b)

   The scientists find that the water flea population has an unusually high total biomass compared with the algae they feed on. Which of the following would best explain this unusual result?

   [1 mark]

   • ASmall fish transfer energy back into the water fleas
   • BWater fleas photosynthesise, adding extra biomass
   • CWater fleas do not lose any energy through respiration
   • DThe biomass of algae was measured at a time when the algae population was unusually low

   (c)

   The water fleas have a total biomass of 40 kg and receive 200 kg of biomass from algae per year. Calculate the percentage efficiency of biomass transfer from algae to water fleas.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A woodland food chain is: oak leaves → caterpillars → blue tits → sparrowhawks. A student suggests that if all sparrowhawks were removed from the woodland, the number of caterpillars would decrease.

   (a)

   Evaluate whether the student's suggestion is correct, explaining the chain of effects that would follow the removal of sparrowhawks.

   [3 marks]

   (b)

   Explain why only a small number of trophic levels, such as the four shown in this woodland food chain, can normally be supported in an ecosystem.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A marine food chain is: phytoplankton → zooplankton → herring → seals. Scientists have found that seal numbers have declined sharply over the last ten years, while herring numbers have increased.

   (a)

   Construct a detailed scientific explanation of how energy transfer between trophic levels limits the length of this food chain, and how this relates to the shape of a pyramid of biomass for the food chain.

   [6 marks]

   (b)

   Suggest and evaluate two possible explanations for why herring numbers have increased at the same time as seal numbers have declined in this marine ecosystem, using ideas about feeding relationships and energy transfer to support your explanations.

   [6 marks]

   Total for question 4: 12 marks

End of questions