Energy transfer between trophic levelsAQA A-Level Biology: Subtopic test

1. 1
   A dairy cow grazes on a pasture. During one day, the chemical energy store in the grass she ingests is 250 MJ. She loses 90 MJ in faeces and urine and releases 110 MJ in respiration.

   (a)

   In the equation N = I − (F + R), what does F represent?

   [1 mark]

   • AThe chemical energy fixed by photosynthesis in the grass
   • BThe energy released as heat in respiration
   • CThe energy stored in the cow's new biomass
   • DThe chemical energy lost to the environment in faeces and urine

   (b)

   What does N, the net production of the cow, represent?

   [1 mark]

   • AThe total energy in the grass she ingests
   • BThe energy lost from her body as heat
   • CThe energy stored in her new biomass, available for her growth and reproduction and to the next trophic level
   • DThe energy lost in faeces and urine

   (c)

   Calculate the net production of the cow in MJ per day and express it as a percentage of the energy she ingests.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   An ecologist studies a food chain in an estuary. The net primary production of the phytoplankton is 20 000 kJ m⁻² year⁻¹. The net production of the zooplankton that feed on them is 2 400 kJ m⁻² year⁻¹, and the net production of the small fish that feed on the zooplankton is 360 kJ m⁻² year⁻¹.

   (a)

   What is the efficiency of energy transfer from the phytoplankton to the zooplankton?

   [1 mark]

   • A8.3 %
   • B12 %
   • C15 %
   • D88 %

   (b)

   Which statement correctly explains why energy transfer between trophic levels is less than 100 % efficient?

   [1 mark]

   • ASome energy is lost as heat in respiration
   • BPhotosynthesis in the phytoplankton is only 1 % efficient
   • CEnergy is destroyed when it is passed to the next level
   • DDecomposers add energy to the food chain

   (c)

   Calculate the efficiency of energy transfer from the zooplankton to the small fish.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In an oak woodland the net primary production of the trees is 14 000 kJ m⁻² year⁻¹. Herbivorous insects consume 1 400 kJ m⁻² year⁻¹ of this. Most of the rest falls to the ground as leaf litter and dead wood, and a small amount is stored as new wood in the living trees.

   (a)

   Calculate the percentage of the trees' net primary production that is consumed by herbivorous insects, and suggest two reasons why most of the net primary production is not consumed by them.

   [3 marks]

   (b)

   Explain what happens to the chemical energy in the leaf litter and dead wood, referring to bacteria and fungi.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An ecologist compares the energy budgets of two consumers feeding on the same grassland over one year. The field vole is a small mammal that keeps its body temperature constant; the grasshopper is an insect whose body temperature varies with its surroundings. For each, per m² per year: energy ingested (I), lost in faeces and urine (F) and lost in respiration (R). Vole: I = 400 kJ, F = 120 kJ, R = 270 kJ. Grasshopper: I = 400 kJ, F = 150 kJ, R = 100 kJ.

   (a)

   Explain how the net production of the vole is calculated, and explain why the transfer of energy from the grass to the vole is not 100 % efficient.

   [6 marks]

   (b)

   The grasshopper converts a greater percentage of the energy it ingests into biomass than the vole. Use the data to explain this, and evaluate whether the data show that insects are always a more efficient source of energy for predators than small mammals.

   [6 marks]

   Total for question 4: 12 marks

End of questions