Productivity and energy transferEdexcel A-Level Biology A: Subtopic test

1. 1
   A forest ecologist studies a plot of tropical forest. Over one year the plants in the plot fix 18 000 kJ m⁻² year⁻¹ of light energy as chemical energy in organic molecules. In the same year plant respiration releases 11 700 kJ m⁻² year⁻¹ of this energy.

   (a)

   What is the net primary productivity of the plot, in kJ m⁻² year⁻¹?

   [1 mark]

   • A6 300
   • B11 700
   • C18 000
   • D29 700

   (b)

   Which statement about net primary productivity is correct?

   [1 mark]

   • AIt is always greater than the gross primary productivity
   • BIt is the total light energy absorbed by the chloroplasts
   • CIt is the energy released by the plants as heat in respiration
   • DIt is the chemical energy stored in plant biomass that is available to consumers and decomposers

   (c)

   Calculate the percentage of the gross primary productivity that is stored as net primary productivity, and explain why herbivores can only use the net primary productivity.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Energy flow through a grassland food chain was measured over one year. The net primary productivity of the grass was 12 000 kJ m⁻² year⁻¹. The energy stored in the biomass of the grasshoppers (primary consumers) that fed on the grass was 1 320 kJ m⁻² year⁻¹, and the energy stored in the biomass of the frogs (secondary consumers) that fed on the grasshoppers was 132 kJ m⁻² year⁻¹.

   (a)

   What is the percentage efficiency of energy transfer from the grass to the grasshoppers?

   [1 mark]

   • A9.1%
   • B11%
   • C0.11%
   • D89%

   (b)

   Which is a reason why not all of the energy in the grass is transferred to the grasshoppers?

   [1 mark]

   • AEnergy is destroyed when it passes between trophic levels
   • BGrasshoppers absorb all the energy they eat, so none is lost in faeces
   • CSome parts of the grass, such as the roots, are not eaten
   • DGrasshoppers do not respire

   (c)

   Calculate the percentage efficiency of energy transfer from the grasshoppers to the frogs.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A beef farmer keeps cattle in a grass field. Over one year the grass in the field has a net primary productivity of 60 000 kJ m⁻² year⁻¹. The cattle take in 24 000 kJ m⁻² year⁻¹ of this energy as food. Of the energy taken in, 7 200 kJ m⁻² year⁻¹ is lost in faeces and 12 600 kJ m⁻² year⁻¹ is lost through respiration and excretion. The rest is stored in the cattle as new biomass.

   (a)

   Calculate the percentage of the net primary productivity of the grass that is converted into new cattle biomass. Show your working.

   [3 marks]

   (b)

   Explain why only part of the energy taken in by the cattle becomes new biomass, and suggest one way in which the farmer could increase the efficiency of conversion of the grass energy into beef.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An ecologist compares two ecosystems, a tropical rainforest and a temperate grassland. In the rainforest the gross primary productivity is 36 000 kJ m⁻² year⁻¹ and plant respiration releases 25 200 kJ m⁻² year⁻¹. In the grassland the gross primary productivity is 12 500 kJ m⁻² year⁻¹ and plant respiration releases 5 000 kJ m⁻² year⁻¹. In both ecosystems assume that the net primary productivity is the energy available to the primary consumers, and that about 10% of the energy in each trophic level is transferred to the next.

   (a)

   Calculate the net primary productivity of each ecosystem. Explain why the net primary productivity is a different percentage of the gross primary productivity in the two ecosystems, and discuss which ecosystem makes more energy available to consumers.

   [6 marks]

   (b)

   Calculate the energy available to the secondary consumers in each ecosystem. Explain why the energy transferred falls at each trophic level, and evaluate which ecosystem could support more top predators.

   [6 marks]

   Total for question 4: 12 marks

End of questions