Productivity and energy transferEdexcel A-Level Biology A: Mind map

Exam questions on Productivity and energy transfer

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.
   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
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.
   Calculate the percentage efficiency of energy transfer from the grasshoppers to the frogs.2 marks
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.
   Calculate the percentage of the net primary productivity of the grass that is converted into new cattle biomass. Show your working.3 marks