Productivity and energy transferEdexcel International A Level Biology: Subtopic test

1. 1
   In a grassland ecosystem the gross primary productivity (GPP) of the plants is 14 500 kJ m⁻² year⁻¹. The plants lose 6 000 kJ m⁻² year⁻¹ as a result of respiration (R).

   (a)

   Calculate the net primary productivity (NPP) of the grassland in kJ m⁻² year⁻¹.

   [1 mark]

   • A20 500
   • B14 500
   • C8 500
   • D6 000

   (b)

   Which statement describes the net primary productivity of a community?

   [1 mark]

   • AThe energy stored in the biomass of the producers and available to the next trophic level
   • BAll the light energy that reaches the leaves of the producers
   • CThe total energy fixed by the producers in photosynthesis
   • DThe energy lost as heat by the producers in respiration

   (c)

   Calculate the percentage of the GPP that is lost in respiration. Show your working.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In a food chain in a freshwater ecosystem, the net primary productivity of the producers is 12 000 kJ m⁻² year⁻¹. The energy stored in the biomass of the primary consumers (zooplankton) is 1 080 kJ m⁻² year⁻¹ and the energy stored in the biomass of the secondary consumers (small fish) is 130 kJ m⁻² year⁻¹.

   (a)

   Calculate the efficiency of energy transfer from the producers to the primary consumers.

   [1 mark]

   • A11.1
   • B1.1
   • C90
   • D9.0

   (b)

   Which statement is a reason why the energy in the primary consumers is less than the NPP of the producers?

   [1 mark]

   • APrimary consumers do not respire
   • BSome of the producer biomass is not eaten
   • CEnergy is destroyed as it passes along a food chain
   • DProducers do not store energy in their biomass

   (c)

   The efficiency of transfer from the primary to the secondary consumers is 12.0%, which is greater than the efficiency from the producers to the primary consumers. Suggest why.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Over one year a field of maize receives 30 000 kJ m⁻² of light energy. The maize fixes 600 kJ m⁻² in gross primary productivity and loses 200 kJ m⁻² in respiration.

   (a)

   Calculate the net primary productivity of the maize and the percentage of the light energy that is converted to net primary productivity. Show your working.

   [3 marks]

   (b)

   Suggest why such a small percentage of the light energy is converted to biomass by the maize.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A farmer has one hectare of land. If it is used to grow wheat, the grain provides 60 GJ of edible energy per year. If it is used instead to grow grass with the same net primary productivity of 60 GJ per year, and the grass is eaten by cattle, 10% of the energy in the grass is transferred to the biomass of the cattle and 70% of the cattle biomass can be eaten by people.

   (a)

   Explain why the efficiency of energy transfer between trophic levels is low, and why food chains rarely have more than four or five trophic levels.

   [6 marks]

   (b)

   Evaluate the suggestion that growing wheat for people to eat, rather than feeding grass to cattle, would feed more people from the same area of land.

   [6 marks]

   Total for question 4: 12 marks

End of questions