QUESTION 127 (5 marks)
A grassland has annual producer GPP of 25 000 kJ m⁻² and producer respiration of 9000 kJ m⁻². Two consumer groups feed directly on separate portions of producer material: grazers on living leaves and detritivores on dead leaves. They do not feed on one another. Their annual new biomass contains 700 and 1300 kJ m⁻² respectively.
a) Calculate annual producer NPP. [1 mark]
b) Calculate the percentage of NPP stored in the combined new biomass of the two consumer groups. [2 marks]
c) Explain why omitting dead leaves from an energy-flow model would omit a source of energy for these consumers. [2 marks]
Practice marking scheme
Answer
Producer NPP is 16 000 kJ m⁻² year⁻¹. The combined new consumer biomass is 2000, equivalent to 12.5% of producer NPP. Dead leaves can transfer chemical energy to detritivores.
Working
NPP = 25 000 − 9000 = 16 000. Because the two groups obtain energy directly from different producer portions, their production can be added without counting a consumer-to-consumer transfer twice. Combined transfer fraction = (700+1300)/16 000×100 = 12.5%. Energy stored in dead organic matter is available to detrital consumers; carbon and nutrients may later be recycled while energy is dissipated as heat.
Marking criteria
- Calculates NPP 16 000 kJ m⁻² year⁻¹. [1 mark]
- Adds consumer production to obtain 2000. [1 mark]
- Calculates combined fraction 12.5%. [1 mark]
- States that dead leaves retain chemical energy in organic matter. [1 mark]
- Links detritivore feeding to transfer of that energy into the detrital food chain. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.