QUESTION 128 (5 marks)
Producer productivity is measured over equal periods in two seasons. Grazers feed on these producers; other limitations on grazers are not measured.
| Season | GPP | Producer respiration |
|---|---|---|
| Cool | 6000 | 2000 |
| Warm | 8000 | 5000 |
a) Calculate NPP in each season. [2 marks]
b) Explain why the season with higher GPP need not supply more newly produced biomass energy to grazers. [2 marks]
c) State why the productivity data alone cannot predict an exact grazer population size. [1 mark]
Practice marking scheme
Answer
Cool-season NPP is 4000; warm-season NPP is 3000 kJ m⁻² per period. Higher warm-season GPP does not imply more producer biomass energy available to grazers because respiration is also higher.
Working
Cool: 6000 − 2000 = 4000. Warm: 8000 − 5000 = 3000. Gross fixation rises but the respiratory increase is larger, so net production falls. NPP describes newly stored producer biomass energy; actual grazer abundance also depends on consumption, other resources, predation and other conditions.
Marking criteria
- Calculates cool NPP 4000. [1 mark]
- Calculates warm NPP 3000. [1 mark]
- Identifies greater respiratory loss in the warm season. [1 mark]
- Explains why net production is lower despite higher gross fixation. [1 mark]
- Names an unmeasured limiting factor or explains that transfer and population response are not supplied. [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.