Explain why the net primary productivity (NPP) of an ecosystem is always less than the gross primary productivity (GPP), and outline how NPP is used to support consumers at higher trophic levels.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
GPP is the total rate of photosynthetic fixation of energy/carbon by producers in an ecosystem.
Producers use a portion of GPP for their own cellular respiration, releasing energy as heat; therefore NPP = GPP minus respiration losses.
NPP represents the net gain in organic biomass by producers per unit time and is therefore the energy/matter available to primary consumers.
Only a fraction of NPP passes to the next trophic level because energy is lost through consumer respiration, undigested material in faeces, and matter entering detritus/decomposers.
Producers use a portion of GPP for their own cellular respiration, releasing energy as heat; therefore NPP = GPP minus respiration losses.
NPP represents the net gain in organic biomass by producers per unit time and is therefore the energy/matter available to primary consumers.
Only a fraction of NPP passes to the next trophic level because energy is lost through consumer respiration, undigested material in faeces, and matter entering detritus/decomposers.
Examiner tips
- Use the exact terms GPP, NPP, cellular respiration, detritus; show the subtraction relationship; explain energy loss at each trophic step.
- Keep answer concise – 4 marks, so one sentence per point is enough.
Common mistakes
- Confusing GPP with NPP; writing that NPP is greater than GPP; omitting the role of respiration in producers or consumers.
Mark scheme (4 marks)
- GPP is the total rate of photosynthetic fixation of energy/carbon by producers in an ecosystem
- Producers use a portion of GPP in their own cellular respiration, releasing energy as heat, so NPP = GPP minus respiration losses
- NPP represents the net gain in organic biomass by producers per unit time and is therefore the energy/matter available to primary consumers
- Only a fraction of NPP passes to the next trophic level because energy is lost through consumer respiration, undigested material in faeces, and matter entering detritus/decomposers
Key terms in this question
gross primary productivity (GPP) · net primary productivity (NPP) · trophic level
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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