A food web in a grassland ecosystem contains the following organisms: grass, grasshoppers, frogs, and hawks. Describe the roles of the grass and the grasshoppers in this food web, and explain why a pyramid of biomass for this food web would be wider at the base than at the top.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In a grassland ecosystem, grass is eaten by grasshoppers, grasshoppers are eaten by frogs, and frogs are eaten by hawks.
Model answer (4 marks)
Grass is the producer, a photosynthetic organism that supplies biomass for all other organisms in the web.
Grasshoppers are primary consumers, herbivores that feed on the grass.
Only about 10 % of the energy or biomass is transferred from one trophic level to the next.
Because of this low transfer efficiency, the dry mass of living material falls from the producer to the tertiary consumer, so the pyramid of biomass is widest at the base (grass) and narrowest at the top (hawks).
Grasshoppers are primary consumers, herbivores that feed on the grass.
Only about 10 % of the energy or biomass is transferred from one trophic level to the next.
Because of this low transfer efficiency, the dry mass of living material falls from the producer to the tertiary consumer, so the pyramid of biomass is widest at the base (grass) and narrowest at the top (hawks).
Examiner tips
- Use the exact terms ‘producer’ and ‘primary consumer’. Show the 10 % transfer efficiency to justify the shape of the pyramid. Keep the answer concise and directly linked to the marks.
Common mistakes
- Confusing producers with consumers. Omitting the 10 % transfer efficiency. Writing a long paragraph instead of concise points.
Mark scheme (4 marks)
- Grass is the producer / photosynthetic organism that provides biomass for all living things / is at the start of the food chain (trophic level 1).
- Grasshoppers are primary consumers / herbivores at trophic level 2 that feed on the producer.
- Only around 10% of the energy / biomass is transferred from one trophic level to the next.
- Therefore the dry mass of living material at each trophic level decreases from producer to tertiary consumer, so the pyramid is widest at the base (grass) and narrowest at the top (hawks).
Key terms in this question
Related
- All Pearson Edexcel International GCSE Biology (4BI1) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →