A farmer converts a large area of species-rich grassland into a monoculture crop field, growing only wheat. Explain how this change reduces biodiversity and suggest why a reduction in biodiversity in this area could affect the stability of the local ecosystem.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Biodiversity refers to the variety of living organisms in an area. Ecosystems with high biodiversity tend to be more stable and resilient than those with low biodiversity.
Model answer (5 marks)
1. Converting the grassland to a wheat monoculture removes many plant species, so the number of plant types in the area falls sharply.
2. With fewer plant species, the variety of habitats and food sources for invertebrates, insects and birds is reduced, causing many of these animals to disappear.
3. The farmer will normally use pesticides and herbicides to keep weeds out; these chemicals kill non‑crop organisms and further lower biodiversity.
4. In a low‑biodiversity ecosystem there are fewer species in each food web, so the system is less able to absorb disturbances.
5. If a species is lost in such a system, there are few or no other species that can replace its ecological role, which can trigger further losses and even collapse of the ecosystem.
2. With fewer plant species, the variety of habitats and food sources for invertebrates, insects and birds is reduced, causing many of these animals to disappear.
3. The farmer will normally use pesticides and herbicides to keep weeds out; these chemicals kill non‑crop organisms and further lower biodiversity.
4. In a low‑biodiversity ecosystem there are fewer species in each food web, so the system is less able to absorb disturbances.
5. If a species is lost in such a system, there are few or no other species that can replace its ecological role, which can trigger further losses and even collapse of the ecosystem.
Examiner tips
- Use the exact terms ‘monoculture’, ‘biodiversity’, ‘food web’, ‘stability’.
- Show the causal chain: loss of plant species → loss of animal species → reduced resilience.
- Mention pesticides as a direct cause of further loss.
- Keep each point concise and focused on the marks given.
Common mistakes
- Confusing ‘biodiversity’ with ‘species richness’ only, ignoring the loss of habitats.
- Failing to mention pesticides/herbicides as a cause of further loss.
- Over‑generalising that any loss will collapse the ecosystem without explaining the role of alternative species.
Mark scheme (5 marks)
- Converting grassland to a monoculture reduces the variety of plant species present
- Fewer plant species means fewer habitats / food sources available, so animal species (invertebrates, insects, birds) are also lost
- Use of pesticides / herbicides to maintain the monoculture further kills non-crop species, reducing biodiversity
- In an ecosystem with low biodiversity, there are fewer species in food webs / food chains, so the ecosystem is less stable
- If one species is lost from a low-biodiversity ecosystem, there are fewer alternative species to fulfil that role, so other species may also be lost / the ecosystem may collapse
Key terms in this question
biodiversity · monoculture · ecosystem · stability
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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