Explain how keystone species contribute to maintaining biodiversity within an ecosystem, and outline why their removal can trigger a trophic cascade.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A keystone species has a disproportionately large effect on ecosystem structure relative to its abundance/biomass.
Keystone species regulate prey or competitor populations, preventing any one species from dominating and thereby maintaining species richness and diversity.
If a keystone species is removed, populations at lower trophic levels increase unchecked, leading to overexploitation of resources at the next trophic level down.
This trophic cascade causes loss of species or a reduction in biodiversity because habitat structure or resource availability is drastically altered.
Keystone species regulate prey or competitor populations, preventing any one species from dominating and thereby maintaining species richness and diversity.
If a keystone species is removed, populations at lower trophic levels increase unchecked, leading to overexploitation of resources at the next trophic level down.
This trophic cascade causes loss of species or a reduction in biodiversity because habitat structure or resource availability is drastically altered.
Examiner tips
- Use the term "keystone species" and explain its disproportionate effect. Show the regulatory role on prey/competitors. Explain the sequence of a trophic cascade. Mention loss of biodiversity as the final outcome.
Common mistakes
- Confusing a keystone species with a keystone predator. Failing to link the cascade to biodiversity loss. Using vague terms like "important" instead of "disproportionately large effect."
Mark scheme (4 marks)
- A keystone species has a disproportionately large effect on ecosystem structure relative to its abundance/biomass.
- Keystone species regulate prey/competitor populations, preventing any one species from dominating, which maintains species richness/diversity.
- Removal of a keystone species leads to a trophic cascade — populations at lower trophic levels increase unchecked, causing overexploitation of resources at the next trophic level down.
- The trophic cascade results in the loss of species / reduction in biodiversity because habitat structure or resource availability is drastically altered.
Key terms in this question
keystone species · trophic cascade · biodiversity
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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