Explain how the presence of a keystone species can affect the structure of a community, including the impact on other species and ecosystem processes.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A keystone species is one whose influence on its ecosystem is disproportionately large relative to its abundance.
Its removal can cause a cascade of changes: species that depend on it for food or habitat may decline or disappear, while competitors or predators may increase, altering the community’s species composition.
Because it regulates population sizes and interactions, the keystone species also shapes ecosystem processes such as nutrient cycling, primary productivity and energy flow.
A classic example is the sea otter in kelp‑forest ecosystems: otters keep sea urchin numbers low, allowing kelp to thrive and supporting a diverse community of fish and invertebrates.
Its removal can cause a cascade of changes: species that depend on it for food or habitat may decline or disappear, while competitors or predators may increase, altering the community’s species composition.
Because it regulates population sizes and interactions, the keystone species also shapes ecosystem processes such as nutrient cycling, primary productivity and energy flow.
A classic example is the sea otter in kelp‑forest ecosystems: otters keep sea urchin numbers low, allowing kelp to thrive and supporting a diverse community of fish and invertebrates.
Examiner tips
- Define keystone species first; show cause–effect chain; give a clear example; keep answer concise to fit 4 marks
Common mistakes
- Confusing keystone with dominant species; not linking removal to community changes; omitting an example
Mark scheme (4 marks)
- definition of a keystone species
- impact on other species in the community
- effect on ecosystem processes
- example or illustration of a keystone species in action