IB Biology SL 4.2.5 Keystone Species
Practise identifying keystone species and explaining how their presence or loss changes community structure and biodiversity.
- Syllabus
- First assessment 2025
- Course
- Biology SL
- Level
- SL
Exam points
- Identify a keystone species from its disproportionate ecological effect.
- Explain how removing a keystone species changes community structure and biodiversity.
D4.2.5—Keystone species question 1
The sea snail Nucella ostrina and the sea star Pisaster ochraceus are predators of the mussel Mytilus trossulus. The mussels live on rocks at the edge of the sea and feed on phytoplankton and zooplankton. The zooplankton feed on the phytoplankton.
Nucella ostrina
Pisaster ochraceus
Mytilus trossulus
Groups of 50 mussels were transplanted to an experimental area and protected from predation until the start of the experiment. Researchers then investigated the effect of the predators on the population of the mussels over a period of 60 days.
Sea stars along the pacific coast of the US are dying from "sea star wasting syndrome". Suggest the effect this will have on the ecosystem.
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