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Practice · Biology · 2025 syllabus · D4.2

IB Biology D4.2 Stability and change: practice questions

5 exam-style questions on D4.2 Stability and change, SL and HL, 1 marked HL: 3 multiple choice, 1 data-based and 1 short answer. Each one comes with the mark scheme, a model answer and the reasoning behind every mark. Free with your email.

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D4.2-01 · Paper 1A style · multiple choice · 1 mark

Keystone species

What is a keystone species? [1]

  1. The species with the largest population and biomass in the community
  2. The first species to colonize bare ground at the start of a succession
  3. Any predator species that occupies the top trophic level of a food chain
  4. A species with an effect on its community far larger than its abundance suggests
Show the answer and the reasoning

Answer: D. D is correct. A keystone species has a disproportionately large effect on the structure of its community, so removing it can cause the ecosystem to collapse, even if the species itself is not common. A confuses importance with abundance. B describes a pioneer species. C is wrong because, although some top predators are keystone species, many keystone species are not predators (for example, some pollinators or ecosystem engineers).

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The 5 questions in D4.2

  1. D4.2-01 Keystone speciesPaper 1A style · multiple choice · 1 mark
  2. D4.2-02 Sequence of eutrophicationPaper 1A style · multiple choice · 1 mark
  3. D4.2-03 Arrested successionPaper 1A style · multiple choice · 1 mark · HL
  4. D4.2-04 Mercury in a lake food chainPaper 2 style · short answer · 6 marks
  5. D4.2-05 A river below farmlandPaper 1B style · data-based · 10 marks

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Every Biology and Chemistry question here is original, written for practice in the style of the papers set since the 2025 syllabus, with invented data. Developed independently from the International Baccalaureate Organization, which has not endorsed it. Found a mistake? Email vytis@limeadmissions.com with the question id.