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D4.3.7—Coral reef threats

Coral reefs are threatened by warming-driven bleaching and carbon-dioxide-driven acidification that reduce symbiosis, calcification, habitat complexity, and marine biodiversity globally.

Syllabus
First assessment 2025
Objective
D4.3.7
Level
SL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceNovember 2024
Most common paperPaper3
Typical marks2–4

Common command terms

  • Describe
  • Suggest
  • Outline
  • Discuss
  • Deduce
  • Explain

Scoring notes

Common mistake
Treating bleaching as colour loss only, without explaining zooxanthellae expulsion and reduced nutrient supply.

Recent exam appearances

November 2024Paper3 ["SL"] · TZ213[ 2 ]D4.3.7—Coral reef threats
November 2023Paper2 ["SL"] · TZ17(a)[ 4 ]D4.3.7—Coral reef threats
May 2023Paper3 ["SL"] · TZ213(b)(ii)[ 2 ]D4.3.7—Coral reef threats
November 2018Paper3 ["SL"] · TZ015(b)[ 4 ]D4.3.7—Coral reef threats
Practice this objective

Coverage 2018–2024 · Updated 16 Jul 2026

Coral Reefs Face Heat and Carbonate-Chemistry Stress

Coral reefs are threatened when warming causes bleaching and altered seawater chemistry reduces calcification, while pollution and overfishing weaken recovery.

Heat disrupts coral–algal symbiosis; acidification lowers carbonate ion availability; local stressors reduce resilience and recruitment.

Separate direct heat stress, chemistry effects and local pressures before evaluating a reef outcome.; separate driver, mechanism, response and timescale

A marine heatwave expels symbiotic algae, bleaching coral; repeated heat before recovery raises mortality risk. This gives a concrete prediction from the stated climate condition.

Bleaching is a stress response, not immediate death; outcome depends on duration, species and recovery conditions. Interpret the result within the stated evidence and scenario limits.

Coral reef threats

Assessment in practice

1–4 marks
How it is assessed

This objective is assessed through structured response, commonly using Describe / Suggest / Outline.

Command terms

Describe / Suggest / Outline / Discuss / Deduce / Explain

What earns marks

Build the answer around this relationship: Coral bleaching occurs when heat stress disrupts the coral-zooxanthellae symbiosis.

Watch for

Treating bleaching as colour loss only, without explaining zooxanthellae expulsion and reduced nutrient supply.

Representative question

Question 1

[Maximum number: 5]

Outline the reasons that climate change is a threat to coral reefs.

Retrieve the SL Climate Chain

Core D4.3 is secure when every climate impact is explained as a chain: human greenhouse-gas sources or feedbacks change climate conditions, which alter habitats, oceans, carbon stores, or species distributions. Carbon sequestration is the mitigation chain that stores atmospheric CO2.

  • human gases and positive feedbacks amplify warming
  • boreal forests, ice habitats, upwelling and reefs shift through specific mechanisms
  • species may move poleward, upslope, contract, or lose ice/reef habitat
  • afforestation, agroforestry, regeneration and peatland rewetting store CO2

Climate Effects on Ecosystems

Core climate-change transfer answers should not list endangered examples. They should identify the climate driver, explain the physical or chemical mechanism, then state the biological consequence. Use this for greenhouse gases, feedbacks, boreal forests, ice-dependent species, upwelling, range shifts, reefs, and carbon sequestration.

  • Link human activities to increased greenhouse gases and enhanced warming.
  • Explain ecosystem impacts using mechanisms such as positive feedback, carbon sink/source shifts, habitat ice loss, reduced upwelling, range shifts, bleaching or acidification.
  • Explain carbon sequestration by naming the storage pathway in biomass, forests, soils or peatlands.

Concept essentials

  • Coral bleaching occurs when heat stress disrupts the coral-zooxanthellae symbiosis.
  • Ocean acidification lowers pH and reduces calcification in corals and other calcium-carbonate organisms.
  • Coral decline reduces habitat complexity and biodiversity across reef ecosystems.
ConceptIB Biology SL