Explain causes of extinction
Link climate change, competition, hunting and habitat loss to population decline and extinction.
Section 18.3 · Conservation
A Level; Cambridge International Biology 9700, 2028–2030. Written responses are self-checked; this does not certify supervised fieldwork or practical competence.
Before you begin
- Classification compares shared characteristics; populations occupy environments and contain variation.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How can habitat loss increase extinction risk?
What you will learn
- Explain how four stated pressures can reduce survival or reproduction
- Connect population decline to extinction risk
Build the explanation
Climate change can alter temperature, rainfall, seasons or other conditions beyond a population’s tolerance. Competition can reduce access to food, space or mates. Hunting directly removes individuals, while habitat degradation or loss removes resources and breeding sites. These pressures can lower survival and reproductive success; small isolated populations may then be more vulnerable.
Use evidence within scope
A threat does not guarantee extinction. Explain the pathway from the stated pressure to survival, reproduction or population size, and consider how severity, duration and species adaptation affect the outcome.
Pause and explain
A species’ habitat is degraded but some individuals persist. Why is extinction not yet certain?
Worked example
A wetland is drained and a specialist frog loses its breeding ponds. Explain the extinction risk.
Show the worked solution
- Habitat and breeding sites are lost.
- Recruitment falls and the population may decline.
Answer Loss of the wetland removes breeding habitat, reducing successful reproduction and recruitment; if decline continues and no suitable habitat remains, extinction risk increases.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain how intensive hunting could cause local extinction.
Give me a hint
Identify the required evidence and sequence the reasoning.
Compare my reasoning
- Mortality rises as individuals are removed.
- The population may fall below replacement.
If hunting removes individuals faster than the population can replace them, abundance can decline until no local individuals remain.
Look for these in your work
- Relevant evidence selected
- Reasoning linked to conclusion
A competitor arrives after a climate shift. Why might the native population be affected by both factors?
Give me a hint
Use the supplied example or data to justify a conclusion.
Compare my reasoning
- Climate may alter suitability or resources.
- Competition can further reduce resource access.
Climate change can reduce suitable conditions or alter resources; competition may then further reduce access to food or habitat, lowering survival and reproduction.
Look for these in your work
- Concept applied
- Limit or assumption stated
A species’ habitat is degraded but some individuals persist. Why is extinction not yet certain?
Give me a hint
Apply the concept in a new context and qualify its limits.
Compare my reasoning
- Persistence shows some individuals survive.
- Outcome depends on continued reproduction and future conditions.
The population may recover if conditions and reproduction remain sufficient; risk depends on how persistent the degradation is and whether enough suitable habitat remains.
Look for these in your work
- New context handled
- Conclusion appropriately bounded
Common mistakes
- Listing a threat without explaining its effect
- Saying every introduced competitor causes extinction
What can you explain now?
How can habitat loss increase extinction risk?
Compare with the explanation
It removes resources or breeding habitat
Survival or successful reproduction can fall.
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Make it stick
How do the four named pressures reduce population survival or reproduction?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.