Lesson focus: extinctionWhen a type of living thing dies out completely, so none are left anywhere. occurs when no living members of a species remain, usually after interacting pressures overwhelm survival, reproduction or recovery.
Explain causes of extinction, distinguish local from global loss and evaluate evidence and conservation responses.
Build on earlier learning
Natural selectionNatural selection can help populations track some changes. Extinction occurs when adaptation, movement and recovery do not keep pace with pressures.
You will be successful when you can…
- Distinguish local extinction from global extinction.
- Explain interacting causes.
- Relate small population size to risk.
- Evaluate conservation choices and evidence.
Words that carry the science
- extinction
- The global loss of a species when no living individuals remain.
- local extinction
- Loss of a species from one area while it survives elsewhere.
- biodiversity
- Variation among living organisms at genetic, species and ecosystem levels.
- habitat fragmentation
- Division of habitat into smaller, more isolated patches.
- genetic bottleneck
- A sharp population reduction that removes genetic variation by chance.
- viability
- The capacity of a population to persist and reproduce over time.
Connect interacting pressures to population viability
01Extinction usually has several interacting causes
Habitat loss, overexploitation, invasive species, pollution, disease, climate change and rare catastrophic events can reduce survival or reproduction. Pressures can reinforce one another: fragmented populations may be less able to recover from fire or disease.
local extinctionWhen a living thing disappears from one area but still lives in other places. can remove a species from one ecosystem without global extinctionWhen a type of living thing dies out completely, so none are left anywhere.. Fossil and historical records are incomplete, so dates and causes often carry uncertainty.
02Small populations face feedback risks
Small populations are vulnerable to chance fluctuations in births, deaths and sex ratio. Inbreeding can increase expression of harmful recessive alleles, while genetic drift can remove useful variation.
A bottleneck can leave low genetic diversity even if numbers later recover, reducing options for future evolutionary response.
03Conservation acts at several levels
Protecting and reconnecting habitat, reducing exploitation, controlling invasive species and restoring ecosystem processes address causes in the wild. Captive breeding, seed banks and cryopreservation can protect some genetic material.
Interventions have trade-offs, costs and uncertainty. Monitor abundance, reproduction, genetic diversity and habitat quality, and adapt management when evidence changes.
Extinction: habitat loss and hunting
A species becomes extinct when no living individuals are left. Several pressures often act together: habitat loss shrinks the number of animals an area can support, while hunting, pollution, disease or invasive species kill more of them. Very small populations struggle to recover.
Each dot is about 20 animals.
Controls
Numbers are illustrative. Conservation can help: protecting habitat, banning hunting, captive breeding and seed banks.
Build a causal extinction-risk chain
A forest specialist remains in three isolated patches after widespread clearing.
- Habitat area and resources have decreased.
- Fragmentation reduces movement and mating between patches.
- Small isolated populations face demographic chance, inbreeding and local disasters.
- A corridor and habitat restoration may improve movement, but monitoring is needed to test effectiveness.
How to make the reasoning visible: A strong explanation combines direct habitat loss with population-level feedbacks rather than naming one cause only.
Prioritise conservation evidence
Match each threat to a response and a measure of success.
| Threat evidence | Possible response | Success measure | Important caution |
|---|---|---|---|
| Isolated habitat patches | Restore habitat corridor | Movement and gene flow | Corridor quality matters |
| Unsustainable harvest | Enforce quotas or protection | Adult survival and recruitment | Displacement of harvest |
| Invasive predator | Targeted control | Native breeding success | Non-target effects |
| Low genetic diversity | Managed breeding or translocation | Heterozygosity and fitness | Outbreeding risks |
| Climate-driven range loss | Protect climate refugia and connectivity | Occupancy and reproduction | Future climate uncertainty |
Why should conservation monitor reproduction as well as adult numbers?
Why can connecting populations reduce risk?
Check your understanding
Answer from memory, then check against the suggested answer.
What is global extinction?
What is habitat fragmentation?
Why is low genetic diversity risky?
A species survives only in a zoo. Is it globally extinct?
Explain why de-extinction technology would not automatically restore a lost ecosystem role.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A proposal suggests conserving an endangered species only by captive breeding in zoos.
Evaluate captive breeding: give one advantage and two disadvantages.
(3)Evaluate relying on captive breeding alone, and give a conclusion.
(3)Total for this question: 6 marks
Command word: For evaluateA command word: weigh up the good points and bad points, then give your overall judgement., balance a real benefit against limitations, then give a justified combined strategy.