1.9.2Inquiry Process
Reasons for Extinction — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 33 of 160
Part 1 of 5Human Causes
Current extinction rates are far higher than the natural background rate, mostly because of human activity.
Habitat destruction
- Clearing forests, draining wetlands and building cities removes the places where species live and feed.
Overexploitation
- Hunting, fishing or collecting faster than populations can recover led to the loss of the passenger pigeon.
Invasive species
- Introduced predators, competitors and diseases can wipe out native species that have no defences against them.
Pollution
- Pesticides, plastics and nutrient runoff poison organisms or change habitats so that sensitive species disappear.
Climate change
- Rapid warming shifts habitats faster than many species can move or adapt.
Part 2 of 5Vulnerable Species
Vulnerable Species
Small range
- Species living in a single island or valley can be eliminated by a single event.
Small population
- Few individuals mean low genetic diversity and a high chance of dying out by chance.
Specialised niche
- Species that depend on one food or habitat cannot switch when it disappears.
Slow reproduction
- Large animals with few offspring cannot recover quickly from losses.
| Species | Main cause | Outcome |
|---|---|---|
| Passenger pigeon | Hunting | Extinct, last died 1914 |
| Dodo | Hunting and invasive species | Extinct, 17th century |
| Golden toad | Disease and climate | Not seen since 1989 |
Part 3 of 5Natural Causes
1
Extinction has always occurred naturally, at a steady background rate of a small number of species per year.
2
Mass extinctions, such as the one about 66 million years ago, wiped out many groups after major environmental change.
3
Causes include asteroid impact, volcanic activity and long-term climate shifts.
4
Many scientists think we are now in a sixth mass extinction driven by human activity.
5
Unlike earlier events, the present loss is caused by one species and could in principle be reduced.
Part 4 of 5Worked Example
Question
- The golden toad of Costa Rica was last seen in 1989, while the dodo disappeared within about 200 years of human arrival on Mauritius.
- Suggest a main cause for each and explain why small ranges increase risk.
- The dodo was flightless and nested on the ground, so hunting and introduced rats, pigs and monkeys that ate its eggs reduced it.
- The golden toad was confined to a small cloud-forest area, and a fungal disease together with climate change affected its habitat.
- A species with a tiny range has all its individuals exposed to one event, so a single change can eliminate it.
Answer
- Dodo: overexploitation and invasive species.
- Golden toad: disease and climate change, made worse by a very small range.
Part 5 of 5Exam Checklist & Tips
1.9.2 – Key Points for the Exam
- Extinction The death of every member of a species.
- Background rate The normal natural rate of extinction.
- Overexploitation Harvesting at a rate above the recovery rate.
- Invasive species A non-native species that spreads and harms ecosystems.
Common Mistakes
- Saying extinction is always caused by humans; natural extinctions also occur.
- Treating habitat loss as a minor factor; it is usually the main one.
- Thinking large populations cannot go extinct.
- Forgetting that invasive species act as predators, competitors or disease carriers.
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