1.6.2Additional lessons
Reasons for Extinction — IB Biology SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 7 of 142
Part 1 of 4Extinction and Biodiversity
Extinction removes a species forever and lowers biodiversity.
Extinction
- Extinction A species is extinct when the last individual has died, so its alleles are lost permanently.
Biodiversity
- Biodiversity Variety of life at three levels: ecosystem diversity, species diversity (richness and evenness) and genetic diversity within a species.
Natural extinction
- Natural extinction Species have always disappeared as environments change or competitors arise, at a low background rate.
Anthropogenic extinction
- Anthropogenic extinction Extinction caused by human activity, now far above the background rate, which reduces biodiversity at every level.
Why it matters
- Why it matters Losing species can weaken ecosystem stability, remove ecosystem services and reduce genetic resources for the future.
Part 2 of 4Human Causes of Extinction
Most modern extinctions combine several pressures acting on a small population at the same time.
Human Causes of Extinction
Overexploitation
- Overexploitation Hunting, fishing or harvesting faster than a population can reproduce drives numbers down until recovery is impossible.
Habitat loss
- Habitat loss Clearing land for farming, housing or roads removes the places species need to feed, breed and shelter.
Invasive species
- Invasive species Introduced predators, competitors or pathogens can outcompete or eat native species that evolved without them.
Pollution and climate change
- Pollution and climate change Toxic chemicals and rapid environmental change can exceed the rate at which a species can adapt or migrate.
| Cause | Example | Effect on the species |
|---|---|---|
| Overexploitation | Hunting of passenger pigeons | Numbers fall faster than breeding can replace them |
| Habitat loss | Forest cleared for farming | Food and nesting sites disappear |
| Invasive species | Introduced rats on islands | Native birds and eggs are eaten |
Part 3 of 4Case Studies
1
Passenger pigeon: huge flocks were shot and trapped for food on a very large scale, and the last known bird died in 1914.
2
Caribbean monk seal: hunted for oil and skins and disturbed by fishing and coastal development, it was declared extinct in 2008.
3
North Island giant moa: large flightless birds hunted by people and affected by loss of forest habitat, with no survivors today.
4
A third case study of your own choice should show a clear human cause, such as the baiji river dolphin affected by fishing and river traffic.
5
For each case, record the cause, the date or period, and the ecological or genetic consequence of losing the species.
Part 4 of 4Exam Checklist & Tips
1.6.2 – Key Points for the Exam
- Extinction The permanent loss of a species when no individuals remain.
- Anthropogenic Caused by human activities.
- Overexploitation Harvesting a species faster than it can replace itself.
- Invasive species A non-native species that spreads and harms native species or ecosystems.
- Genetic diversity The range of alleles within a species or population.
Common Mistakes
- Writing "habitat destruction" without stating how it removes food, shelter or breeding sites for the species.
- Treating all extinctions as human-caused, when natural extinctions also occur at a low background rate.
- Giving a case study with no date, cause or consequence, so no marks are earned for detail.
- Confusing extinct with endangered; endangered species still have living individuals.
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