The big idea: Human population growth is the overarching cause: more people need more food, land, water and materials, and that drives every specific cause of biodiversity loss.
The points to remember
- The human population has grown very fast, especially in the last 100 years.
- More people need more food, land, water, energy and materials.
- That demand drives the specific causes: over-exploitation, urbanization, land clearance, pollution and invasive species.
- So population growth is the overarching cause: the cause behind the others.
Remember it as: More people, more pressure.
Real example: 8 billion people: The human population passed 8 billion in 2022, more than three times the number in 1950. Each extra person needs food, water, space and materials.
Link the levels correctly:
Population growth works through the others: Population growth does not kill species directly. Explain which specific cause it drives, such as clearing land for food.
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The big idea: People take too many wild animals and plants, cover land with towns, and clear forests for farms, destroying natural habitat.
The points to remember
- Over-exploitation: hunting, fishing or harvesting a species faster than it can reproduce.
- Urbanization: towns, cities and roads spread over natural habitat.
- Deforestation and land clearance for agriculture remove whole habitats.
- Loss of natural habitat is the biggest single cause: every species living there is affected.
- Reclaiming or restoring degraded land is not a cause: it helps biodiversity.
Causes of loss
- Towns and cities spreading
- Land cleared for farms
- Species over-harvested
Not causes
- Reclaiming degraded land
- Setting up nature reserves
- Rewilding farmland
Real example: Atlantic cod: Off Newfoundland in Canada, cod were caught faster than they could breed for decades. In 1992 the stock collapsed and the fishery had to be closed.
Read the options carefully:
Restoration is not a cause: Reclamation of a degraded ecosystem restores habitat. In a list of causes, it is the odd one out.
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The big idea: Pollution harms species where they live, and global transport carries pests, diseases and invasive alien species to places they never reached before.
The points to remember
- Pollution: pesticides, fertilizer run-off, plastics, oil and gases from burning fossil fuels.
- Global transport: ships, planes and trade carry species around the world.
- An invasive alien species has no natural predators in its new home, so its numbers grow fast.
- Invasive species eat, outcompete or infect native species, and break food chains.
- New pests and diseases can kill native species that have no resistance to them.
Real example: chytrid fungus: A fungus that infects the skin of frogs and toads was spread around the world by the trade in amphibians. It has caused declines in hundreds of amphibian species.
Name the cause exactly:
Not just 'pollution': 'Pollution' on its own is too vague for a mark. Say which pollution, such as pesticides from farms or carbon dioxide from burning fossil fuels.
How it is tested: Paper 1A: which listed activities cause biodiversity loss. Paper 2: state or outline a cause, or discuss the impact of an invasive species.
IB-style questionOutline[2 marks]
Outline how human population growth is an overarching cause of the biodiversity crisis.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
Be specific: Name the cause exactly: 'nitrate run-off from farms', not just 'pollution'; 'over-fishing of cod', not just 'humans'.