The big idea: Biodiversity — the variety of living things — is falling fast, and human activity is the main cause.
When a species disappears completely it has become extinct — gone forever.
Almost every current threat traces back to five human-driven causes: habitat loss, overexploitation, pollution, invasive species and climate change.
| Human cause | What it means | Example |
|---|---|---|
| Habitat loss | Natural homes are cleared, drained or built over | Cutting down rainforest for farmland |
| Overexploitation | Species taken from the wild faster than they breed | Overfishing a fish stock to collapse |
| Pollution | Harmful substances added to air, water or soil | Pesticides and plastic harming wildlife |
| Invasive species | A non-native species spreads and out-competes natives | Rats eating the eggs of ground-nesting birds |
| Climate change | Human-driven warming shifts conditions faster than species can adapt | Coral bleaching as the sea warms |
The five human drivers all push biodiversity down. Habitat loss is drawn largest because it destroys the most — but overexploitation, pollution, invasive species and climate change all add to the falling trend (memory hook: HIPPO).
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Two words to keep straight: Extinction = the last member of a species dies, so the species is lost permanently.
Biodiversity loss = the overall variety of species is falling — it includes extinctions but also shrinking populations before any species is fully gone.
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Extinctions have happened naturally throughout Earth's history, but today the rate is far higher and is driven by people.
Learn the five causes below — each is a favourite source of exam marks. A handy memory hook is HIPPO (Habitat loss, Invasive species, Pollution, Population/overexploitation, and a changing climate).
- Biodiversity
- The variety of living organisms — the number of different species and the variety within them.
- Extinction
- The permanent loss of a species when its very last member dies.
- Habitat loss
- Destruction or fragmentation of the natural place where a species lives (e.g. deforestation, draining wetlands).
- Overexploitation
- Harvesting or hunting a species faster than it can reproduce, so its population crashes.
- Invasive species
- A non-native species, introduced by humans, that spreads and harms native species by competing with, eating or infecting them.
Habitat loss is the biggest driver: Of the five, habitat loss destroys the most biodiversity. When a forest, reef or wetland is cleared, every species that depends on it loses its home at once — not just one target species.
Invasive species are especially damaging on islands, where native species often have no defences against a brand-new predator or competitor.
Direct causes
- Overexploitation — hunting or fishing a species to collapse
- Invasive species — a new predator eating natives directly
- Fast and obvious — one species is targeted
Indirect causes
- Habitat loss — removing the place a species needs to live
- Pollution and climate change — changing the conditions
- Slower and wider — many species harmed at once
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How this is tested: On Paper 1A (multiple choice) you are asked to identify the human cause behind a named extinction, or to pick which listed factors cause biodiversity loss.
On Paper 2 a short List or Suggest question can ask you to name human activities that threaten a group such as pollinators.
On Paper 3 an extended Discuss question can ask you to work through the impacts of an invasive species in a new ecosystem.
IB-style question — an invasive cane toad
Cane toads were released into a region of Australia to control beetles, but they spread rapidly. The toads are poisonous and have no natural predators there. Discuss the likely impacts of the cane toad on the region's biodiversity. [3]
How to build a 3-mark answer
- Identify the cause. The cane toad is an invasive (non-native) species introduced by humans.
- Give a direct impact. Native predators that eat the toad are poisoned and die, so their populations fall.
- Give a knock-on impact. With no natural predators the toad's numbers explode; it competes with and preys on native species, reducing their populations — so overall biodiversity falls.
Final answer
The cane toad is an invasive species: it poisons native predators that try to eat it, and with no predators of its own it multiplies and out-competes or eats native species, lowering biodiversity.
Why this scores: A Discuss answer needs named impacts with reasoning, not just 'it is bad'. This answer identifies the cause (invasive species), gives a direct effect (poisoning predators) and a knock-on effect (population explosion → out-competing natives → lower biodiversity).