Key Idea: Biodiversity is the variety of life — species, genes and ecosystems. It is being lost fast because of human activity, so this topic is about why it matters and how we conserve it. Everything here links to two simple questions: what threatens biodiversity, and what can we do about it (protect species where they live, breed them elsewhere, repair whole ecosystems, and decide what to save first). It shows up on Paper 1A as quick 'identify the cause / identify the conservation action' MCQs, and on Paper 2 and Paper 3 as longer Suggest / Outline / Discuss questions that want named methods with reasoning.
🌍 Why biodiversity matters & why it is being lost
Biodiversity is the variety of life at three levels: species diversity (how many different species), genetic diversity (variety of genes within a species) and ecosystem diversity (variety of habitats). It matters because more species makes an ecosystem more stable and resilient — if one species declines, others can take over its role, so food webs keep working. Biodiversity also gives humans food, medicines and materials. Since around 1970 wild populations have fallen sharply, driven by human activity — a rate of loss high enough to be called a biodiversity crisis.
Species (number of different species) · Genetic (variety within a species) · Ecosystem (variety of habitats). The stability / resilience point is the safest answer for why biodiversity matters.
🪓 Causes of biodiversity loss & extinction
Extinction is when the last member of a species dies (the species is gone forever); biodiversity loss is the wider fall in variety. Extinctions happen naturally, but today's rate is far higher because of five main human causes — and habitat loss destroys the most biodiversity of all.
| Human cause | What happens | Example |
|---|---|---|
| Habitat loss | natural habitat is destroyed or fragmented | clearing forest for farmland (biggest driver) |
| Overexploitation | species harvested faster than they can replace themselves | overfishing, overhunting |
| Pollution | harmful substances damage or kill organisms | pesticide spraying harming bees |
| Invasive species | introduced species out-compete or prey on natives | cane toads in Australia |
| Climate change | shifting temperatures disrupt habitats and timing | flowering out of step with pollinators |
Of the five, habitat loss wipes out the most biodiversity — when a forest or reef is destroyed, every species that lived there is lost at once.
🏞️ In situ conservation
In situ means 'in place' — protecting a species in its natural habitat, usually with nature reserves and national parks. Its big advantage is that it conserves the species together with its whole ecosystem, keeping its natural behaviour intact, and it is usually cheaper per individual than ex situ. Reserves need active management and good design: a large, rounded reserve has a big sheltered interior and little exposed edge, and wildlife corridors between reserves let animals move, interbreed and recolonise.
| Design feature | Why it helps biodiversity |
|---|---|
| Large, rounded shape | big sheltered interior, little harsh edge → more interior species survive |
| Wildlife corridors | animals move, interbreed (genetic diversity) and recolonise after losses |
| Active management | control invasive species, poaching and human disturbance |
Near a habitat edge it is windier, brighter and drier, with more predators and invasive species — so a thin, all-edge reserve protects far fewer species than a compact one of the same area.
🧬 Ex situ conservation
Ex situ means 'out of place' — conserving a species away from its natural habitat: zoos, botanic gardens, captive-breeding programmes and seed / tissue (gene) banks. It lets a species breed safely away from predators, poachers and habitat clearance, raise its numbers under expert care, preserve genetic variety through managed breeding (studbooks), and provide individuals to reintroduce to the wild later. It is a last resort / back-up: it is expensive, holds few individuals, and animals can lose natural behaviours — so it works best alongside in situ conservation.
| In situ | Ex situ | |
|---|---|---|
| Where | in the natural habitat (reserves, parks) | away from it (zoos, gardens, gene banks) |
| Strength | whole ecosystem & behaviour kept; cheaper per individual | safe from wild threats; controlled breeding |
| Weakness | can't remove threats already in the habitat | costly; few individuals; lost natural behaviour |
🦫 Rewilding & ecosystem restoration
Rewilding helps a degraded ecosystem repair itself by restoring natural processes — then stepping back so the system self-regulates. Methods include restoring natural water flow (re-flooding), letting native vegetation return, reconnecting habitats, and reintroducing keystone species. A keystone species has an effect far bigger than its numbers — for example beavers build dams that restore water flow and create wetland, which supports many more species. Reclamation restores land that was badly damaged (e.g. by mining) back to a functioning ecosystem.
A keystone species has a large effect on its whole ecosystem relative to its numbers. Reintroducing one (e.g. beavers, wolves) can restart natural processes and restore the wider community.
🎯 Choosing what to conserve — the EDGE programme
There are far more endangered species than money, land and time to save them all, so we must prioritise. The EDGE of Existence programme ranks species by two scores: E·D = Evolutionarily Distinct (very few close living relatives — a unique branch of the tree of life) and G·E = Globally Endangered (high risk of extinction). A species that scores high on both is the top priority, because its loss would remove a unique, irreplaceable branch of evolutionary history that no other species can replace.
Evolutionarily Distinct and Globally Endangered. Distinct = few close relatives; endangered = at high risk. Prioritise species that are both.
✍️ Worked examples
IB-style question — why added biodiversity matters
Ecologists found that when otters returned to a river, the number of fish and insect species rose sharply. Suggest why this increase in biodiversity is good for the river ecosystem. [2]
Model answer:
Pick a benefit of more species. More species makes the ecosystem more stable and resilient — there is more 'backup' if conditions change.
Tie it to the data. With more fish and insect species, if one species declines others can take over its role, so food webs keep working and the ecosystem recovers more easily from disturbance.
More species makes the river ecosystem more stable and resilient: if one species is lost, others can carry out its role, so the ecosystem keeps functioning and recovers more easily from disturbance.
IB-style question — in situ vs ex situ
A rare frog is threatened by a deadly fungal disease spreading through its streams. One team proposes a nature reserve (in situ); another a captive-breeding programme in a disease-free facility (ex situ). Discuss which approach is better for saving this frog. [4]
Model answer:
In situ strength: a reserve protects the frog in its natural habitat, keeping its behaviour and ecosystem intact, and is usually cheaper per individual.
In situ weakness here: a reserve cannot stop the fungal disease already in the streams, so the wild frogs may still die out.
Ex situ strength here: a disease-free facility lets the frogs breed safely away from the fungus and acts as a genetic back-up.
Reach a judgement (Discuss): because the threat is a disease the habitat can't exclude, ex situ is the better immediate rescue — but pair it with in situ work for later reintroduction.
In situ protects the natural habitat and is cheaper but cannot remove the fungal disease, so wild frogs stay at risk. Ex situ breeding in a disease-free facility is the better immediate rescue and a genetic back-up — ideally combined with in situ work for later reintroduction.
IB-style question — applying EDGE
A fund can protect only one of two rare species. Species X is the last survivor of an ancient group with no close living relatives. Species Y is one of forty very similar species in its family. Both are endangered. Using EDGE, state which to prioritise and explain why. [3]
Model answer:
Apply the rule. EDGE prioritises species that are both evolutionarily distinct AND globally endangered; both are endangered, so distinctness decides.
Compare them. Species X has no close relatives → highly distinct; Species Y has forty similar relatives → not distinct.
Conclude. Prioritise Species X: its loss would remove a unique, irreplaceable branch of the tree of life, while Y's loss is cushioned by its many close relatives.
Prioritise Species X. Both are endangered, so distinctness decides: X is evolutionarily distinct (no close relatives), so its loss removes a unique branch of the tree of life, while Y's many relatives mean less unique biodiversity is lost.
✅ Quick self-check
Tap each card to check yourself.
What are the three levels of biodiversity? Species diversity (number of different species), genetic diversity (variety within a species) and ecosystem diversity (variety of habitats).
Name the five human causes of biodiversity loss. Habitat loss (the biggest), overexploitation, pollution, invasive species and climate change.
In situ vs ex situ — what's the difference? In situ = protecting a species IN its natural habitat (reserves, parks). Ex situ = protecting it AWAY from the habitat (zoos, botanic gardens, captive breeding, gene banks).
Why does reserve shape matter? A large rounded reserve has a big sheltered interior and little harsh edge, so more interior species survive; a thin, all-edge strip protects far fewer.
What is a keystone species in rewilding? A species with a large effect on its whole ecosystem relative to its numbers (e.g. beavers building dams); reintroducing it restarts natural processes.
What does EDGE stand for and do? Evolutionarily Distinct and Globally Endangered — it prioritises conservation effort on species that are both distinct (few relatives) and at high risk, to save the most irreplaceable biodiversity.
Exam Tips
- For 'why does biodiversity matter?', lead with stability / resilience — the safest mark.
- Habitat loss is the biggest cause of biodiversity loss; name it first when asked.
- In situ = IN the habitat; ex situ = OUT of (away from) the habitat. Don't swap them.
- Ex situ is a back-up / last resort — it works best alongside in situ for later reintroduction.
- Rewilding restores natural PROCESSES; a keystone species has an effect far bigger than its numbers.
- EDGE prioritises species that are BOTH evolutionarily distinct AND globally endangered.
- Discuss / Outline questions want NAMED methods with reasoning — never just 'it helps biodiversity'.