Biodiversity is being lost faster than at any time since the dinosaurs. The biology, the evidence, and what conservation actually does.
Every sub-topic below feeds at least one of these questions.
What factors are causing the sixth mass extinction of species?
How can conservationists minimize the loss of biodiversity?
The required syllabus content for A4.2, in order. Each card is one lesson-sized checkpoint.
Biodiversity as the variety of life in all its forms, levels and combinations
Millions of species have been discovered, named and described but there are many more species to be discovered.
Causes of anthropogenic species extinction
Causes of ecosystem loss
Evidence for a biodiversity crisis
Causes of the current biodiversity crisis
Need for several approaches to conservation of biodiversity
Selection of evolutionarily distinct and globally endangered species for conservation prioritization in the EDGE of Existence programme
Biodiversity isn't one thing. It exists at three nested levels, and you need to measure all three to capture the variety of life properly.
The variety of ecosystems within an area or on Earth. A region with rainforest, coral reef, mangrove and savanna has more ecosystem diversity than a region with only one of those.
The number and relative abundance of species within an ecosystem. Combines richness (how many different species) and evenness (how equal their populations are).
The variety of genes and alleles within a species. Species with high genetic diversity are more resilient to environmental change — there's more variation for natural selection to act on.
Fossil evidence suggests there are more species alive today than at any point in Earth's history. Most haven't been discovered yet.
Catalogued, classified, named.
Most are undescribed — many will probably go extinct before we find them.
End-Ordovician, Late Devonian, Permian–Triassic, Triassic–Jurassic, Cretaceous–Paleogene.
Many scientists consider human-driven extinction rates equivalent to a mass extinction.
Classification is pattern recognition — but the same observations can be classified differently. "Lumpers" focus on similarities and recognise fewer, broader species. "Splitters" focus on differences and recognise more, narrower species. Bird taxonomy is a perennial battleground between the two approaches.
The sixth mass extinction is driven by human activities — six recurring causes. The IB asks for case studies of named extinct species.
Human activities causing extinction include: habitat destruction, climate change, pollution, overexploitation, invasive species, disease.
Dinornis novaezealandiae — the North Island giant moa. Up to 3 metres tall, >250 kg. Flightless. Lived in New Zealand until ~500 years ago. Before humans arrived, the moa had few predators (just the Haast's eagle). When Māori people arrived, they hunted moa for food, clothing and jewellery. Reproduction couldn't keep up with the hunting rate. The giant moa was driven to extinction within a few centuries — a clear case of overexploitation.
Neomonachus tropicalis — the Caribbean monk seal, declared extinct in 2008. Once abundant across the Caribbean. Hunted for oil, meat and pelts since European colonisation in the 1500s; later, overfishing of reef fish depleted its food supply. A combination of overexploitation and ecosystem degradation.
The IB expects you to also study a species that has gone extinct from a region familiar to you. Possibilities: baiji (Yangtze river dolphin), Bramble Cay melomys (first mammal extinction attributed to climate change), Pinta giant tortoise (overhunting + invasive goats).
Two case studies the IB names: mixed dipterocarp forests of Southeast Asia, and another from a region familiar to you.
Mixed dipterocarp forests are tropical rainforests of Southeast Asia (Borneo, Sumatra, the Malay peninsula), dominated by tree species in the family Dipterocarpaceae. Among the most biodiverse ecosystems on Earth. Human-driven causes of their loss:
Mangrove forests grow at tropical and subtropical coastlines, providing nurseries for fish, storm protection for coasts, and habitat for species like manatees. Causes of loss in Florida and elsewhere:
Biodiversity loss is not a feeling — it's a measurement. Established by the IPBES from repeated ecological surveys, peer-reviewed publications, and increasingly, citizen science.
The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) assesses global biodiversity data for policy makers. Its 2019 Global Assessment concluded: "Human actions threaten more species with global extinction now than ever before."
A common quantitative biodiversity measure. D = N(N−1) / Σ n(n−1), where N is the total number of organisms in the sample and n is the number of each species. Higher D = more biodiverse.
Worked example: Ecosystem A has fewer species but more individuals — D ≈ 1.8. Ecosystem B has more species with more even populations — D ≈ 2.6. B is more biodiverse despite having fewer individuals.
Volume of data far exceeding what professional teams can collect. Cost-effective (volunteers). Increases public engagement with conservation. Apps like iNaturalist standardise data collection and identification.
Data quality is variable — volunteers have different skill levels. Sampling can be biased toward easy-access sites or charismatic species. Quality control measures and statistical correction are essential before drawing conclusions.
To be verifiable, scientific evidence usually has to come from a peer-reviewed published source where the methodology can be checked. Citizen-collected data brings unique benefits but also unique methodological concerns.
All causes of the biodiversity crisis trace back to the same root: a human population of 8 billion and rising, consuming the planet's resources faster than they regenerate.
In 1804 the human population was 1 billion. Today it is over 8 billion. Specific drivers, all stemming from population × consumption:
Unsustainable hunting, fishing, logging. Poaching of elephants for ivory. Industrial fishing removing target species and bycatch faster than they reproduce.
People migrating to cities means more land cleared for housing, infrastructure, services. Native ecosystems replaced with urban habitat.
For timber and for agricultural land. Each loss of forest is a loss of habitat and a contribution to climate change.
Displaces natural ecosystems. Fertilisers and pesticides pollute downstream ecosystems. Monocultures replace diverse natural communities.
Plastics, chemicals, heavy metals, light, noise. Industrial and agricultural pollutants poison food webs and habitats.
Carried by global transport networks. Once established in new ecosystems, invasives outcompete natives, disrupt food webs, spread novel pathogens.
Different species and ecosystems need different conservation strategies. The IB names two broad categories — and several specific tools within each.
Conservation outside the species' natural habitat: zoos, aquaria, botanical gardens, seed banks, tissue banks.
Protection from predators and habitat loss. Controlled breeding conditions. Technologies like IVF can boost reproductive success. Espanola giant tortoise: from 15 surviving individuals in the 1960s to >1,500 reintroduced — captive breeding succeeded so well the programme is no longer needed.
Captive populations have limited genetic diversity. Organisms may lose survival skills needed in the wild. Doesn't address the destruction of the original habitat — the species has nowhere to return to.
Conservation within the species' natural habitat. Tools include nature reserves, ecosystem restoration, removal of invasives, reintroduction of locally extinct species, legal protection, controlling access, anti-poaching enforcement, rewilding.
Organisms keep their survival strategies — they're never removed from the wild. Other species in the protected habitat benefit too. Ecosystem-level biodiversity is preserved.
Many endangered species need very large areas. Poaching is hard to prevent at scale. Genetic variety may already be reduced before protection begins. Political and economic pressure on protected land never goes away.
Conservation budgets are finite. The EDGE programme prioritises species that are both evolutionarily distinct (few close relatives) and globally endangered (high extinction risk).
Species with few or no close living relatives — long branches on the tree of life. Their loss would erase a unique evolutionary heritage, not just a tip on the tree. Example: the western long-beaked echidna — the only living member of its genus, a deeply distinct lineage of egg-laying mammal.
Listed as endangered or critically endangered on the IUCN Red List. High risk of extinction within decades without intervention.
Choosing which species to conserve has ethical, environmental, political, social, cultural and economic dimensions. It requires societal debate, not just biological data. EDGE is one defensible framework; not the only one.
If you can't define one of these in a sentence, that's where to revise next.
“In what ways is diversity a property of life at all levels of biological organization?”
“How does variation contribute to the stability of ecological communities?”