Biodiversity is the variety of species, habitats, and genes within an ecosystem. At GCSE level, you need to understand the main human-driven causes of biodiversity loss, the evidence for current extinction rates being far above the natural background rate, and the conservation strategies that can slow or reverse decline.
What is biodiversity and why does it matter?
Biodiversity has three levels:
- Species diversity — the number and relative abundance of different species in an area
- Genetic diversity — the variety of alleles within a species or population
- Ecosystem (habitat) diversity — the range of different habitats and ecological communities
High biodiversity makes ecosystems more resilient — better able to withstand environmental change, disease, or the loss of individual species. If one species disappears from a diverse food web, others can take over its role. In a low-diversity system, losing one species can trigger a cascade of further extinctions.
Biodiversity also has direct practical value: many medicines (including aspirin, penicillin, and numerous cancer drugs) were originally discovered in wild organisms; wild plants and their genes are used to develop new crop varieties; and ecosystems provide services such as clean water filtration, carbon storage, and soil formation.
What are the main causes of biodiversity loss?
Habitat destruction
Habitat destruction is the leading cause of species loss worldwide. When a habitat is cleared, the species living in it lose their food, shelter, and breeding sites. Major forms include:
- Deforestation — tropical rainforests cover less than 6% of Earth's land surface but contain more than half of all known species. Clearing forest for agriculture, cattle ranching, or timber removes these species' only habitat.
- Agricultural intensification — replacing diverse grasslands and hedgerows with monoculture farmland eliminates the food plants, nesting sites, and invertebrate communities that many animal species depend on.
- Urbanisation — converting land to housing, roads, and industry fragments habitats and creates barriers that prevent animal movement and gene flow.
Invasive species
When a non-native species is introduced into a new ecosystem (deliberately or accidentally), it may have no natural predators there and can:
- Outcompete native species for food and space
- Predate native species that have no evolved defences
- Carry diseases to which native species are susceptible
Examples: grey squirrels (introduced from North America to Britain in the 1870s) outcompete native red squirrels and carry the squirrel parapoxvirus, to which they are immune but which is fatal to reds; American mink escape from fur farms and devastate water vole populations.
Pollution
| Type of pollution | Effect on biodiversity |
|---|---|
| Pesticides | Kill non-target insects; reduce food for birds and bats; accumulate up food chains (biomagnification) |
| Eutrophication | Excess fertiliser in waterways causes algal blooms; decomposers deplete oxygen; aquatic life dies |
| Plastic waste | Marine animals ingest or become entangled in plastic; microplastics enter food chains |
| Air pollution | Acid rain damages vegetation and acidifies freshwater habitats |
Climate change
Rising global temperatures and changing rainfall patterns affect species in several ways:
- Habitat shift: the range of suitable climate for a species moves poleward or to higher altitude — but if habitat is fragmented, species cannot move
- Phenological mismatch: the timing of events (e.g. flowering, insect emergence, bird breeding) changes at different rates for different species, breaking important ecological relationships
- Coral bleaching: warming oceans cause coral polyps to expel the symbiotic algae (zooxanthellae) they depend on, turning reefs white and eventually killing them
Overexploitation
Hunting, fishing, and collection at rates faster than populations can reproduce leads to population collapses. The passenger pigeon (hunted to extinction by 1914), Atlantic cod, and numerous shark species illustrate how rapidly this can occur even for formerly abundant populations.
What are the current extinction rates?
The natural background extinction rate — the rate at which species would go extinct without human interference — is estimated at approximately 1–5 species per year per million species. Current observed extinction rates are estimated at 100–1,000 times this background rate. Some scientists describe the current period as the sixth mass extinction event in Earth's history.
The IUCN Red List (International Union for Conservation of Nature) classifies species by their extinction risk: Least Concern, Near Threatened, Vulnerable, Endangered, Critically Endangered, Extinct in the Wild, and Extinct. More than 40,000 species are currently listed as threatened with extinction.
What conservation strategies can slow biodiversity loss?
In situ conservation (conservation in the wild)
| Strategy | How it works |
|---|---|
| Nature reserves and national parks | Protect habitat from development; allow ecosystems to recover |
| Wildlife corridors | Connect fragmented habitats so animals can move and breed between areas |
| Marine protected areas (MPAs) | Restrict fishing and development in key ocean habitats |
| Legislation | Laws such as the UK Wildlife and Countryside Act make it illegal to kill or disturb protected species |
| Reintroduction programmes | Re-establish species in areas where they have been lost (e.g. white-tailed eagle in Scotland) |
Ex situ conservation (conservation outside the wild)
| Strategy | How it works |
|---|---|
| Zoos and captive breeding | Maintain genetically diverse populations; breed animals for reintroduction |
| Seed banks | Store seeds of plant species at low temperature and humidity (e.g. the Svalbard Global Seed Vault) |
| Botanical gardens | Maintain living collections of rare and threatened plant species |
| Gene banks | Store genetic material (sperm, eggs, DNA) of endangered species for future use |
In situ conservation is generally preferred because species can continue to evolve and interact in their natural habitat. Ex situ conservation is a last resort and supplement — a frozen zoo cannot replace a functioning ecosystem.
Frequently asked questions
Why does biodiversity loss matter to humans directly?
Many of the world's most important medicines come from wild organisms — including antibiotics (from fungi and bacteria), anti-cancer drugs (taxol from the Pacific yew tree), and aspirin (originally from willow bark). Wild species are also the genetic reservoir used to breed disease-resistant and high-yielding crop varieties. Losing a species permanently removes a potential source of future medicines, foods, or materials that we may not yet know we need.
What is the difference between in situ and ex situ conservation?
In situ conservation protects species in their natural habitat — through reserves, legislation, and habitat management. Ex situ conservation protects species outside their natural habitat — in zoos, seed banks, or botanical gardens. In situ is preferred because it preserves the species' ecological relationships and allows natural evolution to continue. Ex situ is used when in situ is no longer viable and as an emergency backup.
Can species go extinct for natural reasons as well as human ones?
Yes. Background extinction occurs naturally due to geological change, disease, competition, or rare catastrophic events. However, the current extinction rate is estimated to be 100–1,000 times the natural background rate, and the dominant driver is human activity — particularly habitat destruction, invasive species introduction, overexploitation, pollution, and climate change. The geological record shows five previous mass extinction events caused by natural factors (e.g. asteroid impact, volcanic eruptions); biologists consider the current loss to be a sixth, human-caused event.
How does habitat fragmentation make extinction more likely even if the total habitat area stays the same?
A single large reserve supports larger populations with more genetic diversity and allows animals to move freely to find food, mates, and shelter. Fragmenting the same area into isolated patches creates small populations that are more vulnerable to local extinction through random demographic variation (bad luck), inbreeding (reducing fitness), and the inability to recolonise after a local disaster. Species at the edges of fragments also face harsher, more variable conditions than those in the interior.
For Socratic GCSE biology with Professor Darwin — tracing species through ecosystems and the forces that unite or divide them — visit aitutors.me.