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Spec Mapping — OCR H420 Module 4.2.1 — Biodiversity, content statements covering the factors that affect biodiversity (habitat loss, deforestation, intensive agriculture, climate change, pollution, overexploitation, invasive species) and the reasons for conserving biodiversity (refer to the official OCR H420 specification document for exact wording). The figures and trends cited below are widely documented in textbooks and reports; some are paraphrased — frame uncertain numbers as "estimated" or "current consensus suggests" in exam answers.
Global biodiversity is declining at an unprecedented rate. The Living Planet Index suggests that monitored wildlife populations have fallen by around two-thirds since 1970, and the IUCN Red List identifies tens of thousands of species as threatened with extinction. Understanding why biodiversity is declining is essential both for exam success and for making informed conservation decisions. OCR A-Level Biology A Module 4.2.1 requires you to explain how human activities affect biodiversity and to evaluate the importance of conserving it.
The concept of an ongoing "sixth mass extinction" — comparable to the five geological mass-extinctions identifiable in the fossil record — was made widely known by Elizabeth Kolbert's 2014 book of that name, building on decades of taxonomic work. Theodosius Dobzhansky's framing of biology in the light of evolution applies forcefully: extinction is the long-term endpoint of populations that cannot keep up with the rate of environmental change, whatever its cause. The conservation argument is therefore both ethical and evolutionary — once a lineage's alleles are lost, the evolutionary information they carried cannot be re-derived.
Key Definitions:
- Anthropogenic — caused or produced by human activity.
- Deforestation — large-scale removal of forest cover.
- Monoculture — cultivation of a single crop species over a large area.
- Eutrophication — nutrient enrichment of water, often leading to algal blooms.
- Climate change — long-term shifts in global weather patterns caused primarily by greenhouse gas emissions.
flowchart TD
A[Biodiversity Loss] --> B[Habitat Destruction]
A --> C[Climate Change]
A --> D[Pollution]
A --> E[Overexploitation]
A --> F[Invasive Species]
B --> B1[Deforestation]
B --> B2[Agricultural expansion]
B --> B3[Urbanisation]
D --> D1[Pesticides]
D --> D2[Eutrophication]
D --> D3[Plastic]
Conservation biologists sometimes use the acronym HIPPO to remember the main drivers: Habitat loss, Invasive species, Pollution, Population (human), and Overharvesting. OCR focuses on three in particular: deforestation, agriculture and climate change.
Forests hold around 80% of the world's terrestrial biodiversity, with tropical rainforests especially rich — the Amazon alone is home to at least 40,000 plant species, 2,500 fish, 1,300 birds and millions of invertebrates. Each year, approximately 10 million hectares of forest are cleared worldwide, mainly for:
Palm oil plantations in Borneo and Sumatra have destroyed more than half of orangutan habitat since the 1990s. Population estimates for Bornean orangutans have fallen from over 230,000 in the 1970s to around 100,000 today; the Sumatran and Tapanuli species are critically endangered.
Agriculture now occupies around half of the world's habitable land. While it feeds us, it also represents the single biggest driver of terrestrial biodiversity loss. Intensive farming impacts biodiversity through:
Modern cultivars dominate while land races are lost. Wild relatives of crops — vital for breeding new disease-resistant varieties — decline with their habitats.
The UK Farmland Bird Index has fallen by more than 50% since 1970. Species such as tree sparrow (−96%), grey partridge (−93%) and turtle dove (−98%) have collapsed as hedgerows, winter stubble and insects have disappeared. Agri-environment schemes (see Lesson 6) aim to reverse these trends.
Rising global temperatures, shifting rainfall patterns and more frequent extreme weather events all threaten biodiversity. Effects include:
Species move polewards or uphill as temperatures rise. Many UK butterflies have shifted their ranges northward by 50–100 km in recent decades. Species unable to move fast enough — or with nowhere to go (mountain summits, Arctic islands) — face extinction.
Spring events (leaf-out, insect emergence, bird breeding) are occurring earlier. If predators and prey respond differently, ecological relationships break down. For example, UK pied flycatchers now arrive too late to feed chicks on peak caterpillar numbers.
Arctic sea ice is disappearing in summer, with devastating consequences for polar bears, walruses and ice-dependent algae that underpin the entire food web.
Exam Tip: OCR questions on climate change often ask you to link abiotic change to biological response to biodiversity outcome. A strong answer might read: "Rising sea temperatures (abiotic) expel zooxanthellae from coral polyps (biological response), killing the coral and eliminating the habitat for thousands of reef species (biodiversity outcome)."
Non-native species introduced to new habitats can out-compete, predate or bring diseases to native species. UK examples include:
Biodiversity underpins ecosystem services — the benefits humans derive from nature:
| Service | Example |
|---|---|
| Provisioning | Food, fibre, timber, freshwater, medicines |
| Regulating | Pollination, pest control, climate regulation, flood control |
| Supporting | Nutrient cycling, soil formation, primary production |
| Cultural | Recreation, spiritual value, education |
The Millennium Ecosystem Assessment valued global ecosystem services at tens of trillions of pounds annually. Protecting biodiversity is therefore not just altruistic — it is economically essential.
Many argue that species have intrinsic value — a right to exist — regardless of use to humans. Others emphasise the cultural, spiritual and aesthetic importance of nature. Either way, these are legitimate reasons to preserve biodiversity.
| Driver | Primary mechanism | Trophic level most affected | OCR-credit example | Reversibility on policy timescales |
|---|---|---|---|---|
| Deforestation | Direct habitat loss + fragmentation | Specialists at all levels | Orangutan, Amazon endemics | Slow (decades-centuries) |
| Intensive agriculture | Habitat simplification + chemicals | Insects, farmland birds | UK farmland bird index decline | Moderate (agri-environment schemes) |
| Climate change | Range shifts + phenological mismatch | Coral reefs, polar | Great Barrier Reef bleaching | Very slow (decades-centuries) |
| Pollution | Lethal + sub-lethal physiology | Aquatic invertebrates | Sewage-driven low-D streams | Often fast once source removed |
| Overexploitation | Direct removal | Large vertebrates, fish | North Atlantic cod | Moderate if banned in time |
| Invasive species | Predation, competition, disease | Island endemics | Grey squirrel, signal crayfish | Hard once established |
A long-term ecological monitoring scheme in southern England has tracked invertebrate Simpson's index of diversity (D) on three paired sites:
- Site A — semi-natural chalk grassland (control); D essentially flat at 0.82 (1990) to 0.79 (2024).
- Site B — adjacent intensively-farmed arable; D fell from 0.71 (1990) to 0.31 (2024).
- Site C — formerly farmed, entered agri-environment scheme 2010; D fell to 0.34 by 2010 then recovered to 0.58 by 2024.
(a) Suggest TWO reasons why D at Site B has fallen so steeply. (4 marks) (b) Calculate the percentage recovery of D at Site C between 2010 and 2024, taking Site A as the unspoiled reference. Show your working. (2 marks) (c) Evaluate the contribution of intensive agriculture, relative to climate change, to insect biodiversity loss in southern England. Use the data and your wider knowledge. (9 marks)
| Part | AO1 | AO2 | AO3 |
|---|---|---|---|
| (a) | 2 | 2 | 0 |
| (b) | 0 | 2 | 0 |
| (c) | 3 | 3 | 3 |
(a) 4-mark Grade C response: Pesticides like neonicotinoids will have killed many insects. Hedgerow removal will have destroyed habitats so fewer species can live there.
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