Factors Affecting Biodiversity

Global biodiversity is declining at an unprecedented rate. The Living Planet Index estimates that monitored wildlife populations have fallen by around 69% 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 specification 4.2.1 (g) requires you to explain how human activities affect biodiversity and to evaluate the importance of conserving it.

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.

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The Major Threats

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.

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1. Deforestation

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:

• Agriculture (cattle ranching, soy, palm oil).
• Timber extraction.
• Mining and infrastructure.

Effects on Biodiversity

• Direct habitat loss. Species that depend on closed-canopy forest cannot survive in pastureland.
• Fragmentation. Remaining patches are smaller and more isolated, reducing gene flow and increasing edge effects.
• Species extinction. Many rainforest species have tiny ranges; clearing their habitat means extinction.
• Soil erosion and nutrient loss. Without tree cover, topsoil washes away and nutrients leach.
• Climate effects. Forests store carbon; deforestation releases it and alters local rainfall.

Case Study: Orangutans

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.

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2. Agriculture

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:

Habitat Simplification

• Monocultures replace diverse natural communities with a single species.
• Hedgerow removal between fields (UK lost more than 50% of hedgerows 1950–2000) eliminates wildlife corridors.
• Drainage of wetlands destroys irreplaceable habitats.
• Ploughing damages soil invertebrates and seed banks.

Chemical Inputs

• Pesticides kill target pests but also non-target invertebrates; neonicotinoids are implicated in bee declines.
• Herbicides eliminate wildflowers that would feed pollinators.
• Fertilisers run off into streams, causing eutrophication: algal blooms deplete oxygen and kill aquatic life.

Selective Breeding and Genetic Erosion

Modern cultivars dominate while land races are lost. Wild relatives of crops — vital for breeding new disease-resistant varieties — decline with their habitats.

Case Study: UK Farmland Birds

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.

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3. Climate Change

Rising global temperatures, shifting rainfall patterns and more frequent extreme weather events all threaten biodiversity. Effects include:

Shifting Ranges