Biodiversity and Human Impact

Biodiversity is the variety of all the different species of organisms on Earth, or within a particular ecosystem. High biodiversity is essential for the stability of ecosystems and for human well-being. However, human activities are reducing biodiversity at an alarming rate. This lesson covers what biodiversity is, why it matters, and the ways in which humans are threatening it, as required by AQA GCSE Biology.

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What Is Biodiversity?

Biodiversity refers to the variety of living organisms in an area. It can be measured at different scales:

Scale	What It Measures	Example
Species diversity	The number of different species and the abundance of each species in an area	A meadow with 40 species of wildflower is more biodiverse than one with 5
Genetic diversity	The variation in alleles (versions of genes) within a species	Cheetahs have low genetic diversity; dogs have high genetic diversity
Ecosystem diversity	The range of different ecosystems in an area	A region with forests, wetlands, grasslands and rivers has high ecosystem diversity

Exam Tip: The most commonly tested definition is species diversity. Make sure you say "the variety of different species" and not just "the number of organisms" — a field with 1,000 organisms of one species is less biodiverse than a field with 100 organisms from 20 different species.

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Why Is Biodiversity Important?

Biodiversity matters for many reasons:

Reason	Explanation
Ecosystem stability	Greater biodiversity means more food web connections, making ecosystems more resilient to change
Food security	Humans depend on a diverse range of crops and livestock for food
Medicine	Many medicines are derived from plants and animals (e.g. aspirin from willow bark, penicillin from fungi)
Industrial materials	Organisms provide raw materials such as wood, fibres, rubber and oils
Genetic resources	Wild relatives of crops contain useful alleles for breeding disease-resistant varieties
Pollination	Many crops depend on insect pollinators; declining pollinator biodiversity threatens food production
Aesthetic and cultural value	Nature provides recreation, inspiration and cultural significance
Ethical responsibility	Many people believe all species have a right to exist regardless of their value to humans

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How Are Humans Reducing Biodiversity?

The growing human population demands more resources, more land and more energy. This leads to several activities that reduce biodiversity:

1. Habitat Destruction

Habitat destruction is the single greatest threat to biodiversity worldwide. When habitats are destroyed, the organisms that live there lose their home, food sources and breeding sites.

Type of Habitat Destruction	Cause	Impact on Biodiversity
Deforestation	Clearing forests for agriculture, cattle ranching, logging or development	Loss of thousands of species; fragmented habitats
Hedgerow removal	Removing hedgerows to create larger fields for farming	Loss of habitat for birds, insects and small mammals
Wetland drainage	Draining marshes and bogs for building or farming	Loss of habitat for amphibians, wading birds and aquatic species
Urbanisation	Building houses, roads, factories on previously natural land	Permanent loss of habitat; soil sealing
Mining and quarrying	Extracting minerals and rocks from the ground	Destruction of surface habitats; pollution of waterways

2. Pollution

Pollution introduces harmful substances into the environment, killing organisms or making habitats uninhabitable.

Type of Pollution	Source	Effect on Biodiversity
Water pollution	Sewage, fertilisers (eutrophication), industrial waste, pesticides	Kills aquatic organisms; eutrophication depletes oxygen
Air pollution	Burning fossil fuels (SO2, NO2), vehicle emissions	Acid rain damages leaves and acidifies lakes and soil
Land pollution	Pesticides, herbicides, landfill waste, toxic chemicals	Poisons organisms; makes soil unsuitable for plant growth
Light pollution	Artificial lighting from cities	Disrupts nocturnal animals, migration patterns and breeding cycles
Noise pollution	Traffic, industry, construction	Disrupts animal communication, mating calls and navigation

Exam Tip: Eutrophication is a frequently examined topic. Make sure you can describe the full sequence: excess fertiliser runs into water -> algae bloom -> algae block light -> aquatic plants die -> bacteria decompose dead plants -> bacteria use up dissolved oxygen -> fish and other aquatic organisms die.

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Eutrophication: Step by Step

Eutrophication is the process by which excess nutrients (particularly nitrates and phosphates from fertilisers) cause excessive growth of algae in water bodies, leading to oxygen depletion.

graph TD
    A["Excess fertiliser washed<br/>into rivers/lakes<br/>(by rain)"] --> B["Algae grow rapidly<br/>(algal bloom)"]
    B --> C["Algae block sunlight<br/>from reaching<br/>aquatic plants below"]
    C --> D["Aquatic plants cannot<br/>photosynthesise<br/>and die"]
    D --> E["Bacteria decompose<br/>dead plants and algae"]
    E --> F["Bacteria multiply<br/>and use up<br/>dissolved oxygen"]
    F --> G["Fish and other aquatic<br/>organisms die<br/>(lack of oxygen)"]
    style A fill:#c8e6c9,stroke:#2e7d32
    style B fill:#a5d6a7,stroke:#2e7d32
    style C fill:#fff9c4,stroke:#f9a825
    style D fill:#ffccbc,stroke:#d84315
    style E fill:#d7ccc8,stroke:#5d4037
    style F fill:#ef9a9a,stroke:#c62828
    style G fill:#ef5350,stroke:#c62828,color:#fff

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3. Overexploitation

Overexploitation means taking resources from the environment faster than they can be replaced:

Activity	Impact
Overfishing	Fish populations collapse; marine food webs disrupted
Overhunting	Species driven to extinction or near-extinction (e.g. dodo, passenger pigeon)
Over-harvesting of plants	Rare species depleted; deforestation of valuable timber species
Peat extraction	Destruction of peat bog habitats; release of stored carbon

4. Introduction of Invasive Species

When non-native species are introduced to an ecosystem (either deliberately or accidentally), they can outcompete native species, spread disease, or predate on native organisms.

Invasive Species	Where Introduced	Impact
Grey squirrel	UK	Outcompetes native red squirrel; carries squirrelpox virus
Japanese knotweed	UK	Outcompetes native plants; damages buildings and infrastructure
Cane toad	Australia	Poisons native predators; outcompetes native frogs
Signal crayfish	UK	Outcompetes native white-clawed crayfish; carries crayfish plague

5. Climate Change

Climate change caused by increased greenhouse gas emissions is altering habitats worldwide:

Effect of Climate Change	Impact on Biodiversity
Rising temperatures	Species unable to adapt may die out; ranges shift towards the poles/higher altitudes
Sea level rise	Coastal habitats flooded; islands submerged
Changing rainfall patterns	Droughts in some areas, flooding in others; habitats disrupted
Ocean acidification	CO2 dissolves in seawater, lowering pH; coral reefs and shell-forming organisms affected
Melting ice caps	Loss of polar habitats for species such as polar bears and penguins