AQA GCSE Biology (8461)

4.7.3 Biodiversity and the effect of human interaction on ecosystems

Biodiversity is shrinking. Every habitat AQA asks you about — rainforests, peat bogs, rivers, grasslands — is being damaged by the same handful of human pressures: deforestation, peat extraction, water and air pollution, global warming, overgrazing and the spread of non-native species. This page walks through what biodiversity actually means, why it matters for the stability of ecosystems and for us, the chain of events behind each damaging activity, and the conservation programmes that try to reverse the damage. By the end you'll write the exact mark-scheme phrases AQA wants: 'organisms lose their habitat and cannot survive', 'algae grow rapidly and decomposers use up oxygen', and 'breeding programmes for endangered species'.

From the 8461 specification

Biodiversity is the variety of all the different species of organisms on Earth, or within an ecosystem. A great biodiversity ensures the stability of ecosystems by reducing the dependence of one species on another for food, shelter and the maintenance of the physical environment. The future of the human species on Earth relies on us maintaining a good level of biodiversity. Many human activities are reducing biodiversity and only recently have measures been taken to try to stop this reduction. Rapid growth in the human population and an increase in the standard of living mean that increasingly more resources are used and more waste is produced. Humans reduce the amount of land available for other animals and plants by building, quarrying, farming and dumping waste.

Why this matters

Biodiversity is short for biological diversity — the variety of different species in a place. A tropical rainforest can hold ten thousand species in a single square kilometre; a ploughed wheat field holds essentially one. High biodiversity means lots of interacting species, lots of food-chain redundancy, and lots of resilience: if one species disappears, others can fill the gap. Low biodiversity means a fragile ecosystem that can collapse when conditions change. Humans depend on biodiverse ecosystems for clean air, clean water, pollination of crops, fertile soil, fish stocks and medicines. Since the Industrial Revolution, human population has grown from about one billion to over eight billion, and per-person resource use has climbed even faster. The result is that habitats are being converted to farmland and cities, waste is piling up in landfills, fertilisers leach into rivers, and the climate is warming. Conservation work — protected areas, breeding programmes, reforestation, sustainable farming — tries to slow the loss. The AQA spec wants you to be able to describe both the pressures and the responses.

How to learn this topic

Build on what you already know

GCSE 4.7.1: abiotic and biotic factors that affect organisms.
GCSE 4.7.2: producers, consumers, decomposers and the carbon cycle.
GCSE 4.4.2: aerobic respiration uses oxygen — relevant for the eutrophication chain.
KS3: food chains, food webs, decomposition, photosynthesis.

Define biodiversity and explain why it matters for stability.
Why human population growth + standard of living drives biodiversity loss.
Land use: building, quarrying, farming, dumping waste.
Deforestation — habitat loss for agriculture, timber, biofuels.
Peat bog destruction — habitat loss + stored CO₂ released.
Water pollution and eutrophication — the five-stage chain.
Air pollution: sulfur dioxide and acid rain.
Global warming and climate change consequences.
Overgrazing and introducing non-native species.
Conservation programmes: breeding, reserves, reforestation, sustainable agriculture, recycling.

Key terms

biodiversity: The variety of all the different species of organisms in an ecosystem, or on the whole Earth. (AQA wants 'variety of different species'. Don't write 'number of organisms' — it's about species variety, not population size.)
stability (of an ecosystem): When populations and abiotic factors stay roughly constant over time. High biodiversity supports stability because species depend less on any one other species. (Marking phrase: 'great biodiversity ensures stability of ecosystems'.)
deforestation: The clearing of large areas of forest, usually for agriculture, timber, or biofuel production. (Always link to habitat loss: 'organisms that depend on trees for food and shelter cannot survive'.)
peat bog: A wetland habitat where partly decomposed plant matter (peat) accumulates because waterlogged, acidic conditions prevent full decomposition. Stores large amounts of carbon. (Two-part answer: extracting peat (1) destroys the habitat and (2) releases stored CO₂.)
eutrophication: The process in which excess nutrients (from fertilisers or sewage) in water cause algae to grow rapidly, blocking light. Plants die and decomposers use up the oxygen, killing fish. (Learn the five phrases in order: algae grow rapidly / light cannot penetrate / decomposers respire / use up oxygen / fish die.)
acid rain: Rain made acidic by dissolved sulfur dioxide and nitrogen oxides from burning fossil fuels. Damages trees and acidifies lakes, reducing biodiversity. (Name the gas: sulfur dioxide. Name two effects: kills trees, kills aquatic life.)
global warming: The long-term rise in Earth's average surface temperature caused by increased greenhouse gases (mainly CO₂ and methane) in the atmosphere. (Be ready to list consequences: rising sea levels, melting ice, changing weather patterns, species migration / extinction.)
non-native species: A species introduced (deliberately or accidentally) to an area where it does not naturally occur. Can outcompete native species and reduce biodiversity. (Marking phrase: 'no natural predators in the new habitat — population grows rapidly'.)
breeding programme: A conservation programme in which endangered species are bred in zoos or reserves to increase their numbers, with offspring sometimes released into the wild. (Always give an example: giant panda, condor, Arabian oryx.)
SSSI (Site of Special Scientific Interest): A protected area in the UK designated for its rare or important wildlife, geology or habitat. Development and damage are restricted.

Notes

What biodiversity means and why it matters

Biodiversity is the variety of all the different species of organisms in an ecosystem, or on Earth. A coral reef, a tropical rainforest and an ancient woodland all have high biodiversity — many different species living together. A monoculture wheat field or a desert has low biodiversity.

A great biodiversity ensures the stability of ecosystems. The reason: when there are many species, each species depends less on any one other species for food, shelter or maintenance of the physical environment. If a single prey species disappears, predators have alternatives. If one pollinator dies out, others can pollinate the plants. High biodiversity makes ecosystems resilient — they can absorb disturbances and recover.

This matters for humans too. The future of the human species on Earth relies on us maintaining a good level of biodiversity. We depend on biodiverse ecosystems for clean water, clean air, pollination of crops, fertile soil, fisheries, timber, and medicines. AQA expects you to write this exact phrase — biodiversity ensures stability — and to link it to the survival of humans.

Why human activity is reducing biodiversity

Two things are happening at once:

The human population is growing rapidly — from about 1 billion in 1800 to over 8 billion today.
The standard of living is rising — people use more energy, eat more meat, buy more goods, throw away more waste.

Together these mean increasingly more resources are used and more waste is produced. Humans reduce the amount of land available for other species by building, quarrying, farming and dumping waste. Habitats shrink and species lose the places they live.

The AQA-specified human activities reducing biodiversity are below.

Deforestation

Large areas of forest are being cleared for:

Agriculture — to plant crops (soya, palm oil) and graze cattle.
Timber — for building and paper.
Biofuels — to grow crops that can be turned into bioethanol or biodiesel.

Deforestation removes the trees that thousands of species depend on for food and shelter. When the trees go, the organisms that lived there cannot survive — and biodiversity falls. Deforestation in tropical regions also reduces the rate at which CO₂ is removed from the atmosphere by photosynthesis, contributing to global warming.

Peat bog destruction

Peat bogs are wetland habitats where partly decomposed plant matter (peat) builds up over thousands of years because the waterlogged, acidic conditions stop full decomposition. Peat bogs:

Are habitats for many specialist species — sphagnum moss, rare insects, wading birds, carnivorous plants.
Store enormous quantities of carbon locked in the peat.

Peat is dug up and sold as compost for gardens. When peat is removed, the habitat is destroyed and the species that lived there cannot survive — so biodiversity falls. The peat also dries out and is broken down by decomposers, releasing the stored carbon back to the atmosphere as CO₂, adding to global warming.

Water pollution and eutrophication

Water is polluted by:

Sewage — adds nutrients and pathogens.
Fertilisers — nitrates and phosphates wash off fields after rain.
Toxic chemicals from industry.

The most-asked chain in AQA papers is eutrophication — what happens when fertilisers run into a river or lake. The five marking phrases are:

Algae grow rapidly — extra nutrients fuel a surface bloom.
Light cannot penetrate the water below the algae.
Submerged plants and the algae themselves die because they cannot photosynthesise.
Decomposers respire as they break down the dead plants and use up oxygen in the water.
Fish and other animals die because there is not enough oxygen for aerobic respiration.

Learn those five phrases in that order — they answer almost any eutrophication question.

Air pollution and acid rain

Burning fossil fuels releases sulfur dioxide (SO₂) and nitrogen oxides into the atmosphere. These dissolve in water droplets in clouds to form acid rain (mostly sulfuric and nitric acid). Acid rain:

Kills trees (especially conifer forests) by damaging leaves and acidifying the soil.
Acidifies lakes and rivers, killing fish and aquatic invertebrates.

Fewer trees and fewer fish means lower biodiversity.

Global warming and climate change

Greenhouse gases — mainly CO₂ (from burning fossil fuels, deforestation, peat extraction) and methane (from cattle and rice paddies) — trap heat in the atmosphere. The consequences AQA expects you to know:

Rising sea levels as polar ice melts and warm water expands — coastal habitats flooded.
Melting ice caps and glaciers — loss of polar habitats for species like polar bears.
Changing weather patterns — droughts, floods, more extreme storms.
Species migration and extinction — organisms must move to cooler areas or die out; many cannot move fast enough.

All of these reduce biodiversity by removing or disrupting the habitats that species depend on.

Overgrazing

When too many cattle, sheep or goats are kept on a piece of land, they eat the plants faster than the plants can regrow. The plant species disappear, the soil dries out and erodes, and the animals that depended on those plants — insects, birds, small mammals — lose their food source and decline. Overgrazing converts grassland into bare or desertified land with very low biodiversity.

Introducing non-native species

A non-native species is one brought (deliberately or accidentally) to an area where it did not previously live. Examples include the grey squirrel in Britain, cane toads in Australia, and signal crayfish in UK rivers. Non-native species can:

Have no natural predators in their new home, so populations explode.
Outcompete native species for food and habitat.
Prey on native species that have not evolved defences against them.

The result is that native species decline and biodiversity falls.

Conservation programmes

Governments, charities and scientists run programmes to maintain biodiversity:

Breeding programmes for endangered species — zoos and reserves breed rare animals (giant pandas, condors, Arabian oryx) and release them into protected wild populations.
Protected areas — national parks, nature reserves, Sites of Special Scientific Interest (SSSIs) and marine reserves — where development and damage are restricted.
Tree planting and reforestation to restore lost forest habitats and remove CO₂.
Regenerating wildlife habitats — restoring peat bogs, hedgerows, ponds, wildflower meadows.
Sustainable agriculture — crop rotation, hedgerows, less fertiliser run-off, leaving field margins.
Recycling resources — reduces landfill waste, cuts demand for raw materials, lowers land pollution.

Each action addresses a different pressure. Together they slow biodiversity loss — but they cannot fully reverse it while population growth and resource use keep rising.

Exam tips

Always define biodiversity as the VARIETY of different SPECIES — not the number of organisms. Lose a mark if you write 'number of organisms in an ecosystem'.
For eutrophication (4 marks) the order is: algae grow rapidly → light blocked → plants and algae die → decomposers respire and use up oxygen → fish and other animals die. Memorise the order — examiners check the chain.
Peat questions always need TWO impacts: habitat destroyed AND stored CO₂ released. One impact alone caps you at 1 mark.
If a question mentions a non-native species, write 'no natural predators in the new habitat' as one of your marking points — it's the most common phrase in the mark scheme.
For global warming consequences, name AT LEAST TWO from: rising sea levels, melting ice, changing weather patterns, species migration / extinction. Don't just write 'it gets hotter'.
Conservation programme answers should include AT LEAST TWO different actions (e.g. breeding programmes + protected reserves) — single-action answers cap at low marks.
Whenever a question describes a habitat being destroyed, finish with 'organisms cannot survive so biodiversity decreases'. This phrase wins the final marking point in almost every spec 4.7.3 question.

Mark-scheme phrasing

Common misconceptions

Worked example

Question:

Answer:

Frequently asked questions

Why does biodiversity matter — isn't one species as good as another?

No — variety is the whole point. High biodiversity means many species, each with slightly different roles in the ecosystem. If one species disappears (a disease wipes out a particular tree, or a predator dies out), others can fill its role and the ecosystem keeps working. With low biodiversity there's no backup. A single pest, a drought or a change in temperature can collapse the whole community. Humans depend on biodiverse ecosystems for clean water, fertile soil, pollination of crops, fisheries, timber and medicines — so the long-term survival of the human species depends on maintaining biodiversity.

Why does removing peat for compost matter so much?

Two reasons, and AQA wants both. First, peat bogs are habitats for many specialist species — sphagnum moss, rare insects, wading birds, carnivorous plants like sundew. Digging up the peat destroys their habitat, so these organisms cannot survive and biodiversity falls. Second, peat stores enormous amounts of carbon that built up over thousands of years from partly-decomposed plant matter. When peat is dug up and dries out, decomposers can finally break it down — releasing the stored carbon as CO₂ to the atmosphere and contributing to global warming. So one activity causes both habitat loss AND climate change.

Why do fish die in a polluted river — surely the fertiliser itself isn't toxic to them?

Correct — fertiliser nitrates aren't directly toxic to fish at normal levels. The fish die because of OXYGEN starvation. The chain works like this: nitrates from fertiliser fuel a massive bloom of algae on the water surface. The algae block sunlight, so the plants growing underwater (and eventually the algae themselves) cannot photosynthesise and they die. Bacteria — decomposers — feed on the huge amount of dead plant matter and respire as they do so. Their respiration uses up the dissolved oxygen in the water. Once oxygen levels drop, fish and other aquatic animals cannot get enough oxygen for aerobic respiration, and they suffocate. This whole chain is called eutrophication and it's a five-step marking sequence in AQA papers.

What's so dangerous about a non-native species — surely a new species adds to biodiversity?

In the very short term a non-native species adds one more species to the area, but the longer-term effect is usually the OPPOSITE — biodiversity falls. The reason: in its native range a species has evolved alongside its predators, diseases and competitors, which keep its population in check. In a new place those controls are absent. The non-native species has no natural predators and the local species have not evolved any defences against it. So its population grows very fast, and it either outcompetes native species for food and habitat or eats them directly. Several native species can be driven to local extinction by one invasive arrival. Examples: grey squirrels driving out red squirrels in Britain, cane toads poisoning native predators in Australia.

If conservation programmes work, why is biodiversity still falling?

Conservation is fighting an uphill battle. Conservation programmes — breeding endangered species, protecting reserves, planting trees, sustainable farming, recycling — do slow biodiversity loss in specific places. But globally, the underlying drivers are still growing: human population is rising (around 8 billion and climbing), standards of living are rising (so per-person resource use is rising even faster), and demand for land, food, energy and materials keeps growing. Conservation can preserve patches and rescue individual species, but it cannot fully offset the worldwide pressure from habitat conversion and climate change. That's why AQA emphasises that 'only recently have measures been taken' — and why scientists argue we need to do much more.

Is global warming definitely caused by humans?

Yes — the scientific evidence is overwhelming. The current warming matches the rise in CO₂ from burning fossil fuels (and methane from cattle and rice farming) very closely. Ice cores show that CO₂ levels have not been this high for at least 800,000 years. Multiple independent lines of evidence — global temperature records, ice melt, sea level rise, ocean acidification — all point the same way. For AQA you need to know the causes (CO₂ and methane from human activity), the consequences (rising sea levels, melting ice, changing weather, species migration / extinction), and the link to biodiversity (habitats disrupted, species cannot adapt fast enough, extinctions increase).