Biodiversity and Human Impact

A tropical rainforest hums with thousands of different species; a ploughed field of a single crop holds very few. The difference is biodiversity — the variety of life — and it turns out to be one of the most important things about an ecosystem. A community rich in species is more stable and more able to withstand change, while one that has lost its variety is fragile. Yet human activity is reducing biodiversity in many parts of the world, through the way we use land, the pollution we produce and the demands of a growing population. This lesson, part of Topic B4 of OCR Gateway Science A, explains what biodiversity is and why it matters, surveys the main human impacts on it, and looks at the ways biodiversity can be maintained. Throughout, we describe trends qualitatively — the patterns and directions of change — rather than quoting specific figures.

By the end of this lesson you should be able to define biodiversity and explain why high biodiversity makes ecosystems stable, describe the main ways humans reduce biodiversity, and describe measures used to maintain it.

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What Biodiversity Means

Biodiversity is the variety of different species of organisms living in an ecosystem (or on the Earth as a whole). An ecosystem with high biodiversity contains many different species; one with low biodiversity contains few. A natural woodland, with its many trees, shrubs, herbs, insects, birds and mammals, has high biodiversity; a field planted with a single crop (a monoculture) has low biodiversity.

Biodiversity matters because it makes ecosystems more stable. The reason links directly to the interdependence and food webs you met earlier in B4:

• In a community with many species, there are many feeding relationships (a complex food web). If one species declines, predators can switch to other prey and the effect is buffered — the community does not collapse.
• High biodiversity means species are less dependent on any single other species, so the loss of one species does less damage.
• A varied community is better able to recover from disturbance and to withstand changes in conditions, because some species are likely to cope even if others do not.

So the higher the biodiversity, the less dependent each species is on others and the more stable the ecosystem. A low-biodiversity ecosystem, by contrast, has few links: lose one species and the whole community can be thrown out of balance.

Exam Tip: Link biodiversity to stability explicitly. The marking point is that high biodiversity → less dependence on single species / more food-web links → more stable, better able to withstand change. Do not just say "more species is good" — say why it makes the ecosystem stable.

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Why Human Activity Threatens Biodiversity

The human population has grown rapidly and continues to grow, and as it does, the demand for resources — food, water, energy, land and raw materials — rises with it. Meeting that demand puts pressure on ecosystems in several ways: we take over more land, we extract more resources, and we release more waste and pollution. The overall effect, in many places, is a reduction in biodiversity as habitats are lost or damaged and species decline. The sections below set out the main routes by which this happens.

flowchart TD
  A["Growing human population<br/>+ rising resource use"] --> B["More land taken for<br/>farming, building, quarrying"]
  A --> C["More pollution<br/>(air, water, land)"]
  A --> D["More deforestation<br/>+ peat destruction"]
  B --> E["Habitats lost or damaged"]
  C --> E
  D --> E
  E --> F["Reduced biodiversity"]

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Human Impacts on Biodiversity

Land use

As the human population grows, more land is used for housing, farming, industry and quarrying. Building on or ploughing up natural habitats destroys them, leaving fewer places for wild species to live and reducing biodiversity. Even where land is farmed productively, replacing varied natural vegetation with a single crop (monoculture) sharply lowers the variety of species the land can support.

Deforestation

Deforestation is the large-scale clearing of forests — often to provide land for farming (including grazing cattle and growing crops), for timber, or for building. Removing forests destroys the habitats of the enormous number of species that live there, reducing biodiversity. Deforestation also releases carbon dioxide (when trees are burned and as dead wood is decomposed) and reduces the amount of CO₂ that would have been removed by the trees' photosynthesis — a link to the carbon cycle you study next.

Peat bog destruction

Peat bogs are wet, boggy areas where, because the ground is waterlogged and low in oxygen, dead plant material does not fully decompose and builds up over thousands of years as peat. Peat is dug up for use as a fuel and as compost in gardening. Destroying peat bogs destroys the habitat of the specialised species that live there, reducing biodiversity. It also releases carbon dioxide: the stored peat decomposes or is burned, returning to the air the carbon that had been locked away — adding to atmospheric CO₂.

Pollution

As resource use rises, so does pollution, which can occur in three places:

• Air pollution — gases such as sulfur dioxide (causing acid rain) and smoke from burning fuels, which can damage plants and harm animals.
• Water pollution — sewage, fertilisers and chemicals washing into rivers and lakes; fertilisers in particular can trigger excessive growth of algae that depletes the water's oxygen, killing aquatic animals.
• Land pollution — toxic chemicals, pesticides and waste tipped onto land, which can poison soil organisms and the species that feed on them.

In every case, pollution kills organisms or damages their habitat, reducing the variety of species that can survive — so pollution lowers biodiversity.

Human impact	What happens	Effect on biodiversity
Land use (building, farming, quarrying)	Natural habitats built on or ploughed up	Habitats destroyed; fewer species; lower biodiversity
Deforestation	Forests cleared for farmland, timber, building	Habitats lost; releases CO₂; lower biodiversity
Peat bog destruction	Peat dug up for fuel/compost	Specialised habitats lost; releases CO₂; lower biodiversity
Pollution (air, water, land)	Harmful substances released	Organisms killed or habitats damaged; lower biodiversity
Growing population + resource use	Rising demand for land, food, energy	Drives all of the above; overall pressure on biodiversity

Exam Tip: When asked how a human activity reduces biodiversity, give the mechanism: not just "deforestation is bad" but "deforestation destroys habitats, so the species that lived there decline, reducing the variety of species". Describe the trend qualitatively — there is no need to quote figures, and you should not invent any.

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Maintaining Biodiversity

Many programmes now aim to protect and restore biodiversity. You should be able to describe several approaches:

• Conservation — actively protecting species and habitats, for example through breeding programmes for endangered species and protecting their habitats.
• Setting up protected areas — national parks and nature reserves where habitats are preserved and damaging activities restricted.
• Reforestation — replanting trees in areas that have been deforested, restoring habitats and removing CO₂ from the air.
• Reducing deforestation and protecting peat bogs (and using peat-free compost) so existing habitats and carbon stores are not lost.
• Reducing, reusing and recycling waste — cutting the amount of land used for landfill and the resources extracted, which lessens habitat destruction and pollution.
• Reducing pollution — for example treating sewage and industrial waste before release, and cutting harmful emissions.