Farming's Impact on Biodiversity Revision Notes | A Level AQA Biology

1. Why Farming and Biodiversity Are in Conflict

Agriculture feeds billions of people, but it does so by reshaping ecosystems to serve a single purpose: producing food. Every time land is converted, simplified, or chemically managed to boost yield, the natural variety of life in that area tends to shrink. Understanding exactly how and why this happens is central to the AQA specification — and to any serious conversation about the planet's future.

The core tension is this: farming works best when conditions are controlled and uniform, but biodiversity thrives on complexity and variation. These two goals pull in opposite directions, and the history of modern agriculture is largely a story of productivity winning at biodiversity's expense.

2. How Farming Techniques Reduce Biodiversity

There is no single mechanism at work here — farming affects biodiversity through several distinct routes, each targeting a different part of the ecosystem. Thinking about them separately first makes it much easier to evaluate their combined impact.

Picture a traditional patchwork landscape: meadows, woodland edges, ponds, and mixed hedgerows all within a few kilometres of each other. Now replace it with a single wheat field stretching to the horizon. The structural complexity that supported hundreds of species — different plant heights, flowering times, seed types, and nesting sites — has been replaced by one uniform crop at one uniform height. Species that cannot use wheat as a food source or shelter disappear from that area.

Monoculture also reduces genetic diversity within the crop itself. When farmers plant a single high-yield variety, the genetic variation that once existed across many local varieties is lost. This matters because genetic diversity is the raw material for adaptation — a genetically uniform crop is far more vulnerable to a new pathogen or a shift in climate.

2a. Hedgerow Removal

To create fields large enough for modern machinery, farmers have removed vast lengths of hedgerow across the UK. Between the 1940s and 1990s, an estimated 50% of Britain's hedgerows were lost. This is not a minor aesthetic change — hedgerows function as linear nature reserves.

• Hedgerows provide nesting sites for birds such as yellowhammers, linnets, and whitethroats — all species that have declined sharply since intensive farming expanded.
• They act as movement corridors, allowing mammals, invertebrates, and reptiles to travel between habitat patches without crossing open, exposed ground.
• The shrubs and trees within hedgerows produce berries, seeds, and insects that support food chains extending well beyond the hedgerow itself.
• Root systems stabilise soil and reduce run-off, indirectly protecting aquatic habitats downstream.

When hedgerows are removed, these functions disappear simultaneously. Species that depended on the hedgerow for any one of these resources — food, shelter, or connectivity — lose access to all of them at once. Populations become isolated in remaining habitat fragments, reducing gene flow between them and increasing the risk of local extinction.

2b. Pesticide Use

Pesticides are chemicals applied to crops to kill organisms that reduce yield — insects, weeds, and fungi. From a farming perspective, they are highly effective. From a biodiversity perspective, the consequences extend far beyond the target species.

Herbicides remove the wild plant species that grow among crops. These plants — often dismissed as weeds — are the foundation of many food chains. Poppies, cornflowers, and fat hen provide seeds for farmland birds and nectar for pollinators. When herbicides eliminate them, the insects and birds that depended on them lose their food source, even if the pesticide never directly touched those animals.

Insecticides have a more direct effect. Non-target invertebrates — including bees, beetles, and hoverflies — are killed alongside the pest species. This matters enormously because many of these invertebrates are pollinators or are themselves food for birds and bats. The decline of farmland birds such as the grey partridge is closely linked to insecticide-driven reductions in the invertebrate prey their chicks depend on during the first weeks of life.

2c. Fertiliser Use and Eutrophication

Inorganic fertilisers supply nitrogen and phosphorus to crops, dramatically increasing yield. The problem arises when excess fertiliser is washed off fields by rain — a process called leaching — and enters rivers, lakes, and ponds.

Worked example: Tracing the Eutrophication Sequence

Describe the sequence of events by which nitrate fertiliser run-off leads to the death of fish in a river.

1. Nitrates and phosphates leach from fields into the river. — These nutrients are the limiting factor for algal growth in most natural water bodies, so their sudden increase triggers rapid reproduction.
2. Algae multiply rapidly, forming a dense surface layer. — The bloom blocks sunlight from reaching submerged aquatic plants, which then die because they cannot photosynthesise.
3. Aerobic bacteria decompose the dead algae and plants. — Bacterial respiration consumes dissolved oxygen from the water. This is the critical step — oxygen depletion, not the algae itself, kills the fish.
4. Dissolved oxygen concentration falls to levels that cannot support aerobic organisms. — Fish, invertebrates, and other aerobic aquatic species suffocate. The water body becomes dominated by anaerobic bacteria, drastically reducing species diversity.

Answer: Nitrate run-off → algal bloom → light blocked → aquatic plants die → bacterial decomposition → oxygen depleted → aerobic organisms including fish die.

Notice that eutrophication does not reduce biodiversity by directly poisoning organisms — it does so by altering the physical and chemical environment so that most species can no longer survive. This is an important distinction when writing exam answers: the mechanism is oxygen depletion driven by decomposition, not direct toxicity from the fertiliser.

2d. Drainage of Wetlands and Ploughing of Natural Habitats

Wetlands — marshes, bogs, and flood meadows — are among the most biodiverse habitats on Earth per unit area. They have been drained extensively to create arable land, destroying the specialised communities of plants, invertebrates, amphibians, and wading birds that depend on waterlogged conditions. Once drained, these habitats are effectively irreversible on any human timescale.

Similarly, ploughing permanent grassland or ancient meadow destroys decades or centuries of accumulated soil structure, seed banks, and invertebrate communities. A meadow that has never been ploughed may support over 100 plant species; a newly sown agricultural field typically supports fewer than ten.

3. The Conservation-Farming Balance

Recognising that farming reduces biodiversity is only half the specification requirement. The other half asks you to engage with the genuine tension between conservation goals and the practical demands of feeding a growing human population. This is where evaluation skills become essential.

The case for prioritising food production is not straightforward. Global population continues to rise, and agricultural land is finite. Intensive farming produces far more food per hectare than extensive or organic methods. Reducing yield per hectare to accommodate wildlife would require either more land to be converted to agriculture — harming biodiversity elsewhere — or accepting reduced food security.

The case for conservation is equally compelling. Biodiversity underpins the ecosystem services that agriculture itself depends upon: pollination, natural pest control, soil fertility, and water purification. Destroying biodiversity to maximise short-term yield can undermine the long-term productivity of the land. This is not a purely ethical argument — it is an ecological and economic one.

Several policy mechanisms attempt to navigate this balance. Agri-environment schemes in the UK pay farmers to manage land in ways that benefit wildlife — leaving field margins unsprayed, planting wildflower strips, or maintaining ponds. These schemes acknowledge that conservation has an economic cost that farmers should not bear alone.

3a. Conservation Strategies in Practice

Conservation strategies range from small-scale on-farm measures to national and international policy frameworks. At the farm level, the most effective approaches tend to restore habitat complexity without taking large areas out of production entirely.

• Unsprayed field margins: strips of land around field edges left free of pesticide and fertiliser, allowing wild plants and invertebrates to persist alongside the crop.
• Wildflower strips and beetle banks: deliberately sown areas of native flowering plants that provide nectar for pollinators and shelter for predatory beetles that naturally control pest populations.
• Hedgerow restoration and management: replanting removed hedgerows and managing existing ones by rotational cutting to maintain berry and flower production.
• Organic farming: avoids synthetic pesticides and inorganic fertilisers entirely, typically supporting higher species diversity on-farm, though at lower yields per hectare.
• Set-aside: taking a proportion of farmland out of production entirely and allowing natural vegetation to develop — a policy used in the EU from the 1990s onwards.
• Buffer zones along waterways: uncultivated strips beside rivers and streams that intercept fertiliser run-off before it reaches the water, reducing eutrophication risk.

Each of these strategies involves a trade-off. Unsprayed margins reduce the total cropped area slightly. Organic farming reduces yield. Set-aside removes land from production entirely. The question of how much productivity society is willing to sacrifice for biodiversity is ultimately a political and ethical one, but biology provides the evidence base for making that decision rationally.

4. Exam Technique: Writing High-Quality Answers on This Topic

This topic appears frequently in AQA papers as extended-response questions worth 6 or more marks, often using the command words 'evaluate', 'explain', or 'discuss'. The marks are structured to reward both sides of the argument and a reasoned conclusion — not listing farming harms.

A common pattern in student responses is to describe farming techniques accurately but then fail to explain the mechanism by which each technique reduces biodiversity. Examiners award marks for the causal chain, not the observation alone. For eutrophication, for example, stating that 'fertilisers cause water pollution' earns no marks; tracing the sequence from nutrient run-off through algal bloom to oxygen depletion earns full credit.

The farming-biodiversity conflict sits at the intersection of ecology, economics, and ethics. For AQA, the key is demonstrating that you understand the specific biological mechanisms by which each farming technique reduces biodiversity, and that you can evaluate conservation responses with the same precision. Vague answers about 'pollution' or 'habitat loss' will not reach the top band — named mechanisms, named strategies, and a reasoned conclusion will.