AQA GCSE Biology: Pollution and Waste

As the human population grows and living standards rise, we produce more waste and pollution. This guide explains why, and works through the three kinds of pollution and their effects on biodiversity. The recurring point is that pollution kills organisms and so reduces biodiversity.

Why waste is increasing

Humans are using more resources and producing more waste because of the rapid growth in the human population and the rise in the standard of living. Unless waste and chemicals are handled properly, more pollution is caused.

Water pollution

• Fertiliser run-off makes algae grow, which blocks light so plants below die and decompose; the decomposers use up the oxygen in the water, so aquatic animals die.
• Sewage may contain pathogens, and has the same oxygen-depleting effect.
• Toxic chemicals, such as pesticides and herbicides, build up in food chains and poison consumers.

The first of those is the one that comes up most, and it has to be given as a sequence rather than as a fact. Fertiliser washes off fields into a river. The extra mineral ions let algae grow fast across the surface. That layer blocks the light, so the plants underneath cannot photosynthesise and they die. Decomposers feed on all that dead material and multiply, and because they respire aerobically they strip the oxygen out of the water. The fish and insects suffocate. Each of those five steps is a mark, and answers that jump from fertiliser to dead fish collect one of them.

The detail students lose is why the fertiliser causes a problem at all, since it is not poisonous. It does not kill anything directly. It feeds algae, and what kills is the oxygen those algae indirectly take away after they and the plants below them die.

Air pollution

• Smoke contains particulates that can damage lungs and block light, reducing photosynthesis.
• Acidic gases such as sulfur dioxide and nitrogen oxides, from burning fossil fuels, damage lungs and cause acid rain, which acidifies soil (harming roots and leaves) and water (harming fish gills).

Land pollution

• Landfill from household waste takes up land.
• Toxic chemicals, such as pesticides and nuclear waste, build up in food chains and poison consumers.

The impact on biodiversity

Acid rain is worth separating out, because it reaches things a long way from where the gas was burned. Sulfur dioxide and nitrogen oxides dissolve in cloud water and fall somewhere else entirely, which is why acid rain is a problem between countries rather than within one. In soil it lowers the pH, which damages roots and releases aluminium ions that would otherwise stay locked in the rock. In lakes it lowers the pH of the water and damages fish gills. Trees can die hundreds of miles from the power station.

Toxic chemicals and food chains

The same phrase appears under both water and land pollution, and it describes one mechanism. A pesticide that is not broken down and not excreted stays in the body of whatever eats it. A small fish eats many contaminated insects and keeps the pesticide from all of them. A bird eats many of those fish and keeps the pesticide from all of them in turn. The concentration therefore rises at every trophic level, and the organisms that suffer first are the top predators, even though they were never sprayed.

That is why the answer to "why were the birds of prey affected most?" is never "they were nearest the pesticide". They were furthest from it.

The impact on biodiversity

All of these forms of pollution kill plants and animals, which reduces biodiversity.

Notice that the three kinds of pollution are organised by where the pollutant ends up, not by what it is. The same toxic chemical appears under water pollution and land pollution because it can arrive by either route, and sulfur dioxide damages lungs as a gas and damages soil after it has fallen as rain. When a question asks you to classify a pollutant, it is asking where it acts.

How this fits together

Pollution is one way humans harm ecosystems. Others are the ways we use and clear land, covered in the land use, deforestation and global warming guide.