Pollution kills plants and animals which can reduce biodiversity.

The idea

Pollution is the release of harmful substances into an environment. When pollutants enter air, water or soil, organisms may be poisoned, suffocated or have their habitats altered so that they cannot survive. For example, fertiliser washed from fields into a lake causes eutrophication: algae grow rapidly, then decompose, and the oxygen concentration falls so fish and other animals die. Similarly, sulfur dioxide from burning fossil fuels can damage leaves and acidify soils, while oil spills coat feathers and gills. Because different species tolerate pollution differently, sensitive species disappear first. The community becomes dominated by a few tolerant species, so species richness and genetic diversity fall. This is a reduction in biodiversity. The effect depends on the pollutant, its concentration, how long it persists and how quickly it is broken down or dispersed.

What earns the marks

• Pollution is the release of harmful substances into the environment, including air, water and soil.
• Pollutants can kill organisms directly by poisoning them or indirectly by damaging habitats and food sources.
• A named example, such as fertiliser causing eutrophication or sulfur dioxide damaging leaves, shows how a pollutant kills organisms.
• Killing plants and animals removes species from the community, so species richness decreases.
• The surviving community may contain only a few pollution-tolerant species, so biodiversity is reduced.
• The scale of the reduction depends on pollutant concentration, persistence and the sensitivity of different species.

Common mistakes and how to put them right

• Treating pollution as only visible litter.

  Put it right: Pollution includes gases, excess nutrients, pesticides, oil and other harmful substances that may be invisible.

• Assuming all species are affected equally.

  Put it right: Species differ in tolerance, so sensitive species are lost first and tolerant species may survive.

• Saying biodiversity increases because a few tolerant species become abundant.

  Put it right: Biodiversity depends on the variety of species, not the number of individuals of one species.

Exam tips

• Name a specific pollutant and a specific affected organism to make the link between pollution and death concrete.
• Use the chain pollution → kills organisms → fewer species → reduced biodiversity rather than stopping at the word pollution.
• When describing eutrophication, keep the sequence in order: nutrient enrichment, algal growth, decomposition, oxygen depletion, death of animals.