The human population

• Increasing human population requires an increased food yield.
• This can involve the use of fertilisers and pesticides.
• Fertilisers provide chemicals such as nitrates which increase crop yield.
• Plants and animals which reduce crop yield can be killed by pesticides.

Nitrates

• Nitrates dissolved in soil water are absorbed into plants.
• Nitrates are used to produce amino acids which are synthesised into plant proteins.
• Animals consume plants or other animals to obtain amino acids for protein synthesis.
• Fertilisers can be added to soil to increase the nitrate content of the soil.

Fertilisers

• Fertilisers can leach into fresh water, adding extra, unwanted nitrates.
• This will increase algal populations which can cause algal blooms.
• Algal blooms reduce light levels, killing aquatic plants.
• These dead plants, as well as dead algae, become food for bacteria which increase greatly in number.
• The bacteria use up large quantities of oxygen, reducing the oxygen availability for other organisms.
• Genetically modified (GM) crops can be used to reduce the use of fertilisers.

Pesticides

• Pesticides sprayed onto crops can accumulate in the bodies of organisms over time.
• The build-up of toxic substances in living organisms is known as bioaccumulation.
• As they are passed along food chains, toxicity increases and can reach lethal

Alternatives to Pesticides

• Biological control and GM crops may be an alternative to the use of pesticides.
• Biological control is a natural predator of the pest.