Pesticides
Chemical pesticides are substances that are sprayed onto crops to kill organisms that can reduce plant growth such as weeds, insects and fungi. Weeds compete with the crop plants for resources, fungi can cause plant diseases and insects may consume the plants and damage them.
Pesticides can have adverse effects on the environment if they are not biodegradable as they can accumulate in the bodies of organisms over time.
Because the animals tend to eat lots of organisms from the level below in food chains, the concentration of pesticide in the bodies of organisms increases at higher levels of food chains.
This can result in the toxicity of the pollutant reaching fatal levels in the organisms at the top of the food chain. This build-up of toxic substances in living organisms is known as bioaccumulation.
An example of a non-biodegradable, or persistent, pollutant is the insecticide DDT which was used in the 1940s to kill mosquitoes. It accumulates in body tissues because it is not very soluble in water so cannot be excreted.
The concentration of DDT in producers is very low, but increases greatly at higher levels in the food chain. This can cause top-level consumers like ospreys to lay eggs with much thinner shells that are more likely to break.
