Pollution is loss of purity and contamination of any form of life. There are many types of chemical which, when added to air, water or soil (whether by accident or on purpose) can have a harmful effect on plants and animals.   

Insecticides and herbicides are widely used by farmers. Unfortunately some of these chemicals are very persistent - they do not 'break down' or decay in nature. One result of this is that they tend to be passed along food chains. A caterpillar containing a very tiny quantity of a poison may be eaten by a small bird and this, in its turn, may be taken by a bird of prey. Both owls and peregrine falcons have been found with very large amounts of certain insecticides in their bodies.     

Often their eggs have not hatched, so that their numbers have declined. This may imbalance a whole community of plants and animals. 

Chemicals used on farmland are often washed out of soils into streams and rivers where they contaminate the fish. Fish-eating birds such as the kingfisher and certain predaceous fish have suffered in a similar way to the owl and the peregrine falcon. Sometimes a drum that contained a poisonous chemical is left not quite empty in a drainage ditch, so that the water may be poisoned for many kilometres downstream.

Factories sometimes release poisonous waste into rivers, or the sea, but laws are being passed to stop this. The control of ships at sea is more difficult. Sometimes oil is released deliberately or when a ship is damaged. When an oil tanker, the Torrey Canyon, foundered on rocks off the Cornish coast in March, 1967, many thousands of tonnes of crude oil were released. Thousands of sea-birds were 'oiled', became unable to fly, and died. Shellfish and other seashore creatures along the coasts of south-west England and northern France were also killed by this pollution.

The air is also often polluted. The burning of petrol and coal releases gases such as sulphur dioxide into the air. These are poisonous to many plants and animals. Where the air is pure, away from cities for example, in western Scotland and Ireland, lichens of many types cover the branches of trees and the rocks, but only a few hardy types survive in large towns.

The passing of laws to reduce air pollution, as well as making our country cleaner to live in, is thus an important part of conservation.

Small birds such as chaffinches, and mammals such as hedgehogs, that live in the hedgerows, may be killed by passing vehicles. Carrion crows are frequently seen eating creatures that have perished in this way.