In this book, various types of pollution have been described which affect the quality of the air, land and water. The pollutants can usually be measured by analytical chemistry and, in Chapter 12, different methods of chemical analysis are described. However, chemical analysis tells you only the amount of the pollutant present and nothing about its effect on the environment. For this, we need biological methods because living organisms are exposed to pollutants and react according to the length of the exposure and their sensitivity to the pollutant. However, before we can assess whether the organisms are affected by a pollutant, we need first of all to look at an unaffected community. This is an essential part of any scientific investigation, whether it be testing a new medicine or a weedkiller: you have to know what is the state of the unaffected environment before you can draw any conclusions about the impact of the pollutant.

When studying pollution of the environment, the usual technique is to look at those species most sensitive to pollution. For example, for assessing air pollution, there are numerous plants and trees that can tolerate a variety of pollutants at quite high levels, as is shown by the numbers of trees and flowers in city centres where the air is of poor quality because of traffic fumes. However, the lichens that grow on stonework are much more sensitive and there are few, if any, in polluted city centres, although they are numerous in clean air areas. The lichens are known as indicator organisms because a study of the number and variety present gives an indication of the quality of the air.

When assessing water pollution, there are various aquatic organisms that can be studied – fish, water weeds, plankton and invertebrates. The variety of fish life, however, is limited and they can often swim away from pollution when it occurs. The water weeds cannot move away to avoid pollution but they usually die off in winter so can be studied only during summer months. The study of plankton is a very specialist subject so they are not considered to be suitable indicator organisms.