So what makes a good bioindicator? First, they should be easily identifiable - there is little chance for the broad application of a bioindicator if only a few specialists can identify them. Also, the species should be abundant in their preferred habitats over a wide geographical range, and should exhibit similar degrees of tolerance throughout their range. They should be fairly long-lived, and fairly immobile so that they cannot flee easily. And just as the presence of several ancient woodland indicator plants is a better indicator than a single one, so the best bioindicators tend to be "species assemblages".
The use of bioindicators to monitor pollution is now well established and well respected; indeed, it is integral to the maintenance of river quality in the UK. The Environment Agency uses four methods of study: an aesthetic survey; a chemical quality assessment; nutrient measurements; and biological surveys.
In biological surveys the bioindicators used are macro-invertebrates, a wide range of readily visible snails, worms, fly larvae and other species that are very prevalent in many of our water courses. Their suitability is partly explained by their ubiquitous nature, and partly their sedentary nature, and partly the tendency of some species to be affected by very low levels of pollutants. Most importantly, whereas a chemical analysis can only record the conditions present at the exact time a sample was taken, bioindicators can reveal if some pollution event has occurred at any stage in the recent past. It is on the wildlife communities that pollution events, so quickly washed downstream, leave their indelible and disastrous mark.