What do these freshwater critters indicate?
The invertebrate community you find at a particular site is shaped by a combination of physical habitat and water quality factors. There are many “sensitive” taxa that are strongly associated with “natural” (typically bush-covered) habitats with good water quality, and there are several “tolerant” taxa that can thrive in highly modified streams with poor water quality. The presence of tolerant taxa in a sample does not necessarily mean the stream is in poor condition because most tolerant taxa can survive quite happily in high quality and poor quality streams. Therefore it’s better to assess a community using the numbers of “sensitive” taxa and their abundances in your sample (or the proportion of the community made up of sensitive taxa).
The tolerance values (ranging from 0 to 10) listed in the Guide give an indication of which are the sensitive taxa (values of 8 or more) and which are the tolerant taxa (values of 3 or less). Some examples of sensitive and tolerant taxa are shown below: