We use the U.S. Environmental Protection Agency's Rapid Bioassessment Protocols to collect the sample. These organisms spend months to years living on the stream bottom and their presence and the overall composition of the benthic community indicate changes in water quality and habitat degradation in the stream over time. Biologists use metrics to analyze samples of benthic invertebrates taken from a stream. Metrics are measures that summarize the numbers of organisms, the types of organisms, and the pollution tolerance of organisms. Taxa Richness , EPT and HBI are commonly used metrics.
Taxa richness is a measure of the number of distinct families of critters in the stream.
For example, a healthy stream has many different organisms. A healthy stream often has 20-25 families of different organisms. An impacted stream tends to have fewer types of organisms, and most of the critters present are tolerant to pollution and can live in poor habitat. As stream health improves, the number of different organisms increases and taxa richness generally increases. Taxa richness is also seasonal .