Flexi Says: Stream health is often assessed using an invertebrate species index (ISI). Benthic (bottom-dwelling) invertebrates are good indicators of the environmental quality of a stream because they are widespread, long-lived, individuals tend to stay in one place making them easier to count, and many species are sensitive to pollutants. This type of index contains a large number of species from a relatively healthy, undisturbed environment. Scientists then compare the numbers and species of invertebrates from test sites to the reference index. This provides an assessment of biodiversity and overall stream health for the test site.