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 .
The number of different organisms usually peaks in the spring, before the first hatch of mature insects occurs. Mayflies, stoneflies and caddisflies are sensitive to changes in water quality and habitat. The Latin names for the orders of these insects are Ephemeroptera, Plecoptera and Tricoptera. Healthy streams often have 10-15 different families of mayflies, stoneflies and caddisflies present. EPT is a subset of total taxa richness, and it also peaks in the spring.