How does NAWQA assess condition of biological communities?
Information about the composition of stream communities can be used to assess the degree to which biological communities differ from a natural state. In the NAWQA study, biological condition was assessed by comparing observed (O) community characteristics (such as the number of taxa) to those expected (E) if the community was minimally disturbed by human activities. Although observed condition (O) was derived from a sample collected at a site, expected (E) condition was estimated with data from a set of environmentally similar reference sites. Because natural variation in environmental settings is accounted for in this approach, departures of observed condition from expected condition are likely the result of human activities. Biological condition is defined as the ratio of observed to expected (O/E) condition as expressed as a percentage of the reference condition scaled from 0 percent to 100 percent. The lower the percentage, the more altered the biological community, whereas a highe
