1. We used a recently developed Index of Centres of Density (ICD) to assess conservation value of thirty-nine sites in the upper Clinch River drainage of Virginia and Tennessee, U.S.A. The ICD incorporates information on the population density of each species at a site relative to sites throughout the region.
2. Value assessments based on the ICD were compared to those based on species richness. Species richness at sites was not related to ICD scores, but collective species lists from high-ranking sites were similar for both approaches. All sites with rare species had either a high ICD score or high species richness.
3. Four community types (defined by physiography and stream size) were better represented in sites with high-ranking ICDs than in sites with high-ranking species richness. Sites with high ICD scores were distributed uniformly throughout the region.
4. The ICD is a more powerful tool than species richness for assessing conservation value because the ICD identifies areas with regionally rare species, especially viable populations, or distinctive communities, all of which are key components of a region’s biodiversity.