Hotspots of Biodiversity in the United States

Rarity-Weighted Richness Analysis of Critically Imperiled and Imperiled (G1 or G2) SpeciesData, Maps & Tools


A 2014 version will soon update this 2008 map of Hot Spots of Rarity-Weighted Richness for Critically Imperiled and Imperiled SpeciesThe “Hotspots” analysis can assist resource managers and planners by pointing to the places on the landscape that not only are rich in vulnerable species, but also offer an opportunity for a clear conservation impact on these species.



Conservation actions in areas identified as “irreplaceable” can be expected to have a more measurable impact on the long term well-being of a species, than those in an area that is one of many such areas.


Features & Benefits

The Rarity-Weighted Richness (RWR) analysis of critically imperiled and imperiled (G1 or G2) species conducted by NatureServe provides a picture of “hotspots” that represent concentrations of limited-range species and highlights locations with species composition different from adjacent areas. This approach helps address biases that occur in simple count-based analyses because widely distributed species, found in many different locations, have a disproportionate impact in creating areas of high diversity, with clusters of high-diversity areas that represent the same set of species.

By combining overall species richness and the relative rarity (based on restricted distributions) of the species, this analysis points to locations that are essentially “irreplaceable,” and which present conservation opportunities that are found in very few other places.

The RWR analyses are an update of analyses first completed for the publication, Precious Heritage (2000). The method is well described in that source: 

The logic behind this method is that a hexagon assumes increased conservation significance if a number of species occur only in that cell, since protection of these unique species cannot be accomplished elsewhere. At least with regard to those species, the hexagon is “irreplaceable” from a conservation perspective. On the other hand, if a hexagon contains a high diversity of species, all of which can be found in other hexagons as well, that cell would not be irreplaceable to the protection of those species (although it might still be extremely important in their conservation). Employing this concept of irreplaceability, two hexagons with the same number of imperiled species may differ considerably in their conservation significance. Each species is assigned a score—or weight—based on the inverse of the number of hexagons in which it occurs This score may be thought of as the hexagon’s index of irreplaceability. In addition, we have created a continuous surface map of rarity-weighted richness across the country.