Small‐ranged species in biodiversity coldspots: maximum rarity, rarity‐weighted richness, and complementarity‐based priorities in the contiguous United States

Species with small ranges tend to occur with other range‐limited species in biodiversity hotspots, a pattern that has motivated both macroecological theory and practical conservation planning. However, not all range‐limited species follow this pattern. Some occur in otherwise species‐poor ‘coldspots' where maps of rarity‐weighted richness (RWR) may systematically undervalue their habitat. I used maps of suitable habitat for 1719 species of mammals, birds, amphibians, and reptiles in the contiguous United States at ~ 1 and ~ 50‐km resolution to investigate the relationship between range size of species and co‐occurring RWR. I compared the ability of RWR to efficiently represent species to a complementarity‐based algorithm (core area zonation, CAZ), and a new metric I introduce here based on maximum rarity of species (maxR), which ranks locations based on occurrence of the most range‐limited species. Range size was negatively correlated with co‐occurring RWR across all taxa and resolutions, confirming that species with small ranges tend to co‐occur with other range‐limited species. Despite high correlations among prioritization methods, they differed modestly in performance. CAZ most efficiently represented species across nearly all taxa, while maxR generally outperformed RWR when counting species based on presence or low thresholds of proportional habitat. When representation of a species was based on higher proportional thresholds of each species range, RWR tended to more efficiently represent more species than maxR. Comparisons between maxR and RWR methods revealed range‐limited species that occur in relative coldspots, including 53 species listed as ‘Threatened' or ‘Endangered' under the U.S. Endangered Species Act. Mapping maximum rarity should not replace complementarity‐based algorithms for spatial conservation planning but does offer an easily computed diagnostic for detecting range‐limited species in coldspots of biodiversity for use in biodiversity mapping.