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When do you call a species "rare"? 

Wildlife biologists in Virginia have devised a new method for designating species as common or rare based on quantitative analysis of geographic distribution, abundance, and habitat specificity. The approach is designed to provide a rigorous framework that managers and conservationists can use to define priorities and strategies for protecting imperiled species.

Although developed originally with freshwater fishes in mind, the analytical method can be used with other taxonomic groups. Jeremy Pritt and Emmanuel Frimpong from Virginia Tech published the method and results in a recent article in Conservation Biology.

The new method addresses the fact that species rarity can be defined in multiple ways. A species may be regarded as rare because its local populations are small, it has very specific habitat requirements, and/or it has a small geographic range. The multiple dimensions of rarity add complexity and potential for flawed outcomes in what might appear to be relatively straightforward management decisions.

Pritt and Frimpong incorporated the three facets of species rarity into a single, quantitative index. By applying the new method, resource managers may uncover species that should be viewed as rare but are not yet recognized as such, or they might find that species currently designated as rare do not merit that status.

Using their method, Pritt and Frimpong analyzed the status of 399 freshwater and anadromous fish species in North America. For all of the species, they compiled data on local population sizes, geographic ranges, and habitat specificity. Based on the data, they defined quantitative thresholds for rarity, and they categorized each species as rare or common in each of the three characteristics.

The scientists compared their analytical findings to a list of imperiled species that is maintained by the American Fisheries Society (AFS). As expected there was much overlap. However the method developed by Pritt and Frimpong highlighted 30 fish species that are rare in all three dimensions but that were not listed by the AFS as imperiled. The scientists also found three species that were listed as imperiled by the AFS but which were categorized as common in all three dimensions.

The findings suggest that without a comprehensive, quantitative analysis of rarity, conservation practitioners may make misguided judgments about which species deserve protection. According to the authors,

“Our approach could be applied to other taxa to aid conservation decisions and serve as a useful tool for future revisions of listings of fish species.”

-- Reviewed by Peter Taylor

PRITT, J., & FRIMPONG, E. (2010). Quantitative Determination of Rarity of Freshwater Fishes and Implications for Imperiled-Species Designations Conservation Biology DOI: 10.1111/j.1523-1739.2010.01488.x

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