NatureServe conservation status assessments: methodology for assigning ranks

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NatureServe and its member programs and collaborators use a suite of factors to assess the extinction or extirpation (regional extinction) risk of plants, animals, and ecosystems (or “elements” of biodiversity). By researching and recording information on a set of conservation status factors, biologists can assign a conservation status rank to these elements at both global and regional (i.e., national/ subnational) scales. The protocol for assigning a conservation status rank is based on scoring an element against ten conservation status factors, which are grouped into three categories based on the characteristic of the factor: rarity (six factors), trends (two factors), and threats (two factors). Once assigned, scores for the individual factors within each of these categories are pooled and the resulting three summary scores are combined to yield an overall numeric score, which is translated into a calculated rank. This calculated rank is reviewed, adjusted if deemed appropriate by the evaluator (with reasons documented), and recorded as the final assigned conservation status rank, using a G1-G5 scale for global element status, or the equivalent scale for national or subnational assessments. The conservation status factors that comprise each category help guide the consistent and rigorous recording of information to facilitate the assignment of a conservation status. Weights assigned to individual factors reflect their perceived influence on extinction risk for the element. The computation for the calculated status score relies on information from all assessed factors to assign a rank rather than from any single factor, and weights rarity factors the most, followed by trends factors, and then threats. This approach reflects the view of many conservationists that rarity has the most important, but not sole, influence on the probability that a species or ecosystem will become extinct. The set of factors used to assess conservation status, by category, are the following: • Rarity: Population Size, Range Extent, Area of Occupancy, Number of Occurrences, Number of Occurrences or Percent Area with Good Viability/ Ecological Integrity, and Environmental Specificity (used only when the Number of Occurrences and Area of Occupancy are unknown); • Trends: Long-term and Short-term Trend in population size or area; • Threats: Threat Impact (generated by considering the scope and severity of the major threats), and Intrinsic Vulnerability (used only if Threat Impact is unknown). Information for all ten conservation status factors is not required to assign a status. At a minimum, information for only two of eight “core” factors is needed, as long as these factors include either two rarity category factors (one of which must be either Range Extent or Area of Occupancy) or one rarity factor and one factor from the trends or threats category. Depending on the precision of the information supplied and the number of factors for which information exists, a specific conservation status rank (e.g., G1, S3) or range rank (e.g., G1G2, N2N4) can be derived. If the conservation status factor information does not meet the minimum necessary to assess conservation status, or the information for multiple factors is too imprecise, a “U” status rank is assigned (e.g., GU, NU). When assessing each of the ten conservation status factors, ratings are selected from either a logarithmic or categorical scale of values. For conservations status factors that are used by both NatureServe and IUCN (e.g., Area of Occupancy), break points for these scales coincide with those in the IUCN Red List criteria. If available information for a species or ecological type does not permit a single value for a factor to be determined, a range of values can be selected. NatureServe has developed this rank calculator to facilitate the process of assigning conservation status ranks through automation (NatureServe 2009). The calculator works in combination with NatureServe’s data management system, Biotics 4, which contains the element database, including the rank factor information and assigned conservation status ranks for all elements. The updated ranking system and new calculator represent a major upgrade of NatureServe ranking methods. They provide rank standardization—helping to increase the consistency, objectivity, and transparency of the conservation status assessments, facilitate maintenance of the ranks, promote NatureServe network collaboration, incorporate fields that were added previously to lend robustness to ranking, and provide utility in generating global ranks as well as national or subnational ranks. The revised factor definitions and values used in NatureServe’s updated (2009) conservation status assessment protocol are also designed to ensure international compatibility, including the ability to calculate IUCN ranks.

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