Conservation Status Assessment

Identifying Threatened Species and EcosystemsNormes et Méthodes

Vue d'Ensemble

This method assesses the conservation status of species and ecosystems–specifically the extinction risk of species and elimination risk of ecosystems at global scales, and their extirpation risk at national and subnational scales. NatureServe and Natural Heritage Program staff across North America collect and evaluate data for species and ecosystems of concern using these methods and tools to ensure that assigned status ranks are accurate and consistent, based on current field and remote sensing information.

Conservation status assessments are completed to produce conservation status ranks that measure extinction or extirpation risk at three geographic scales: global, national, and subnational. Global, National, and Subnational Ranks (or “G-Ranks,” N-Ranks” and “S-Ranks”) are widely used throughout the conservation community and are regarded as highly credible by scientists, government agencies and private-sector organizations. These assessments are also a valuable resource for government agencies responsible for administration of Federal, state and provincial species conservation laws.

Global Rank Definition                                                                                  
Global Rank Definitions
GX Presumed Extinct (species)/Eliminated (ecological communities and systems) — Species not located despite intensive searches and virtually no likelihood of rediscovery. Ecological community or system eliminated throughout its range, with no restoration potential.
GH Possibly Extinct (species)/ Eliminated (ecological communities and systems) — Known from only historical occurrences but still some hope of rediscovery. There is evidence that the species may be extinct or the ecosystem may be eliminated throughout its range, but not enough to state this with certainty.
G1 Critically Imperiled—At very high risk of extinction due to extreme rarity (often 5 or fewer populations), very steep declines, or other factors.


Imperiled—At high risk of extinction or elimination due to very restricted range, very few populations, steep declines, or other factors.
G3 Vulnerable—At moderate risk of extinction or elimination due to a restricted range, relatively few populations, recent and widespread declines, or other factors.
G4 Apparently Secure—Uncommon but not rare; some cause for long-term concern due to declines or other factors.
G5 Secure—Common; widespread and abundant.


Determining which plants, animals and ecosystems are thriving and which are rare or declining is crucial for targeting conservation of the species, habitats and ecosystems in greatest need. NatureServe and its member Natural Heritage Programs have developed a rigorous and consistent method for evaluating the relative imperilment of both species and ecosystems based on the best available science.

The purpose of the conservation status ranks developed by NatureServe is to assess the relative risk facing a species or ecosystem and does not imply that any specific action or legal status is needed to assure its survival. NatureServe is a non-advocacy organization and is committed to providing objective and reliable scientific data.

Caractéristiques & Avantages

NatureServe and its member programs and collaborators use a rigorous, consistent, and transparent methodology to assess the conservation status (extinction or extirpation risk) of species of plants, animals, and fungi, as well as the conservation status (elimination or extirpation risk) of ecosystems (ecological communities and systems).

Rank Factors

  • Eight core status rank factors are identified as relevant to risk assessments of extinction/elimination, or extirpation.
  • Descriptions of each factor include the basis for its use, and its evaluation and rating criteria.


  • Factors are organized into three categories (rarity, threats, trends).
  • Conditional rules for use of factors are applied to ensure that adequate information is used for assessing status.
  • Factors are scaled and weighted according to their impact on risk.
  • Consistent factor scaling and weighting allow the use of points to effectively score the contribution of each factor to risk.
  • Scores are weighted and combined by category resulting in an overall calculated rank, which is reviewed, and a final conservation status rank assigned.


  • A rank calculator automates the process of assigning conservation status ranks.
  • NatureServe’s Biotics database provides management for all conservation status information.

Conservation status is summarized as a series of ranks derived at global, national, or subnational (state/provincial) levels on a five-point scale from critically imperiled (G1, N1, S1) to secure (G5/N5/S5).

NatureServe’s methods, which have been evolving since 1978, are used by its network of Natural Heritage Programs and Conservation Data Centers throughout North America. In recent years, NatureServe has worked with IUCN to standardize the ratings for shared information fields, such as Range Extent, Area of Occupancy, Population Size, and Threats. This standardization permits the sharing of information among organizations and countries, and allows the information to be used in both IUCN as well as NatureServe assessments.

NatureServe conservation status assessment methodology contains a number of features, most notably that it (1) considers all of the status factor data collectively in assigning a status; (2) may produce “range-ranks,” (e.g., G1G3 = globally critically imperiled to vulnerable) to transparently reveal the degree of uncertainty in a status when the available information does not permit a single status rank; (3) explicitly considers threats in the assessment; (4) assesses conservation status for both species and ecosystems; and (5) is sufficiently complete for North American species that global, national, and subnational ranks are routinely linked to facilitate conservation priority setting.