Element Occurrence Rank Definitions

The information below is provided as a summary for Recovery Document authors to help with assigning Element Occurrence (EO) rankings within the Species Status Assessment (SSA). For more detailed instructions and examples, please visit: NatureServe’s Ranking Species Occurrences Guidelines

A - Excellent viability. Robust population, signs of recruitment, no/low threats. Population is likely to remain stable for at least 20-30 years. 

B - Good viability. Robust population, signs of recruitment, moderate threats. Population is likely to remain stable for at least 20 years. Without management, the population will decrease after 20 years.

C - Fair viability. Population is small, has limited or no recruitment, and moderate threats OR the population is somewhat robust but has limited or no recruitment and high-extremely high threats which indicate immediate declines unless management is continued/initiated.  Population may last 20 years, but not likely to expand.

D - Poor viability. Population is very small, there are no signs of recruitment, and there are moderate-extremely high threats for which immediate management and reintroductions are likely needed for the population to survive. 

E: Verified extant Occurrence recently has been verified as still existing, but sufficient information on the factors used to estimate viability of the occurrence has not yet been obtained. Should only be used when a A-D rank cannot be assigned.

H: Historical. Recent field information verifying the continued existence of the occurrence is lacking. In the absence of known disturbance and with the habitat still extant, H is generally recommended for occurrences that have not been reconfirmed for 20 or more years (depending on the life history or habitat this time period may be shorter or longer). With very few exceptions, occurrences are to be regarded as H after 40 years without confirmation, even with no effort to locate the species.

F: Failed to find. Occurrence has not been found despite a search by an experienced observer at a time and under conditions appropriate for the Element at a location where it was previously reported, but the occurrence still might be confirmed to exist at that location with additional field survey efforts.

X: Extirpated. Adequate surveys by one or more experienced observers at times and under conditions appropriate for the species at the occurrence location, or other persuasive evidence, indicate that the species no longer exists there or that the habitat or environment of the occurrence has been destroyed to such an extent that it can no longer support the species.

EO ranks can be combined or may convey uncertainty. EO rank possibilities that can be used are listed below:

• A - Excellent estimated viability/ecological integrity
• A? - Possibly excellent estimated viability/ecological integrity
• AB - Excellent or good estimated viability/ecological integrity
• AC - Excellent, good, or fair estimated viability/ecological integrity
• B - Good estimated viability/ecological integrity
• B? - Possibly good estimated viability/ecological integrity
• BC - Good or fair estimated viability/ecological integrity
• BD - Good, fair, or poor estimated viability/ecological integrity
• C - Fair estimated viability/ecological integrity
• C? - Possibly fair estimated viability/ecological integrity
• CD - Fair or poor estimated viability/ecological integrity
• D - Poor estimated viability/ecological integrity
• D? - Possibly poor estimated viability/ecological integrity
• E - Verified extant (viability/ecological integrity not assessed)
• F - Failed to find
• F? - Possibly failed to find
• H - Historical
• H? - Possibly historical
• X - Extirpated
• X? - Possibly extirpated
• U - Unrankable
• NR - Not ranked