Understanding Ranks

Sub-national (provincial/state) ranks

Conservation Data Centres and NatureServe use existing information and expertise, for ranking species rarity or conservation status. Ranks help them identify gaps in knowledge for species for which element occurrence data are maintained; typically information is maintained for species ranked extremely rare (S1) to uncommon (S3) in given jurisdictions. Individual CDCs are responsible for developing sub-national ranks for their area. The Atlantic CDC works with provincial and federal experts to develop rarity ranks for species in each of the following provinces: New Brunswick, Nova Scotia, Prince Edward Island, and Newfoundland & Labrador.

Factors considered when ranking include: number of element occurrences, distribution, population size, abundance trends, and threats.

Sub-national element rank definitions

The following definitions refer to species and community ranks at sub-national (provincial/state) levels.  Sub-national ranks are specific to a province or state.  Therefore, a species that is common (S4) in New Brunswick, could be ranked as extremely rare (S1) in Prince Edward Island.


Extremely rare: May be especially vulnerable to extirpation (typically 5 or fewer occurrences or very few remaining individuals).


Rare: May be vulnerable to extirpation due to rarity or other factors (6 to 20 occurrences or few remaining individuals).

S3 Uncommon, or found only in a restricted range, even if abundant at some locations (21 to 100 occurrences).


Usually widespreadfairly common, and apparently secure with many occurrences, but of longer-term concern (e.g., watch list) (100+ occurrences).


Widespread, abundant, and secure, under present conditions.


Numeric range rank: A range between two consecutive ranks for a species/community. Denotes uncertainty about the exact rarity (e.g., S1S2).


Historical: Previously occurred in the province but may have been overlooked during the past 20-70 years. Presence is suspected and will likely be rediscovered; depending on species/community.


Unrankable: Possibly in peril, but status is uncertain - need more information.


Extinct/Extirpated: believed to be extirpated from its former range.


Unranked: not yet ranked.


Accidental: Accidental or casual, infrequent and far outside usual range. Includes species (usually birds or butterflies) recorded once or twice or only at very great intervals, hundreds or even thousands of miles outside their usual range.


Exotic: An exotic established in the province (e.g., Purple Loosestrife or Coltsfoot); may be native in nearby regions.


Exotic numeric: An established exotic that has been assigned a rank.

SNR Unranked: Provincial conservation status not yet assessed.
SNA Not Applicable: A conservation status is not applicable because the species is either: a) exotic, b) not definitively known to occur in the province or c) a hybrid not considered to be conservation significance. A notes field giving the reason or reasons for the SNA rank is included on AC CDC downloadable species lists.


Potential: Potentially occurs, but no occurrences have been reported.


Reported but without persuasive documentation (e.g., misidentified specimen).


Reported falsely: erroneously reported and the error has persisted in the literature.


Zero: not of practical conservation concern because there are no definable occurrences, although the species is native and appears regularly. An SZ rank is generally used for long distance migrants that pass through the province occasionally.



Breeding (Migratory species)


Non-breeding (Migratory species)


Inexact of uncertain (the "?" qualifies the character immediately preceding it in the S-rank)


Captive of cultivated

National and Global Ranks

Information supporting S-ranks in turn supports broader-scale ranking, national (N-rank) and global (G-rank). Canadian CDCs, from the Atlantic to British Columbia, help develop and update N-ranks for Canadian plants and animals.  Although many believe that National ranks offer great value, there is increasing interest in categories used by COSEWIC and General Status Assessments, outlined briefly below. 

Global ranks are assigned by staff specialists at NatureServe in consultation with CDC specialists and other science experts. Global rank definitions are similar to sub-national rank definitions but they refer to the entire range for species or communities regardless of national boarders.  For instance, G1= Critically Imperiled – extremely rare and extremely vulnerable to extinction due to natural or human causes (5 or fewer global occurrences or less than 1000 individuals), while G5 = Demonstrably secure.  See the NatureServe explorer for additional details.

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COSEWIC and General Status of Wild Species

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) uses: extinct, extirpated, endangered, threatened, vulnerable, special concern, insufficient information, and secure to describe the status of species (but not communities) in Canada. The General Status of Wild Species in Canada, uses a ranking system similar to that used by NatureServe and all CDCs.  (See Wild 2000 – the general status of species in Canada – for additional details)

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