About the Heritage Network Ranking System

The classification scheme that CNHP and The Natural Heritage Network use to track rare species and natural communities is a standardized ranking system that allows the Heritage Network members and cooperators to target the most at risk species and ecosystems for inventory, protection, research, and management. Species and ecosystems are ranked on the Global (G), National (N), and Subnational/State/province (S) levels. The basic ranks used to classify species and ecosystems are:

1 = Critically Imperiled (Example: G1 = Globally Ranked Critically Imperiled)
2 = Imperiled (Example: N2 = Nationally Ranked Imperiled)
3 = Vulnerable to Extirpation (Example: S3 = State Ranked Vulnerable to Ext.)
4 = Apparently Secure
5 = Demonstrably Widespread, Abundant, and Secure

There are numerous additional ranks and associated criteria used by the Heritage Network, and they can be seen in the tables below.

Accepted Global (G), National (N), and Subnational/State/Province (S) Ranks

Presumed Extirpated or Extinct - Element is believed to be extirpated from the nation or subnation, or globally extinct. Not located despite intensive searches of historical sites and other appropriate habitat, and virtually no likelihood that it will be rediscovered.
Possibly Extirpated or Extinct (Historical) - Element occurred historically, and there is some expectation that it may be rediscovered. Its presence may not have been verified in the past 20 years. An element would become GH, NH, or SH without such a 20-year delay if the only known occurrences were destroyed or if it had been extensively and unsuccessfully looked for. Upon verification of an extant occurrence, NH or SH-ranked elements would typically receive a G1, N1, or S1 rank. These ranks should be reserved for elements for which some effort has been made to relocate occurrences, rather than simply using this rank for all elements not known from verified extant occurrences.
Critically Imperiled - Critically imperiled because of extreme rarity or because of some factor(s) making it especially vulnerable to extirpation or extinction. Typically 5 or fewer occurrences or less than 1000 remaining individuals.
Imperiled - Imperiled because of rarity or because of some factor(s) making it very vulnerable to extirpation or extinction. Typically 6 to 20 occurrences or between 1,000 and 3,000 remaining individuals.
Vulnerable - Vulnerable either because rare and uncommon, or found only in a restricted range (even if abundant at some locations), or because of other factors making it vulnerable to extirpation or extinction. Typically 21 to 100 occurrences or between 3,000 and 10,000 remaining individuals.
Apparently Secure - Uncommon but not rare, and usually widespread. Possible cause of long-term concern. Usually more than 100 occurrences and more than 10,000 individuals.
Secure - Common, widespread, and abundant. Perpetually secure under present conditions. Typically with considerably more than 100 occurrences and more than 10,000 individuals.
Unranked - Rank not yet assessed.
Unrankable - Currently unrankable due to lack of information or due to substantially conflicting information about status or trends.
Range Rank - A numeric range rank (Example: S2S3) is used to indicate the range of uncertainty about the exact status of the element. Ranges cannot skip more than one rank (Example: SU is used rather than S1S4).
HYB Hybrid - Element not ranked because it represents an interspecific hybrid, not a species.
Exotic - An exotic established in the nation or subnation; may be native in nearby regions (Example: house finch or catalpa in eastern U.S.).
Exotic Numeric - An exotic established in the nation or subnation that has been assigned a numeric rank to indicate its status, as defined for N1 or S1 through N5 or S5.
Accidental - Accidental or casual in the nation or subnation, in other words, infrequent and outside usual range. Includes species (usually birds or butterflies) recorded once or only a few times at a location. A few of these species may have bred on the one or two occasions they were recorded. Examples include European strays or western birds on the East Coast and vice-versa.
Zero Occurrences - Present but lacking practical conservation concern in the nation or subnation because there are no definable occurrences, although the taxon is native and appears regularly in the nation or subnation. An NZ or SZ rank will generally be used for long distance migrants whose occurrences during their migrations have little or no conservation value for the migrant, as they are typically too irregular (in terms of repeated visitation to the same locations), transitory, and dispersed to be reliably identified, mapped, and protected. In other words, the migrant regularly passes through the nation or state, but enduring, mappable Element Occurrences cannot be defined. Typically, the NZ or SZ rank applies to a non-breeding population in the nation or subnation - for example, birds on migration. An NZ or SZ rank may in a few instances also apply to a breeding population, for example, certain Lepidoptera which regularly die out every year with no significant return migration. Although the NZ or SZ ranks typically apply to migrants, it should be used discriminately. NZ or SZ only apply when the migrants occur in an irregular, transitory, and dispersed manner.
Potential - Potential that element occurs in the nation or subnation but no extant or historic occurrences are accepted.
Reported - Element reported in the nation or subnation but without a basis for either accepting or rejecting the report, or the report not yet reviewed locally. Some of these are very recent discoveries for which the program hasn't yet received first-hand information; others are old, obscure reports.
Synonym - Element reported as occurring in the nation or subnation, but the national or state data center does not recognize the taxon; therefore the element is not assigned a national or subnational rank.
* N or S rank that Has been assigned and is under review. Contact the individual subnational Natural Heritage Program for the assigned rank.
Not Provided Species known to occur in this nation or subnation. Contact the individual subnational Natural Heritage Program for assigned rank.

Other Subrank Qualifiers

Subrank Definition
B Breeding - Basic rank refers to the breeding population of te element in the nation or subnation (Example: S2B = Subnational Imperiled - Breeding Population).
N Nonbreeding - Basic rank refers to non-breeding population of the element in the nation or subnation (Example: S3N = Subnational Vulnerable - Non-Breeding Population).
? Inexact or Uncertain - Denotes inexact or uncertain numeric rank (Example: SE? = Uncertain Subnational Exotic Rank).
C Captive or Cultivated - Present populations are only found in captivity or cultivation, or as a reintroduced population not yet established (Example: G1C = Globally Critically Imperiled in Captive or Cultivated Populations Only).
T Infraspecific Taxon (trinomial) - The status of the subspecies or varieties (taxa) are indicated by a "T-rank" following the species' global rank (Example: G2T1 = Globally Imperiled Species with Subspecies or Variety in Question Critically Imperiled).