5 or fewer occurrences, or 1,000 or fewer individuals). 2 — Imperiled — (typically having 6 to 20 occurrences, or 1,001 to 3,000 individuals). 3 — Vulnerable — (rare; typically having 21 to 100 occurrences, or 3,001 to 10,000 individuals). 4 — Apparently secure — (uncommon but not rare, but with some cause for long-term concern; typically having 101 or more occurrences, or 10,001 or more individuals). 5 — Secure — (common, widespread, abundant, and lacking major threats or long-term concerns). Letters X - Presumed extinct or extirpated (not located despite extensive and intensive searches, with rediscovery not reasonably expected). Note that extinction is here considered a global (range-wide) phenomenon, while extirpation applies to loss within a particular national or subnational area, with the entity still extant elsewhere. H - Possibly extinct or extirpated (of historical occurrence but not known recently extant, with some reasonable hope of rediscovery). R or ? - Recorded within a nation or subnation, but local status not available or not yet determined. When combined with a global rank of G1 to G3, local status is 'Indeterminate,' but the entity is nevertheless presumed vulnerable, if still extant. Thus, for example, a G3 species is "globally vulnerable", and an N2 species is "nationally imperiled" for the particular country the rank is assigned. Species with G, N, or S rankings of 4 or 5 are generally not the basis for major conservation actions. Ranks for additional cases Several less frequent special cases are addressed through other notation in the NatureServe ranking system, including: Subspecies and plant varieties T - When desired, infraspecific taxa (subspecies, plant varieties, and other designations below the level of the species) may be assigned global T-ranks. A T-rank is appended to the G-rank for the including species. N-ranks and S-ranks presented with T-ranks apply to the particular infraspecific taxon, not its including species. Most taxa given such ranks have trinomial (three-word) rather than binomial (two-word) scientific names. Non-native (exotic) taxa E - Used at the national or subnational levels, E indicates taxa not native in the specified area, even historically, but currently or historically present there due to direct or indirect human intervention; such taxa are often termed exotic, escaped, non-native, adventive, or waif. Note, however, that regionally native species or other taxa that have recently arrived in the area of interest by natural means (such as wind, floods, or birds), without direct or