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 indirect human intervention, are ranked by the same methodology and notation as for other native taxa.
Interspecific hybrids •
HYB - Modern
interspecific hybrids, typically encountered as isolated individuals, are rarely themselves targets of conservation attention, are generally given a placeholder global rank of HYB, and not ranked at the national or subnational levels. However, reproducing or other self-maintaining, population-forming
species known or suspected to be of hybrid origin are ranked using the same methodology and notation as for other species. For example, many fertile
polyploid species of
ferns formed by interspecific hybridization followed by
chromosome doubling. Some of these hybrid-derived species are quite rare (ranked G1), but others are so widespread, abundant, and secure as to deserve a G5 rank.
Taxa extant only in captivity or cultivation •
C - When appended to X or H, the letter 'C' indicates species or other taxa extant in captivity or cultivation, although otherwise extirpated or extinct.
Variant ranks •
#x# – Range of ranks due to uncertainty, where x would be a repetition of the initial letter for Global, National, or Subnational rankings, e.g. G2G3 would mean a global rank ranging from G2 to G3. Limited to two ranks of difference, beyond which the status would be U for Unrankable (i.e. N1N4 would instead be listed as NU). •
U – Unrankable, due to conflicting or absent information. •
NR – Not ranked, i.e. not yet assessed. •
NA – Not applicable, meaning not suitable for conservation activities, typically used for hybrids with no conservation value, or non-native ecosystems (e.g. agricultural fields). ==Combinations of ranks==