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 Any NatureServe rank may be used alone, or G-, T-, N-, and S- ranks may be combined in that sequence, such as a G5N3S1 rank for a particular species (or ecological community) within a particular subnational unit of a particular nation. An entity has only a single global rank (G-rank alone, or G-rank and T-rank combination), but may have different N-ranks or S-ranks for different nations or subnations within its geographical range. See also Conservation status Listing priority number References External links https://explorer.natureserve.org/ Biota by conservation status Biota of North America by conservation status Biota of Mexico by conservation status Biota of the United States by conservation status Flora of the United States by conservation status Fauna of the United States by conservation status es:Estado de conservación#NatureServe