The NatureServe conservation status system, maintained and presented by NatureServe in cooperation with the Natural Heritage Network, was developed in the United States in the 1980s by The Nature Conservancy (TNC) as a means for ranking or categorizing the relative imperilment of species of plants, animals, or other organisms, as well as natural ecological communities, on the global, national and/or subnational levels. These designations are also referred to as NatureServe ranks, NatureServe statuses, or Natural Heritage ranks. While the Nature Conservancy is no longer substantially involved in the maintenance of these ranks, the name TNC ranks is still sometimes encountered for them.
NatureServe ranks indicate the imperilment of species or ecological communities as natural occurrences, ignoring individuals or populations in captivity or cultivation, and also ignoring non-native occurrences established through human intervention beyond the species' natural range (as, for example, with many invasive species).
NatureServe ranks have been designated primarily for species and ecological communities in the United States and Canada, but the methodology is global, and has been used in some areas of Latin America and the Caribbean. The NatureServe Explorer website presents a centralized set of global, national, and subnational NatureServe ranks developed by NatureServe or provided by cooperating U.S. Natural Heritage Programs and Canadian and other international Conservation Data Centers.
Most NatureServe ranks address the conservation status of a plant or animal species or a natural ecological community using a one-to-five numerical scale (from most vulnerable to most secure), applied either globally (world-wide or range-wide) or to the entity's status within a particular nation or a specified subnational unit with a nation. Letter-based notations are used for various special cases to which the numerical scale does not apply, as explained below. Ranks at various levels may be concatenated to combine geographical levels, and also to address infraspecific taxa (subspecies and plant varieties).
NatureServe conservation statuses may be applied at any or all of three geographical levels:
The most commonly encountered NatureServe conservation statuses at the G-, N-, or S-level are:
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.
Several less frequent special cases are addressed through other notation in the NatureServe ranking system, including:
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.
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.
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.