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 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. Introduction 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 within 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). Global, national, and subnational levels NatureServe conservation statuses may be applied at any or all of three geographical levels: G - Ranks designated at the global (or range-wide) level (G-rank), N - Ranks designated at a national level (N-rank) for a particular nation, and S - Ranks designated at a subnational level (S-rank) for a particular next-lower geographical unit within a nation, such as a state in the US. Commonly encountered ranks The most commonly encountered NatureServe conservation statuses at the G-, N-, or S-level are: Numbers 1 — Critically imperiled — (typically having