What is a Rare Plant?
Rare plants may be scarce because the total population of the species may have just a few individuals, or be restricted to a narrow geographic range, or both. Some rare plants occur sparsely over a broad area. Other rare plants have many individuals, but these are crowded into a tiny area; in some cases, a single county or canyon. A third kind of rare plants are those with both few individuals AND a narrow geographic range: these are the very rarest plants.
NatureServe, a non-profit conservation organization that provides the scientific information and tools needed to help guide effective conservation action, has developed a consistent method for evaluating relative imperilment of species, with an emphasis on rare species. NatureServe's global conservation status ranks are based on a one to five scale, ranging from critically imperiled (G1/T1) to demonstrably secure (G5/T5). These status assessments are based on the best available information, and consider a variety of factors:
- Total number and condition of occurrences (e.g., populations)
- Population size
- Range extent and area of occupancy
- Short- and long-term trends in the above factors
- Scope, severity, and immediacy of threats
- Number of protected and managed occurrences
- Intrinsic vulnerability
- Environmental specificity
NatureServe Global Conservation Status Ranks for Species (G-ranks)
Rank |
Definition |
G1 |
Critically Imperiled - At very high risk of extinction due to extreme rarity (often 5 or fewer populations), very steep declines, or other factors. |
G2 |
Imperiled - At high risk of extinction due to very restricted range, very few populations (often 20 or fewer), steep declines, or other factors. |
G3 |
Vulnerable - At moderate risk of extinction due to a restricted range, relatively few populations (often 80 or fewer), recent and widespread declines, or other factors. |
G4 |
Apparently Secure - Uncommon but not rare; some cause for long-term concern due to declines or other factors. |
G5 |
Secure - Common; widespread and abundant. |
Infraspecific Taxon Conservation Status Ranks (T-ranks)
Infraspecific taxon status ranks (T-ranks) refer to subspecies, varieties and other designations below the level of the species.
Rank |
Definition |
T1-5 |
The status of subspecies or varieties is indicated by a "T-rank" following the species' global rank. Rules for assigning T-ranks follow the same principles outlined above for global conservation status ranks. For example, the global rank of a critically imperiled subspecies of an otherwise widespread and common species would be G5T1. A T-rank cannot imply the subspecies or variety is more abundant than the species as a whole-for example, a G1T2 cannot occur. |
Visit NatureServe's website for more information about their conservation status ranking criteria.
Examples
![USDA NRCS PLANTS Database range map for Eriogonum ovalifolium.](https://webarchive.library.unt.edu/web/20121017182529im_/http://www.fs.fed.us/wildflowers/rareplants/images/EROV_map.gif)
Range of Eriogonum ovalifolium.
Cushion buckwheat, Eriogonum ovalifolium, is a common and widespread plant of western North America. It has a NatureServe Global Conservation Status Rank of G5.
![Cushenbury buckwheat buckwheat, Eriogonum ovalifolium ssp. vineum.](https://webarchive.library.unt.edu/web/20121017182529im_/http://www.fs.fed.us/wildflowers/rareplants/images/eriogonum_ovalifolium_vineum.jpg)
Cushenbury buckwheat, Eriogonum ovalifolium ssp. vineum. Photo by Scott Eliason, San Bernardino National Forest.
![USDA NRCS PLANTS Database range map for Eriogonum ovalifolium ssp. vineum.](https://webarchive.library.unt.edu/web/20121017182529im_/http://www.fs.fed.us/wildflowers/rareplants/images/EROVV_map.gif)
Range of Eriogonum ovalifolium ssp. vineum.
Cushenbury buckwheat, Eriogonum ovalifolium ssp. vineum, on the other hand, is a very rare subspecies only found on special soils in mountains of southern California. It was listed as Endangered in 1994 and is threatened primarily by limestone mining. Cushenbury buckwheat has a NatureServe Global Conservation Status Rank of G5T1, the same G5 for the species Eriogonum ovalifolium, with T1 showing the rarity of the subspecies vineum.
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