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
|Critically Imperiled - At very high risk of extinction due to extreme rarity (often 5 or fewer populations), very steep declines, or other factors.
|Imperiled - At high risk of extinction due to very restricted range, very few populations (often 20 or fewer), steep declines, or other factors.
|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.
|Apparently Secure - Uncommon but not rare; some cause for long-term concern due to declines or other factors.
|Secure - Common; widespread and abundant.
|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.
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, 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.
A “Rarest of the Rare” Species
Rare plants may be scarce because the total population of the species may have few individuals AND restricted to a narrow geographic range; these are the very rarest plants. One such species, budding monkeyflower (Mimulus gemmiparus), was recently surveyed on the Arapaho and Roosevelt National Forests in Colorado.
Population dynamics, rarity, and risk of extinction for populations of Mimulus gemmiparus (budding monkeyflower) on National Forests of Colorado (PDF, 896 KB) - A Research Report Submitted to USDA Forest Service Arapaho and Roosevelt National Forests and Pawnee National Grassland, Fort Collins, Colorado August 22, 2013, by Mark Beardsley, EcoMetrics, LLC, and David A. Steingraeber, Colorado State University.