What Makes a Plant Rare?
The Center for Plant Conservation works to save plants –specifically rare plants. In 1981, Deborah Rabinowitz described seven forms of rarity –each depending on geographical distribution, habitat specificity, and/or local population size.
A plant can be broadly distributed over a wide range, but still be rare because it occurs only in specialized habitats and has small populations, as with climbing vine fern (Microgramma heterophylla). Very narrow range-endemics are also often very rare, such as the cliff palace milkvetch (Astragalus deterior) found only on the south rim of Mesa Verde in the white zone of the upper cliff house sandstone formations. Still others may have populations of large numbers, such as San Diego thornmint (Acanthomintha ilicifolia) with a few population searching tens of thousands in good years, but is restricted to specific habitats in small area –in the thornmint’s case clay soils in coastal San Diego county.
The reason why a plant is rare relates to its biogeographical history and the threats it has faced. Historically, some species were once broadly distributed, but are now restricted to small fragments due to habitat destruction. Other species are naturally rare, meaning they never had a broad distribution, never had large numbers of individuals. They are vulnerable to habitat destruction and climate change, especially if they have highly specific habitat requirements. They may be more susceptible to unpredictable events, such as landslides and fires, which cause catastrophic loss. NatureServe, a CPC network partner, gathers information on plants throughout their range to evaluate their rarity and the degree of threat they face. They have developed a consistent method for evaluating the imperilment of a species, ranging from critically imperiled to demonstrably secure.
The roles rare plants play within their ecosystems –past, present, and future –is rarely understood, largely because it is unexamined. CPC partners are learning more about these plants as they work to save them. We do know that each has a unique story and we hope you enjoy reading some of them here and in exploring plants in CPC’s National Collection.