Plant taxa can be considered rare for a variety of reasons. Species that could all be called rare inlcude those that occur very infrequently but are distributed over a wide geographic area, are locally abundant but occur over a restricted range, or, in the most extreme case, occur infrequently over a restricted range.

Since the 1920’s, rarity in plants has been the subject of numerous scientific debates. Explanations as to why plants are rare include that they are newly evolved and haven’t had time to expand their ranges, that they are old and have lost competitive ability due to decreased genetic diversity (heterozygosity), and that they are restricted physiologically to a narrow range of environmental conditions (e.g., soil type, water regime, temperature, etc.). Depending on the species, any of these may be accurate.

Authors of the various theories as to how a plants come to be rare generally cite one or more species that offer evidence in support of their hypotheses. In 1958, Harlan Lewis and Peter Raven offered their own ideas of how plants speciate and come to have restricted ranges. In their theory of catastrophic selection, or saltational speciation, they observed repeated episodes of extinction and recolonization events by closely related species within the genus Clarkia, especially among populations occurring at the geographical periphery of their ranges.

One of the species Lewis and Raven chose as an example of catastrophic selection was Presidio clarkia (Clarkia franciscana), a species first described by them. This slender, branched herb is a very rare member of the evening-primrose family (Onagraceae). It is found only on serpentine derived soils supporting coastal scrub and coastal prairie. Presidio clarkia is a small annual, self-pollinating herb with finely hairy stems reaching 18 inches in height. Its leaves are narrow, lance-shaped, and less than 2 inches long. Flowers develop from erect buds from May through July. Petals are wedge-shaped with a blunt tip, to one-half inch long, and are lavender-pink shading to white near the middle and with a bright reddish purple base.

The historic distribution of Presidio clarkia is unknown. For a long time, it was thought to only occur at two extant populations in the Presidio; a third has been extirpated. In 1988, disjunct populations of Presidio clarkia were found in the Oakland Hills. Genetic comparisons made between the Presidio and Oakland populations concluded that at least one of the Oakland populations is indigenous and was not planted by a well-meaning but potentially misguided botanist. Although the discovery of the Oakland populations has greatly increased the likelihood that Presidio clarkia can be protected, its numbers are still so low that extinction remains a distinct possibility.

Presidio clarkia was listed as Endangered by the state of California in 1978 and it was proposed for listing as Endangered by the U.S. Fish and Wildlife Service in 1992; the federal listing is still pending. It is also on the CNPS List 1B:3-3-3, indicating that it is endangered in California, distributed in one to several highly restricted occurrences, is endemic to California and endangered throughout its range. Under the federal Endangered Species Act of 1973, a species qualifies for listing as Endangered when it is in danger of extinction throughout all or a significant portion of its range. Under the California Endangered Species Act of 1984, a species is considered eligible for listing as Endangered when its prospects for survival and reproduction are in immediate jeopardy from one or more causes. It is against federal and state law to pick or in any way disturb Presidio clarkia.

Despite improved management of Presidio clarkia by the National Park Service, populations in San Francisco continue to be threatened by illegal off-road vehicle activity, encroachment by invasive weed and horticultural species, and some proposed GGNRA projects. For eight years, CNPS Chapter members worked with the Army to remove exotics from serpentine grasslands on the Presidio and they continue to be active in assisting the National Park Service with its attempts to preserve this very rare species. For information on volunteer work parties, check our Habitat Restoration