California, and the Bay Area specifically, are places of great biological diversity owing to the wide variety of environmental conditions found here. On the San Francisco Peninsula, conditions leading to high biological diversity and species with limited distributions (endemic species) include 1) the interaction of maritime weather and Mediterranean climate to produce localized climatic zones, 2) the development of diverse habitats and barriers to dispersal by mountains and bay, and 3) the presence of a variety of geologic and soil conditions, such as nutrient-poor sand dunes and serpentine-derived soils. This combination of conditions allowed for a flourishing adaptive radiation of many closely related species.

The biologically diverse, native plant communities that evolved on the San Francisco Peninsula are now threatened, however, by urban development and the introduction of strongly competitive non-native species, such as annual grasses and eucalyptus trees. The few places where natural conditions remain are now refuges for rare species. Amongst these, the Presidio is probably the most important single refuge in San Francisco, because it has both a diversity of environmental conditions and several undeveloped areas.

Twelve plants found at the Presidio have been designated as rare, threatened or endangered by the United States Fish & Wildlife Service and/or the California Native Plant Society. These special status species are commonly referred to as rare plants.

The Golden Gate National Recreation Area has adopted a policy that all rare plants found within its boundaries will be treated as though they have full protection of the Endangered Species Act. This is unless the Superintendent judges, on a case by case basis, that evidence against the endangerment of a particular plant is sufficient to allow a specific action.

Because of their special status, monitoring of the Presidio's rare plants has been going on for many years. The results of these pioneer efforts have provided valuable baseline information of plant species "on the brink." Park stewards comb the far reaches of the Presidio's remaining natural areas to census and map their populations every one to three years. The resulting data provide a clear picture of shifts in distribution and population size of these critical plants.

Rare plants in the Presidio are found in either dune communities or serpentine communities because invasive non-native plants are not able to dominate these nutrient-poor areas.

In the dunes, we find three rare annual species:

	San Francisco spineflower (Chorizanthe cuspidata var. cuspidata)
	Dune gilia (Gilia capitata ssp. chamissonis)
	San Francisco lessingia (Lessingia germanorum)

Two dune perennial species are considered rare:

	San Francisco wallflower (Erysimum franciscanum) - also grows on serpentine
	San Francisco campion (Silene verecunda ssp. verecunda)

In areas with serpentine soils, we find three rare annual species:

	San Francisco owl's clover (Triphysaria floribunda)
	Marin dwarf flax (Hesperolinon congestum)
	Presidio clarkia (Clarkia franciscana)

The four rare perennial species that exist on serpentine are:

	Raven's manzanita (Arctostaphylos hookerii ssp. ravenii)
	Franciscan thistle (Cirsium andrewsii)
	San Francisco gumplant (Grindelia hirsutula var. maritima)
	Coast rock cress (Arabis blepharophylla)

Plant populations can vary in size according to many factors, including weather, competition, and management. Variation among years can be great, particularly for annual species, and population trends must be monitored over several years before any confident conclusions about the relative success of management practices can be made. That being said, populations of rare dune species and perennial serpentine species have generally increased over the recent years, whereas populations of the serpentine annuals are stable or perhaps have even declined. The ranges of all twelve rare plants have remained the same or expanded through nursery propagation and outplanting efforts.