Global Range: Ring Mountain, Tiburon Peninsula, northwestern side of San Francisco Bay, Marin County, California.
Catalog Number: US 2673271
Collection: Smithsonian Institution, National Museum of Natural History, Department of Botany
Verification Degree: Verified from the card file of type specimens
Preparation: Pressed specimen
Collector(s): E. J. Hill
Year Collected: 1972
Locality: Tiburon Peninsula, north slopes of Ring Mt., Marin, California, United States, North America
Elevation (m): 350 to 350
- Isotype: Hill, A. J. 1973. Madrono. 22: 100.
Comments: A serpentine grassland; open, rocky slopes.
Number of Occurrences
Note: For many non-migratory species, occurrences are roughly equivalent to populations.
Estimated Number of Occurrences: 1 - 5
Comments: The Nature Conservancy preserve is the only known occurrence.
Life History and Behavior
National NatureServe Conservation Status
Rounded National Status Rank: N1 - Critically Imperiled
NatureServe Conservation Status
Rounded Global Status Rank: G1 - Critically Imperiled
Reasons: Narrowly endemic to serpentine grasslands on a single summit on the Tiburon Peninsula in Marin County, California. The single population is located within a preserve.
Date Listed: 02/03/1995
Lead Region: California/Nevada Region (Region 8)
Listing status: T
For most current information and documents related to the conservation status and management of Calochortus tiburonensis, see its USFWS Species Profile
Degree of Threat: Very high - medium
Comments: Even though this species is protected with a fence, it is susceptible to random events either natural or human due to the fact there is only one known occurrence. The preserve it inhabits is located very near a residential community. Random events such as fire, disease outbreak, landslides, or vandalism could have catastrophic effects on this species. This species is also threatened by potential overuse of the preserve due to its proximity to residential communities, and specifically by, bicycling, motorbikes, and pedestrians (USFWS 1995).
Calochortus tiburonensis, the Tiburon Mariposa Lily, is a rare member of the genus Calochortus in the family Liliaceae. It is endemic to Marin County, California, where it is known only from one population on Ring Mountain. There it occurs on a single serpentine outcrop in grasslands from 50–150 metres (160–490 ft) in elevation. 
Calochortus tiburonensis is a perennial growing from a bulb to 10-60 cm in height with a single leaf. It produces anywhere from 2 to 7 erect flowers annually on a branching stem. The ciliate, light yellow-green petals are streaked with purplish-brown. When mature, ovaries form capsule full of small, dark brown seeds.
The plant was federally listed as a threatened species in 1995. It is considered threatened due to loss of surrounding habitat to recreational activities, to competition from invasive plant species, to its proximity to a dense population center, and to its confinement to a single population of plants. The population grows on land owned by the Marin County Open Space District, an area previously owned and preserved by The Nature Conservancy. It was first brought to the attention of science in 1971, and was described to science in 1973. It is protected along with a number of other rare serpentine soils endemics that grow on and around Ring Mountain.
The plant has probably always been rare and limited to its current distribution on the Ring Mountain outcrop. A 1991 estimate placed the total population size at about 40,000 individuals. While the land is protected, the flora upon it are still vulnerable to events such as wildfires or drought, and to damage from off-leash dogs, hikers, cyclists, vandals, and wildflower collectors.
- Jepson Manual Treatment . accessed 2.12.2013
- Calochortus tiburonensis. Jepson Flora Project: Jepson Interchange. 1993 by the Regents of the University of California.
- USFWS. Determination of endangered status for ten plants and threatened status for two plants from serpentine habitats in the San Francisco Bay region of California. Federal Register February 3, 1995.
- C. Michael Hogan. 2008. Ring Mountain, The Megalithic Portal. ed. Andy Burnham
- Hill, A. J. (1973). A distinctive new Calochortus (Liliaceae) from Marin County, California. Madroño 22:100-104.
- The Nature Conservancy
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