Tiburon Mariposa Lily

Calochortus tiburonensis

Description

Tiburon mariposa lily, Calochortus tiburonensis, is a member of the lily family (Liliaceae) with a single persistent, basal, linear-oblong leaf 1-2 ft (30.5-61 cm) long. The flowering stem, about 20 in (50.8 cm) tall, is usually branched and bears erect flowers in twos or threes at the ends of the branches. The three petals and three sepals are light yellow-green with reddish or purplish-brown markings. The capsule is triangular in cross-section, and about 2 in (5 cm) long. The long slender hairs on the upper surface and margins of the petals and the lack of wings on the capsule distinguish C. tiburonensis from the other two Calochortus species that axe also found on the Tiburon Peninsula.

Tiburon mariposa lily is a bulbous perennial. Individuals are thought to live ten years or more. The basal leaf appears above ground after the onset of winter rains. The species flowers from May to June. On average, each reproductive adult bears two or three flowers, but large individuals may produce eight flowers. Protandry likely limits self-pollination in the species although seed can be produced upon hand self-pollination. In nature, Tiburon mariposa lily appears to be primarily outcrossing (pollen from one plant going to a flower of a different plant without inbreeding) and dependent upon insects for pollination. The flowers are thought to be pollinated primarily by bumble bees. Tiburon mariposa lily appears to be reproductively isolated from the co-occurring Oakland star-tulip by flowering later and by having different pollinators. Tiburon mariposa lily flowers during March and April and is thought to be pollinated by sweat bees.

Often individuals of Tiburon mariposa lily do not reproduce until they are five years old. During the hot, dry portion of the year, the bulbs are dormant, forming a bulb bank that persists from year to year. Seeds germinate at the onset of the rainy season. Seed loss may be the major stage of mortality in the life cycle; there is no evidence of a dormant soil seed bank.

Tiburon mariposa lily appears to have low seed survival and seedling establishment, low adult mortality, and slow growth. Vegetative reproduction, through production of bulblets, occurs in the greenhouse but probably not in nature.

Habitat

Tiburon mariposa lily grows on rocky serpentine slopes and serpentine derived soils at an elevation of approximately 460 ft (140 m). The colonies are in open areas in a serpentine bunchgrass community associated with serpentine reedgrass, Tiburon buckwheat, Tiburon paintbrush, and Marin dwarf-flax.

Distribution

Tiburon mariposa lily is known only from Ring Mountain on the Tiburon Peninsula in southern Marin County. It is an example of a rare species that is restricted in distribution but relatively abundant where it does occur. The single population is distributed in three major colonies separated by 0.25-5.4 mi (0.4-8.7 km). The number of individual plants observed has ranged from the hundreds in 1986 to an estimated 40,000 individuals in 1991. The number of flowering plants counted was 5,783 in 1989, 3,443 in 1990, and 19,875 in 1991.

Threats

The single known population of Tiburon mariposa lily is threatened by chance events such as fire, severe drought, pest or disease outbreak, or other natural or human-caused disasters. The species is also vulnerable due to its proximity to human population centers and intensive development activities. The proximity of the plant to a large human population, along with high visitor use and minimal supervision, increases the likelihood that human-caused disasters, acts of vandalism, and recreational use will affect the plants or their habitat. Unrestricted collecting for scientific or horticultural purposes or excessive visits by individuals interested in seeing rare plants may also be a concern because Tiburon mariposa lily is a strikingly unusual member of this much-collected genus. Non-native invasive plants may be an additional threat. Reportedly, Harding grass and fennel are increasing in numbers on the lower slopes of Ring Mountain.

Conservation and Recovery

Between 1982 and 1995, Tiburon mariposa lily was protected from development because the land on which it occurs was owned and managed by the Nature Conservancy, a group whose management goals are the maintenance of biodiversity and the protection of rare and endangered species. In 1988, the Nature Conservancy developed and implemented an annual monitoring plan to provide data on reproductive success and herbivore damage for Tiburon mariposa lily. The Ring Mountain property was transferred from the Nature Conservancy to Marin County Parks as Open Space in 1995. The Nature Conservancy retained a conservation easement on the property and expects that Matin County will continue monitoring the rare species on the preserve. At this point, the county has not developed a monitoring plan and is depending on volunteers from the Nature Conservancy and California Native Plant Society for monitoring. The preserve is fenced to reduce the incidence of four-wheel drive vehicle and motorcycle use, but is still accessible to bicycles, equestrians, and hikers.

Contact

U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N.E. 11th Ave.
Portland, Oregon 97232-4181
(503) 231-6121
http://pacific.fws.gov/

Reference

U.S. Fish and Wildlife Service. 1998. "Recovery Plan for Serpentine Soil Species of the San Francisco Bay Area." Portland, Oregon. 330+ pp.