|Listed||October 13, 1998|
|Description||Perennial herb with dark-brown, fibrous-coated corms; bears saucer-shaped violet flowers.|
|Habitat||On gentle hillsides, valleys, and flood-plains in mesic, southern needle grass grassland and alkali grassland plant communities in association with clay, loamy sand, or alkaline silty-clay soils.|
|Threats||Urbanization or agricultural conversion.|
Thread-leaved brodiaea, Brodiaea filifolia, is a perennial herb with dark-brown, fibrous-coated corms. The flower stalks are 8-16 in (20.3-40.6 cm) tall with several narrow leaves that are shorter than the scape. The flowers bloom from May to June and are arranged in a loose umbel. The six perianth segments are violet, spreading, and 0.4-0.5 in (1.0-1.3 cm) long. The broad and notched anthers are 0.1-0.2 in (0.25-0.51 cm) long. The fruit is a capsule.
Thread-leaved brodiaea can be distinguished from the other species of Brodiaea that occur within its range by its narrow, pointed staminodia (non-pollen producing stamen), saucer-shaped flower, and a thin perianth tube, which is split by developing fruit.
B. filifolia is known to hybridize with B. orcuttii, B. terrestris, and possibly B. jolonensis, where these species coexist. At least one major population in the vicinity of Miller Mountain (San Diego County) in the Cleveland National Forest appears to represent a hybrid swarm between B. orcuttii and B. filifolia. The Miller Mountain population alone occupies nearly 45% of reported occupied habitat for B. filifolia. Hybridization among these Brodiaea species is a natural phenomenon. However, these plants relied on relatively species-specific native bee species for pollination in the past and the introduction of non-native honeybees, which tend to be species-generalist, may have increased the potential for hybridization.
This species typically occurs on gentle hillsides, valleys, and floodplains in mesic, southern needle-grass grassland and alkali grassland plant communities in association with clay, loamy sand, or alkaline silty-clay soils. Sites occupied by this species are frequently intermixed with, or near, vernal pool complexes, such as near San Marcos (San Diego County), the Santa Rosa Plateau, and southwest of Hemet in Riverside County.
In San Diego County, California, the majority of the thread-leaved brodiaea populations are concentrated within the cities of San Marcos, Vista, and Carlsbad, and are highly correlated with the distribution of clay soils and soils with clay subsoils. From 1974 to 1994, nearly 65% of the clay soils and about 75% of the soils with clay subsoils had been developed or urbanized in these three cities and were no longer available for thread-leaved brodiaea or its associated habitat.
The historical range of thread-leaved brodiaea extends from the foothills of the San Gabriel Mountains at Glendora (Los Angeles County), east to Arrowhead Hot Springs in the western foothills of the San Bernardino Mountains (San Bernardino County), and south through eastern Orange and western Riverside Counties to Carlsbad in northwestern San Diego County.
Forty-six populations of thread-leaved brodiaea have been reported. At least nine of these populations have been extirpated, primarily in San Diego County, California. Thirty-seven populations are presumed extant. Nearly half of these remaining populations are clustered in the growing cities of Vista, San Marcos, and Carlsbad (nine populations) and in the vicinity of the Santa Rosa Plateau in southwestern Riverside County, California (six populations). The remaining 22 populations are scattered within the counties of Orange, Los Angeles, Riverside, San Bernardino, and San Diego.
The population of thread-leaved brodiaea reported to have the largest number of individuals is on private land in the City of San Marcos. The populations with the largest extent of potentially suitable habitat are on the Santa Rosa Plateau, where only about 38 acres (15 hectares) of the plateau is reported as occupied by thread-leaved brodiaea, but about 300 acres (121 hectares) is potentially suitable habitat. These lands are primarily managed by the Nature Conservancy. The only populations of thread-leaved brodiaea known to occur on federal land are on the Camp Pendleton Marine Corps Base in San Diego County, where three populations were recently discovered in an abandoned weapons impact area. Six populations were recently discovered in Orange County. Most of the recently discovered populations of thread-leaved brodiaea in Orange County are relatively small. The largest population (Forster Ranch) supports about 60% of the thread-leaved brodiaea individuals and about 80% of the occupied habitat in Orange County. Only two of the Orange County populations (Casper's Regional Park and Aliso-Woods Canyon Regional Park), with fewer than 1,000 individuals combined, are on lands managed by the county government. Thread-leaved brodiaea has also been found on the San Jacinto Wildlife Management Area in Riverside County.
Thread-leaved brodiaea, in its entire range, occupies about 825 acres (334 hectares) of suitable habitat (mesic needlegrass grassland, mixed native-non-native grassland with clay soils, or alkali annual grassland with alkaline silty clay soils). The total number of individuals of this species and the extent of occupied habitat vary on an annual basis in response to the timing and amount of rainfall, as well as temperature patterns. Fewer than 2,000 individuals have been observed at most populations. Most of these populations occupy less than 13 acres (5 hectares). The largest extant population in Riverside County, Santa Rosa Plateau, has been estimated to contain over 30,000 observed individuals and occupies about 38 acres (15 hectares) of habitat. In San Diego County, the largest confirmed population is on an isolated 40 acre (16 hectare) parcel in San Marcos, California. This population may support as many as 342,000 individual plants. The number of observed individuals often does not correlate with the number of corms present at a site. For example, at one residential development site in 1992, 20 individuals of thread-leaved brodiaea were reported, but more than 8,000 corms were found during the effort to transplant thread-leaved brodiaea to another site.
More than 25% of thread-leaved brodiaea populations have been eliminated by urbanization and agricultural conversion. Over the last 15 years, nearly 150 acres (61 hectares) of occupied habitat containing more than 80,000 plants have been eliminated in the cities of San Marcos and Vista. Urbanization continues to be the most significant threat to this species. About 20% (about eight) of the remaining populations of thread-leaved brodiaea in San Diego and Riverside counties are currently within proposed or approved development projects. Another 10% (four) of the populations are zoned for urbanization or threatened by discing for fire suppression activities or dryland farming. Suitable habitat is at even greater risk. It is probable that the only known population of thread-leaved brodiaea reported for San Bernardino County in nearly 70 years will be removed by a major pipeline project.
Most of the recently discovered populations of thread-leaved brodiaea in Orange County, California are relatively small and are not at immediate risk (two are on protected land). However, the largest population known in Orange County is within the proposed grading footprint of a 1,600-unit residential development in the city of San Clemente. This population occupies about 15 acres (6 hectares) and supports about 60% of the reported thread-leaved brodiaea individuals and about 80% of the habitat occupied by this species in Orange County. As currently proposed, nearly the entire native population at this site would be affected.
The largest reported population of thread-leaved brodiaea occurs on 40 acres (16 hectares) of habitat located near downtown San Marcos in San Diego County, which is zoned for industrial development. Other populations in San Marcos, although not as extensive, are also threatened. For example, a 20-acre (8-hectare) parcel near the largest site is proposed for recreational development.
The only populations of thread-leaved brodiaea known to occur on federal land are on Camp Pendleton Marine Corps Base in San Diego County. Several populations have recently been discovered in an abandoned weapons impact area. While no populations are currently reported as directly threatened by development on the base, a recently-proposed project may alter up to 134 acres (54 hectares) of highly suitable habitat that is immediately adjacent to known occupied habitat.
Vernal wetlands that support five of six populations of thread-leaved brodiaea within the San Jac-into River flood plain and Old Salt Creek near Hemet are threatened by alteration of hydrology (duck ponds), channelization, discing for dry land farming and fire suppression practices, and urbanization. These populations represent about one third of the populations and over 40% of the potential habitat for this species in Riverside County.
At least 12 of the remaining 37 populations of thread-leaved brodiaea within San Bernardino, Orange, Riverside, and San Diego Counties are threatened by the destruction of habitat that will result from urbanization, discing for dry land farming or fire suppression. These populations include a significant portion of the occupied habitat and the largest populations of thread-leaved brodiaea within San Diego and Orange Counties. The reduction of these populations will result in a significant decline in the species.
Conservation and Recovery
In 1991, the State of California established the Natural Communities Conservation Planning program to address conservation needs of natural ecosystems throughout the state. The focus of the current planning program is the coastal sage scrub community in southern California, although other vegetation communities are being addressed in an ecosystem-level approach. Thread-leaved brodiaea and spreading navarretia are currently being considered under the program.
Thread-leaved brodiaea is found in the Cleveland National Forest, managed by the U.S. Forest Service. The Forest Service protects sensitive species and attempts to establish these species in suitable or historic habitat. The Forest Service also encourages land ownership adjustments to acquire and protect sensitive plant habitat.
Thread-leaved brodiaea occurs on private land, including lands managed by the Nature Conservancy. Two populations are on lands managed by the county government and also on the San Jacinto Wildlife Management Area in Riverside County, managed by the California Department of Fish and Game. The only populations of thread-leaved brodiaea known to occur on federal lands are managed by the Department of the Navy. Thread-leaved brodiaea is protected at Miramar Marine Corps Air Station in vernal pool management zones through the Integrated Natural Resource Management Plan. This plan is an example of the permanent protective measures promoted by the Act. Camp Pendleton Marine Corps Base has a similar Draft Isolated Ephemeral Wetlands Management Plan.
U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N.E. 11th Ave.
Portland, Oregon 97232-4181
U. S. Fish and Wildlife Service. 13 October 1998. "Determination of Endangered or Threatened Status for Four Southwestern California Plants from Vernal Wetlands and Clay Soils." Federal Register 63 (197): 54975-54994.