Santa Cruz Island Rockcress

Sibara filifolia

Description

Sibara filifolia, a slender annual herb in the mustard family (Brassicaceae or Cruciferae) that flowers in April, is 5-15 in (12.5-37.5 cm) tall. The flowers are pink to purplish with spoon-shaped petals 0.12-0.25 in (3-6 mm) in length. The pinnately lobed leaves are 1-2 in (2.5-5 cm) long, with narrow linear lobes. The fruit is a slender pod (silique), 0.6-1 in (15-25 mm) long, that contains many wingless seeds. Sibara filifolia is distinct from S. virginica, which has narrowly winged seeds, and from S. rosulata and S. deserti, which have white petals. No other species of Sibara occur on the Channel Islands.

Sibara filifolia was first collected by E. L. Greene in 1886, described by him the following year as Cardamine filifolia, and then transferred by him to Arabis filifolia in a later 1887 publication.

In 1886, Greene proposed the new genus Sibara to accommodate this species. Sibara has been retained by Munz and Keck in 1959, Munz in 1968 and 1974, and Rollins in 1993.

Habitat

Sibara filifolia is confirmed to exist only on San Clemente Island in a sea terrace on the southern part of the island, near Pyramid Head. It grows on volcanic rock scree (talus) in association with Opuntia prolifera (cholla), Selaginella bigelovii (spike-moss), and Lotus argophyllus var. adsurgens (San Clemente Island birds-foot trefoil). During the 1996 season on San Clemente, there were fewer than 40 of these plants growing on a dry rocky saddle with thin soil.

Distribution

Green reported in 1887 that Sibara filifoli " … is to be sought in shady places on the northward slope" on Santa Cruz Island. The type locality apparently has not endured; last seen there in 1936, the plant was not relocated during the 1985 survey of the island. The species is thought to have once been both common and wide-ranging, since it was also collected on the distant islands of Santa Catalina and Santa Cruz. Trask collected S. filifolia in 1901 on Santa Catalina Island, where she reported it to be common in two locations.

The most recent find of Sibara filifolia on Santa Catalina in 1973 was the first in 70 years. The voucher specimen is at the Rancho Santa Ana Botanic Garden Herbarium, but its existence remained unknown until 1996. Although the status of the population on Santa Catalina is not precisely known, the species has not been reported from there since this 1973 collection. M. Hoefs, one of the original collectors, did not relocate any specimens at the original site during a search for Sibara filifolia, but he noted that the habitat and associated species appear to be in good condition. Although Sibara filifolia was not been observed on Santa Catalina Island in the last quarter the twentieth century, its extirpation has not been confirmed, and for that reason the Service believes there is a possibility that it still may be present there.

Sibara filifolia, originally known from historical collections on Santa Cruz Island and Santa Catalina Island, was unknown on San Clemente Island until two plants were discovered near Pyramid Head by Beauchamp in 1986. Prior to this discovery, the species was thought to be extinct. The extent of its original range on San Clemente Island is unknown; there is now only one location with less than 40 individuals.

Threats

Sibara filifolia is threatened by destruction and degradation of habitat, erosion, predation by feral animals, competition with exotic plant species, and fire. Overcollection is another potential threat.

Sibara filifolia may have been extirpated from Santa Cruz Island by overgrazing. Although some areas have been fenced, sheep and pigs continue to re-invade these areas and their numbers appear to be increasing. Although Sibara filifolia could be re-discovered on Santa Cruz Island, grazing by non-native animals may prevent its re-establishment or proliferation.

Invasive exotic plant species, as a consequence of habitat degradation on the islands, now occupy disproportionate positions in their vegetation regimes. The disparity between the reported historical occurrences of Sibara filifolia on shady north-facing slopes and the current presence of the species on grass-free, south-facing slopes, suggests that alien grasses may prevent the expansion of S. filifolia into otherwise suitable habitat.

Considerable fire damage apparently destroyed the known population of Sibara filifolia on San Clemente Island in 1995. Chance fires could drastically reduce or eliminate all of the remaining individuals of the species and destroy the seed bank as well, preventing reestablishment of the last confirmed extant population.

Sibara filifolia is so rare on Santa Catalina Island that any unauthorized collection or even unintentional overutilization brought about by increased publicity following its Federal listing as endangered could result in extinction.

Conservation and Recovery

The Santa Cruz Island rockcress only survives in perilously small populations on San Clemente and Santa Catalina Islands. Its habitats are being conserved by the Nature Conservancy (a private environmental organization) and by the National Parks Service. However, the rare plant is still threatened by competition and habitat changes caused by invasive alien plants, by introduced mammalian herbivores on Santa Catalina Island, and other factors. (Introduced pigs and goats were removed from San Clemente Island in 1992.) The survival of this endangered plant requires strict protection from the feeding of mammals. This could be done by securely fencing known plants, or by eradicating the mammals from the critical habitat. The abundance of competing non-native plants should also be managed. The populations of the Santa Cruz Island rock-cress should be monitored, and research undertaken into its basic biology and ecological requirements. It should be propagated in captivity, to provide stock for out-planting to supplement their small wild populations, and to establish new ones in suitable habitat, perhaps to Santa Cruz Island, from which it has been extirpated.

Contact

U.S. Fish and Wildlife Service
Carlsbad Field Office
2730 Loker Avenue West
Carlsbad, California 92008-6603
Telephone (760) 431-9440
Fax: (760) 431-9624

Reference

U.S. Fish and Wildlife Service. 8 August 1997. "Endangered and Threatened Wildlife and Plants; Determination of Endangered Status for Three Plants From the Channel Islands of Southern California." Federal Register 62 (153): 42692-42702.