|Listed||July 31, 1997|
|Description||A herbaceous, perennial wildflower with white to lavender flowers, comprised of four petals.|
|Habitat||Rocky ledges on canyon walls.|
|Threats||Habitat degradation by soil erosion, feeding by introduced mammals, and competition with alien plants.|
Arabis hoffmannii (Hoffman's rock-cress) was described by Philip Alexander Munz as Arabis maxima var. hoffmannii in 1932 from specimens collected earlier that year by Ralph Hoffmann at the "sea cliffs east of Dick's Harbor," now known as Platts Harbor, on Santa Cruz Island. T.S. Brandegee had collected this rock-cress as early as 1888 from an unspecified location on Santa Cruz Island. Reed Clark Rollins elevated the taxon to species status by publishing the name Arabis hoffmannii in 1936. This nomenclature was retained in the most recent treatment of the genus.
Arabis hoffmannii is a slender, herbaceous, monocarpic (flowering once then dying) perennial in the mustard (Brassicaceae or Cruciferae) family. The one to several stems reach 2 ft (0.6 m) high, and have slightly toothed basal leaves. The white to lavender flowers, comprised of four petals 0.4 in (1 cm) long, are found at the tips of the stems. The slightly curved fruits are borne on long stalks (siliques). The only other rock-cress that occurs on the islands, Arabis glabra var. glabra, is a taller plant with cream-colored flowers.
A 1966 study on reproductive strategies of Arabis hoffmannii shows that individual plants in cultivation may reproduce within two years following establishment, with some plants surviving for at least five years. Individual rosettes are monocarpic, but some plants have more than one rosette. Arabis hoffmannii does not appear to be dependent upon pollinators for seed set, and individual plants may produce as many as 3,000-4,000 seeds. However, the small sizes of natural populations indicate that establishment success of new plants is low.
The Arabis hoffmannii population at Lobo Canyon on Santa Rosa Island is located on a rocky shelf overhanging the canyon, and these plants are associated with giant coreopsis (Coreopsis gigantea ), Greene's dudleya (Dudleya greenei ), Indian pink (Silene laciniata ), and non-native grasses. The canyon bottom below the shelf is heavily grazed and trampled by deer, cattle, and elk.
Santa Cruz island has three small populations of this taxon, one of which occurs in specialized habitat near Centinela Grade on lands owned and managed by The Nature Conservancy (TNC), where individuals grow on Santa Cruz Island volcanoes in association with giant coreopsis (Coreopsis gigantea ), Santa Cruz Island buckwheat (Eriogonum arbore scens ), and coastal prickly pear (Opuntia littoralis ).
Since Brandegee's collection of 1888, few collections of Arabis hoffmannii have been made. On Santa Cruz Island, Moran made a collection from the "Central Valley" in 1950, and McPherson collected the plant near Centinela Grade, possibly the same location, in 1967. It was not until 1985 that Steve Junak relocated a population at this location. Hoffmann's original collection site, near Platts Harbor on Santa Cruz Island, was for many decades in "an area of intense feral animal (sheep) disturbance," and no plants could be found. The evidence of absence seemed so complete a notice of review considering this species extinct was published in the Federal Register (48 FR 53640) in 1983. However, surveys conducted by TNC in 1985 were successful in relocating the plant near Platts Harbor.
According to Moran's field notes, he collected Arabis hoffmannii from Anacapa Island in 1941 "on the slopes above Frenchy's Cove." Despite this documentation, no specimens from this collection have been found in herbaria with known collections of island species, and recent surveys have failed to relocate the plant on Anacapa Island. Hoffmann reported the plant from "the bank above Water Canyon" on Santa Rosa Island in 1930, but numerous surveys have failed to locate any plants from that location. In 1996, a new population of the plant was discovered near the mouth of Lobo Canyon on Santa Rosa Island. This population consists of eight plants, three of which were flowering and the remaining five were vegetative rosettes.
In addition to the lone population on Santa Rosa Island, Arabis hoffmannii is also currently known from three small populations that collectively cover less than 1 acre (0.4 hectares) on Santa Cruz Island. The population near Platts Harbor is located on rocky volcanic cliffs along a north-facing canyon on lands owned by TNC. Because of inaccessibility and the loose structure of the volcanic rock, the cliff site has not been thoroughly surveyed. Only a few dozen plants have been directly observed, although the cliffs may support additional individuals. The population near Centinela Grade numbered about 30 when Junak relocated this population; TNC has monitored this population since 1990, with fewer than 30 plants observed each year. The third population on Santa Cruz Island was located in 1995 near Stanton Ranch, and numbered 16 plants the next year.
Monitoring results at Centinela and Stanton suggest poor establishment success because of a lack of favorable seed germination sites, a high rate of seedling mortality, or a combination of both factors. At these two sites, surviving plants tend to be found in the shade of shrubs where there is a low cover of annual species, suggesting that Arabis hoffmannii cannot tolerate competition with a high cover of annual species. Less than 100 plants in total were present in the three studied populations.
The major threats to Arabis hoffmannii are loss of soil, habitat degradation, trampling of potential seed germination sites by non-native ungulates, predation resulting from feral pig rooting, and competition with annual plants.
Specific examples of browsing or grazing by alien mammals on Arabis hoffmannii have been observed. The zone below an Arabis hoffmannii population on Santa Rosa Island is inhospitable to seed germination because of cattle trampling and soil churning. Seed-rain from that population falls onto areas that are highly trampled and churned, eliminating any chance for population expansion from its precarious cliff location. Arabis hoffmannii is monocarpic and damage from trampling may delay flowering, or even preclude reproduction of trampled individuals. Flowers produced later in the season out of synchrony with pollinator activity results in lower seed productivity. The Nature Conservancy has been monitoring population sizes for Arabis hoffmannii on Santa Cruz Island since 1990. Only 19 individuals were observed in the Centinela population in 1993; this represented a net loss of 13 individuals from the previous year, with mortality of nine of those plants "directly attributed to pig rooting".
The collection of whole plants or reproductive parts of Arabis hoffmannii could adversely affect the genetic viability and survival of this taxa.
Conservation and Recovery
The Hoffman's rockcress only survives in perilously small numbers on a few, inaccessible cliff-ledges on Santa Rosa and Santa Cruz Islands. The broader habitat is being conserved in a relatively natural condition in the Channel Islands National Park, and by the Nature Conservancy, a private conservation organization. Nevertheless, the endangered rockcress and other rare plants are severely threatened by the feeding of sheep and other introduced mammals. The protection of the Hoffman's rockcress requires that these herbivores be reduced or eliminated from its habitat. The abundance of invasive alien plants should also be reduced or eliminated, as these are providing intense competition to native species. The populations of the Hoffman's rockcress should be monitored, and research undertaken into its biology and habitat needs, including methods of beneficial management. A captive-propagation program should be considered, to provide stock for out-planting to supplement the tiny natural population.
U. S. Fish and Wildlife Service
Ventura Fish and Wildlife Office
2493 Portola Road, Suite B
Ventura, California 93003
Telephone: (805) 644-1766
Fax: (805) 644-3958
U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121
U.S. Fish and Wildlife Service. 31 July 1997. "Endangered and Threatened Wildlife and Plants; Final Rule for 13 Plant Taxa From the Northern Channel Islands, California." Federal Register 62 (147): 40954-40974