Santa Rosa Island Manzanita
Santa Rosa Island Manzanita
|Listed||July 31, 1997|
|Description||A medium-to low-growing shrub.|
|Habitat||Occurs in chaparral and woodland communities.|
|Threats||Soil loss by erosion, herbivory by introduced mammals, and low reproductive success.|
Arctostaphylos confertiflora (Santa Rosa Island manzanita) was described in 1934 from a collection made four years earlier "in a sheltered dell south of Black Mountain" on Santa Rosa Island. The new combination Arctostaphylos subcordata var. conferti-flora was published in 1958. However, the original taxonomy was used in subsequent treatments of the genus in 1968 and 1993.
Arctostaphylos confertiflora is a perennial shrub in the heath (Ericaceae) family that grows 4 in to 6.5 ft (10 cm to 2 m) high. The plant has smooth, dark red-purple bark, densely hairy branchlets, bracts, and pedicels, and light green, round-ovate leaves. The flowers are borne in numerous dense panicles that mature into flattened reddish-brown fruits. The only other manzanita that occurs on Santa Rosa Island, Arctostaphylos tomentosa, forms a fire-resistant burl at the base of the stems. Arctostaphylos conferti-flora is not burl-forming and is considered an obligate seeder, requiring fire for regeneration. It occurs in prostrate and upright forms, the former most likely due to climatic and herbivorous influences.
Arctostaphylos confertiflora is found on Santa Rosa Island on sedimentary substrates of Monterey shales and soft volcanoclastic sediments derived from San Miguel volcanics. Near the southern tip of the island, a few individuals are scattered on the slopes above South Point on sandstone outcrops. The taxon occurs as a component of mixed chaparral, mixed woodland, Torrey pine woodland, and island pine woodland communities.
Arctostaphylos confertiflora is known only from two areas on Santa Rosa Island. All but a few plants occur in the northeast portion of the island, near and east of Black Mountain. Individual plants have been observed at scattered sites from upper Lobo Canyon east to the Torrey pine groves along Beechen's Bay, a distance of about 3 mi (5 km). The total habitat for the plant comprises only a few acres, with the taxon occurring in low numbers. During 1994 surveys, three small patches were mapped within the Torrey pine groves, two in canyons on the north side of Black Mountain and one plant near South Point. Additional surveys of potential habitat were begun in 1996 by United States Geological Survey Biological Resources Division (BRD) staff, but few shrubs have been found to date. Observed shrubs have had recent twig growth browsed off by deer, and no seedlings or young plants have been noted. Ungulates have access to more than 90% of the fewer than 400 remaining plants, all restricted to nearly vertical canyon walls in eight populations in the Black Mountain vicinity. Despite the steepness of the slopes, deer and elk are capable of traveling along trails which provide access to various portions of the populations. A few individuals are also known from Johnson's Lee on the south side of the island. Researchers observed that elk and deer bed down in the shade of larger shrubs, including Arctostaphylos confertiflora, causing compaction and erosion of soils, and exposing the roots of the plants.
Arctostaphylos confertiflora is threatened by soil loss, low reproductive success, and herbivory by elk and deer that has contributed to reproductive failure. The seed bank is either absent or so depleted as a result of soil loss that a catastrophic fire could eliminate the species because recruitment is dependent upon fire treated seed.
A 1990 report noted that most individuals of Arctostaphylos confertiflora are browsed severely by elk and deer. During a recent population survey it was observed that more than 90% of all individuals of Arctostaphylos confertiflora were accessible to ungulates and were browsed at the growing tips. The shape of individual shrubs has been modified as a result of browsing. Short-statured shrubs have been hedged to the point that they do not grow above a certain height. On shrubs that attained a taller stature before browsing pressure became severe, all lower limbs and leaves have been stripped, resulting in a "lollipop" or tree-shaped shrub. Browsing pressure on this species appears to have affected its ability to reproduce, since not a single seedling was observed during a 1988 survey. Wherever shrubs of Arctostaphylos confertiflora have been browsed to form a canopy, the understory is heavily trampled by deer and elk and the bedrock is eroding away around the roots. A. confertiflora does not have a root crown burl that allows some mainland species to tolerate low levels of defoliation; without protection from non-native mammals, continued recruitment failure and reduced genetic vigor may prove catastrophic for this species. This condition was noted in a 1989 letter to Dr. Peter Raven from the leading authority on the genus Arctostaphylos, Dr. Phillip Wells, who expressed his concern that the time remaining for the grazing operation would precipitate the extinction of Arctostaphylos confertiflora if some protection from non-native mammals was not implemented.
Conservation and Recovery
The Santa Rosa Island manzanita only survives in small numbers in two areas on Santa Rosa Island. Although the island is owned by the National Park Service, a cattle ranching operation and a subleased commercial deer and elk hunting operation are operating under 5-year special-use permits, which are renewable until the year 2011 (feral pigs are not part of this agreement, and the National Park Service has eradicated these animals from the island). Consequently, the Santa Rosa Island manzanita and other rare plants are still severely threatened by the feeding of introduced mammals and associated factors. The survival of this endangered plant requires strict protection from the feeding of mammals. Until the special-use permits expire, this could be done by securely fencing the plants. Afterwards, the mammals should be eradicated from the critical habitat. The populations of the Santa Rosa Island manzanita should be monitored, and research undertaken into its biology and ecological requirements. It should be propagated in captivity, to supply stock for out-planting to supplement the extremely small wild population.
U.S. Fish and Wildlife Service
Ventura Fish and Wildlife Office
2493 Portola Road, Suite B
Ventura, California 93003-7726
Telephone (805) 644-1766
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.