Del Mar Manzanita

Arctostaphylos glandulosa ssp. crassifolia

Description

The Arctostaphylos glandulosa ssp. crassifolia (Del Mar manzanita), a member of the heath family (Ericaceae), belongs to a group of six recognized subspecies occurring in California and northwest Baja California, Mexico. This particular plant is an erect shrub, generally 3.3-4 ft (1-1.2 m) tall, but occasionally higher when influenced by other subspecies.

The Del Mar manzanita is distinguished from other subspecies of Arctostaphylos glandulosa by its shorter stature, other subspecies are up to 8.2 ft (2.5 m) in height, and by its dark gray-green leaves that are glabrate above and tomentulose beneath. The branchlets and leaf-like bracts are nonglandular and tomentulose with scattered long hairs or bristles. Generally, A. glandulosa (Eastwood manzanita) is a relatively open, smooth, and dark red-barked shrub characterized by a basal burl and scarcely leaf-like bracts that are shorter than the hairy flower-stalks. Four of six subspecies of A. glandulosa lack nonglandular, tomentulose hairs and scattered white bristles on the branchlets, bracts, and leaves. Of the remaining two subspecies, A. g. ssp. mollis of the western Transverse Ranges has smooth, shiny, and bright green leaves, as well as more uniformly distributed long white bristles, while A. g. ssp. glaucomollis of the San Gabriel and San Bernardino Mountains lacks leaf-like bracts.

Habitat

The Del Mar manzanita, considered an indicator species for the plant community it occurs in, is restricted almost exclusively to sandstone-derived soils in southern maritime chaparral along the south-central coast of San Diego County, California. It also occurs in disjunct populations in northwestern Baja California, Mexico, at least as far south as Mesa el Descanseo, 25 mi (40 km) north of Ensenada. This plant is known to grow in Mexico up to an elevation of 2,400 ft (730 m).

Distribution

The Del Mar manzanita extends from the south-central coast of San Diego County south into northwestern Baja California, Mexico. It is restricted to sandstone terraces and bluffs from Carlsbad south to Torrey Pines State Park, extending inland to Rancho Santa Fe and Del Mar Mesa in San Diego County. An additional population has been reported just south of the San Dieguito River southwest of Lake Hodges. This species has been reported from five localities in northwestern Baja California, ranging from just east of Tijuana along the United States border to Cerro el Coronel and Mesa Descanseo 25 mi (40 km) south of the United States border. These populations may no longer be extant due to considerable urban and agricultural development in the Tijuana vicinity. The most recent collection in the San Diego Museum of Natural History was made by Reid Moran in 1982.

Approximately 16,600-17,600 individuals of Del Mar manzanita were known to be distributed over about 26 population centers in 1982. A significant number of these populations have been severely curtailed since then; a good example of this loss occurred in 1987, when one population of nearly 500 individuals near San Dieguito Creek and the surrounding southern maritime chaparral habitat was cleared and converted to agriculture. Cultivation at this site was active for one season and has not been continued. In addition, nearly 50% of the individuals known from the vicinity of Miramar Reservoir in 1982 were eliminated by the Scripps Ranch development between 1989 and 1992. About 9,400-10,300 individuals, scattered roughly throughout the historic distribution of the species in San Diego County, are known to be extant. About 75% of all remaining individuals are located within six concentrations. The majority of the 26 populations are found on private land; none are known from federal lands and only four occur in state, county, and local parks. The number of individuals in Mexico is not known, although populations in Baja California, based on the limited availability of habitat, are likely to be smaller than in the United States.

Four Arctostaphylos glandulosa populations of about 3,000 total individuals in the vicinity of Miramar Reservoir have been attributed to Del Mar manzanita, but these plants may be intermediate with other subspecies of A. glandulosa and cannot be definitely placed. Even including these populations as Del Mar manzanita would not significantly alter the jeopardized status of this species.

Threats

While 25 of 26 populations of the Del Mar manzanita are still extant, the majority of them have been greatly reduced in numbers and significantly fragmented in makeup by urban and agricultural development, most of which has occurred since the 1980s. There has been about a 50% decline in the number of stands and the number of individuals since 1982. The majority of the remaining individuals are distributed in highly fragmented habitat along the margins of residential development.

Over 75% of the Del Mar manzanita in the United States occurs in six concentrations located in Carlsbad, Encinitas, Del Mar, and Torrey Pines State Park. Four of the six populations, located in Carlsbad and Encinitas, are partly threatened by approved or proposed development projects. These projects will result in the direct elimination of over 1,900 Del Mar manzanita individuals that occur within these six populations. This very substantial loss will only be compounded by the likely additional loss of 1,000 individuals through fuel modification, edge effects, and other indirect but damaging impacts. These two sequences of development-induced effects will in all likelihood reduce the six Del Mar manzanita populations by 35% and 20%, respectively. Several of the smaller populations of this species in Encinitas, Carlsbad, Carmel Valley, and on Carmel Mountain are also threatened by development and associated indirect impacts. There are 10 development projects that may eliminate about 3,000 individuals. This loss, if realized, would eliminate close to 40% of the total population. This rate of decline would be close to the rate of historical losses incurred from the early 1980s through the early 1990s.

The effects of habitat fragmentation and isolation, harmful enough in themselves, are only exacerbated in areas containing this plant that are adjacent to residential development. Most of the populations of this species are relics of larger historic populations. Nearly 15% of extant Del Mar manzanita individuals occur in fragmented, isolated, and poorly configured parcels of open space that are smaller than 50 acres (20 hectares). These disjunct populations, very often surrounded by encroaching developments, are subject to the edge effects of invasion by exotic plants and physical disturbances from nearby residents. They also face threats from the fuel modification activities of discing, reduction of flammable material through thinning, and the maintenance of fire breaks. The effect of isolation and habitat size reduction also retards natural fire and successional cycles within Del Mar manzanita habitat.

Of the six largest populations of this species, 20% of the individuals are within 200 ft (61 m) of existing development and are threatened by edge effects. This is exemplified by Crest Canyon Preserve, where nearly 50% of the approximately 1,000 Del Mar manzanita individuals are within 200 ft (61 m) of development. The populations here are affected by trampling associated with recreational activities and edge effects. This plant is also threatened by trampling in other areas where trails have been cut through populations by recreationalists and farm workers.

Even the populations that are located in "protected" areas are still subject to adverse impacts. Of the larger and more significant occurrences of this plant, only the population located in Torrey Pines State Park is protected and managed for long-term preservation of biological resources; however, this population is located within a 200-acre (81-hectares) parcel that is completely surrounded by development. While another population at the upper end of Agua Hedionda is also under public management, it is subject to incremental clearing impacts as a result of adjacent airport operations, road-widening activities, and clearing related to mulching and agriculture. This population is also bisected by numerous footpaths. At least 15% of this population is situated within 200 ft (61 m) of development. A small population of Del Mar manzanita located within San Dieguito County Park is threatened by edge effects and trampling from recreational activities.

The Del Mar manzanita also occurs within Oak Crest Park in Encinitas. The situation in this publicly owned and managed park shows how often local land-use regulations fail to protect rare species. Although a portion of the park was originally set aside for conservation purposes by the County of San Diego, recreational development here has eliminated southern maritime chaparral habitat and Del Mar manzanita individuals, as well as plants of two other listed species. One recently developed park area encompassed a natural preserve site set aside under an agreement between the city and the California Coastal Commission. Current recreational development plans for Oak Crest Park, including the construction of a community center, swimming pool, and numerous walking paths, will further damage this species by eliminating up to one-third of its southern maritime chaparral habitat. This will also expose the population there to greater threats from invasive exotic plants and pedestrian trampling. The Del Mar manzanita is also known to occur in areas where highway projects have been proposed.

The status of this species and its habitat in extreme northwestern Baja California is not well documented. However, Del Mar manzanita only extends some 25 mi (40 km) south of the United States border, and this region represents one of the most environmentally compromised areas in Baja California. The habitat-destroying factors of urban and agricultural development that have affected the status of this plant in the United States are also clearly having an impact south of the border.

Conservation and Recovery

Four of the 26 surviving populations of the Del Mar manzanita occur in state, county or local parks. Only one population is protected and managed for long-term preservation (in Torrey Pines State Park); however, this population is located within a 200-acre (80 hectares) parcel that is completely surrounded by development. All of the publicly owned habitats of the rare manzanita should be protected against agricultural or residential development. In addition, the largest populations on privately owned land should be protected. This could be done by acquiring the land and designating ecological reserves, or by negotiating conservation easements with the landowners. Additional surveys should be done to document the populations of the Del Mar manzanita in Baja California. Its known populations should be monitored, and research undertaken into its biology and habitat needs.

Contacts

U.S. Fish and Wildlife Service
Carlsbad Fish and Wildlife Office
2730 Loker Ave.
West Carlsbad, California 92008
Telephone: (619) 431-9440
Fax: (760) 431-9624

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

Reference

U.S. Fish and Wildlife Service. 7 October 1996. "Endangered and Threatened Wildlife and Plants; Determination of Endangered or Threatened Status for Four Southern Maritime Chaparral Plant Taxa from Coastal Southern California and Northwestern Baja California, Mexico." Federal Register 61(195):52370-52384.