Catalina Island Mountain-mahogany

Cercocarpus traskiae

Description

Catalina Island mountain-mahogany, Cercocarpus traskiae, a member of the rose family (Rosaceae), is an evergreen shrub or small tree that flowers from March to May. The flowers lack petals and occur in clusters of four to ten. The hypanthium (floral structure derived from the fused lower portions of sepals, petals, and stamens) is densely white-woolly, and is approximately 0.5 in (1.2 cm) long. The fruit is an achene with a persistent plumose style, which dries in a spiral, typical of the genus.

The leathery, clustered leaves are simple, serrate (toothed), and range from 1-2.5 in (2.5-6.3 cm) long. The upper surface of the leaf is glabrous (smooth); the lower surface is densely white-woolly. C. betuloides var. blancheae is relatively common on Santa Catalina Island, though distinct from C. traskiae. It is differentiated from C. traskiae by the strigose (with stiff, sharp, appressed hairs) undersides of its leaves and by the pubescence of the floral tube. In addition, the leaves of C. betuloides var . blancheae are not leathery.

Cercocarpus traskiae was first described by Alice Eastwood in 1898 from a specimen collected by Blanche Trask the year before. Its classification changed several times during the twentieth century. C. traskiae was reduced to a variety of C. betuloides in 1940. Ten years later, this taxon was included as a variety of C. montanus, but in 1959, its name was reverted back to C. betuloides var. traskiae. In 1968, C. betuloides var. traskiae was returned to the species rank of C. traskiae. C. traskiae is recognized at the species level by current taxonomy.

Habitat

Cercocarpus traskiae is endemic to a particular soil type derived from sausserite gabbro parent material. C. traskiae occurs there in coastal sage scrub containing Eriogonum fasciculatum (California buckwheat), Salvia mellifera (black sage), and Rhus integrifolia (lemonade berry). The Santa Catalina Island Conservancy (SCIC), a private corporation which owns 86% of the land on Santa Catalina Island, owns all of the habitat occupied by C. traskiae.

Distribution

Cercocarpus traskiae, one of California's rarest trees, occurs almost exclusively in Wild Boar Gully—a steep-sided, narrow arroyo—on the southwestern portion of Santa Catalina Island. When C. traskiae was originally discovered here a century ago there were an estimated 50 individuals in that population; there are now only six mature trees left. The SCIC has planted C. traskiae seedlings in test plots, but the results of this planting are not yet known.

A single individual of Cercocarpus traskiae was discovered in the Santa Monica Mountains by David Carroll in 1993; although additional individuals may exist in the Santa Monica Mountains, this taxon is not likely to be widespread or common. The single mainland specimen may represent a remnant of an ancestral or sister population of C. traskiae, or a hybrid between C. traskiae and its mainland variety C. betuloides. It is also possible that this specimen was planted.

Threats

Cercocarpus traskiae is imperiled by destruction and degradation of habitat, erosion, predation by feral animals, competition with exotic plant species, fire, and hybridization.

The decline of Cercocarpus traskiae to the point of being endangered is primarily due to grazing by feral goats and pigs on Santa Catalina Island. Such grazing severely damages the habitat that supports this taxon, and severe browsing may kill plants directly and prevent successful reproduction by surviving individuals. Non-native mule deer are also known to consume Cercocarpus traskiae.

A goat and pig management program has reduced the number of feral herbivores, but the threat to C. traskiae still remains.

Pigs continue to degrade the habitat of this plant by preventing surface litter from accumulating. Surface litter holds moisture and seeds on the steep slopes. Pigs also create a network of bare trails with compacted soils. The vegetation loses its tiered, overlapping structure because shrubs become isolated by surrounding trails. Then the pigs severely limit the recovery of this taxon because they uproot new seedlings while searching for food. A noticeable increase in seedlings of all types has been observed since the numbers of pigs and goats have declined.

Before the number of goats and pigs were reduced, their combined impact on the island's habitat nearly drove Cercocarpus traskiae to extinction. Fencing was installed around two individuals in the late 1970s, then improved and enlarged to exclude pigs in 1985; perimeter fencing was also added to limit access by other non-native animals. Seedling counts, as a result of this enhanced protection, increased from just one in 1984 to 70 seedlings in 1988. By 1994, however, a total of only 54 seedlings was found. Most of the C. traskiae trees do not have individual pig-proof fencing around them and the perimeter fencing does not exclude pigs. Approximately 2,000 pigs remained on Santa Catalina Island in 1994. It appears that the SCIC pig removal program has waned since then, as the current estimate of the numbers of pigs on Santa Catalina Island is 2,000 to 3,000 animals.

Although managers for the Santa Catalina Island Conservancy have removed more than 8,000 goats from the island, 300 to 400 goats remained on the island in 1994. Due, in part, to decreased management since 1994, the current estimate of the goat population on Santa Catalina Island is now between 1,000 to 1,500 animals. Populations of introduced mule deer are increasing as well, currently estimated at 500 to 700 animals. Although reduced predation by goats resulted in successful basal sprouting of Cercocarpus traskiae, a continued increase in goat and deer populations would likely reverse this trend. Perimeter fencing along Wild Boar Gully limits the access of deer and goats to C. traskiae, but it does not entirely exclude them.

Cercocarpus traskiae would likely suffer high mortality from fire on Santa Catalina Island. Members of the genus Cercocarpus are long-lived, a trait typical of shrubs in low fire frequency areas. The effects of a severe fire on this species would be significant because so few mature individuals remain and the species is not known to be a stump-sprouter following fire events. Grazing by feral herbivores would inhibit the establishment of any new shoots which might sprout following fire.

Cercocarpus traskiae is threatened by hybridization with the locally common C. betuloides var . blancheae. Because only six mature individuals of C. traskiae are known to exist, genetic swamping of the species would be the probable outcome of hybridization. The uniqueness of the species would be compromised or lost due to the influx of genetic variability from the larger population.

Field biologists in 1989 recommended elimination of mature hybrids as a means of preserving the species.

Cercocarpus traskiae 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

All of the critical habitat of the Catalina Island mountain-mahogany is owned by the SCIC. This agency tries to maintain the habitat in a natural condition. In the mid-1990s, the wild population of the rare tree was only six mature trees. The SCIC has been raising seedlings in captivity and out-planting them to supplement the tiny natural population. In 1993, a single individual of the Catalina Island mountain-mahogany was discovered in the Santa Monica Mountains of the California mainland; additional work is needed to determine whether this represents a natural population or a planted one. It is essential that the introduced populations of goats, pigs, and deer be eliminated from Santa Catalina Island. If this is not done, then the longer-term prospects of the rare mountain-mahogany and other endangered endemics are extremely risky. In the meantime, surviving individuals of the Catalina Island mountain-mahogany must be enclosed within herbivore-proof fences. Invasive plants in its habitat must be managed, as well. The population of the mountain-mahogany should be monitored, and research conducted into its biology and habitat needs. Additional work on out-planting is urgently needed.

Contacts

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
http://pacific.fws.gov/

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.