Bakersfield Cactus

Opuntia treleasei

Description

Like other beavertail cacti, Bakersfield cactus, Opuntia treleasei, has fleshy, flattened, green stems (pads) that vary from rounded, heart-shaped, or diamond-shaped to nearly cylindrical. A single plant may consist of hundreds of pads, which originate both at ground level and from the tips of other pads. The number of individuals in a population may be difficult to determine because pads from adjacent plants often overlap. Thus, cactus populations usually are described by the number of clumps (groups of pads that are rooted at the same point) rather than as a number of individuals. Clumps of Bakersfield cactus can grow up to 14 in (35 cm) high and 33 ft (10 m) across. The pads and fruits are dotted with eyespots, which are rounded structures that contain barbed bristles. Tiny leaves are produced on the youngest pads of beavertail cacti but are shed quickly. Bakersfield cactus has showy magenta flowers. The dry fruits are the size and shape of small eggs and may contain grayish-white seeds. Bakersfield cactus is unique among the varieties ofO. basilaris in that the eyespots contain spines in addition to the bristles. Other features of Bakersfield cactus that differentiate it from related beavertail cacti include the smooth pad surfaces, cylindrical pad bases, nonsunken eyespots, and longer leaves. Bakersfield cactus has also been classified as O. basilaris var. treleasii.

Habitat

Bakersfield cactus is a perennial. The life span of wild plants has not been determined, but clumps in cultivation survived for 48 years, until extremely wet winter weather caused the pads to rot. Bakersfield cactus typically flowers in May. Reproductive biology of this taxon has not been studied, but certain other Opuntia species require cross-pollination for seed-set and many are pollinated by bees. One potential pollinator of Bakersfield cactus is the native solitary bee Diadasia australis ssp. california, which is known to occur in Kern County and which specializes in collecting pollen from Opuntia species. Vegetative reproduction, which is the production of new plants from sources other than seed, is typical in Bakersfield cactus and several related species. Fallen pads root easily if sufficient water is available, but Bakersfield cactus does not survive prolonged inundation. Cactus seeds require warm, wet conditions to germinate, a combination that is extremely rare in the Bakersfield area. Pads may be dispersed by flood waters, but seed dispersal agents are unknown.

Soils supporting Bakersfield cactus typically are sandy, although gravel, cobbles, or boulders also may be present. Known populations occur on floodplains, ridges, bluffs, and rolling hills. Bakersfield cactus is a characteristic species of the Sierra-Tehachapi Saltbush Scrub plant community, but populations near Caliente are in Blue Oak woodland and the Cottonwood Creek population is in riparian woodland. Many Bakersfield cactus sites support a dense growth of red brome and other annual grasses. Sand Ridge is characterized by sparse vegetation and a preponderance of native species such as California filago and yellow pincushion. Historical records indicate that the majority of Bakersfield cactus occurred at elevations of 460-850 ft (140-260 m). The highest-elevation population is at 1,800 ft (550 m) near Caliente and the lowest remaining is at 396 ft (121 m) at Fuller Acres.

Distribution

Bakersfield cactus is endemic to a limited area of central Kern County in the vicinity of Bakersfield. Approximately one-third of the historical occur-rences of Bakersfield cactus have been eliminated, and the remaining populations are highly fragmented. The range was extended to the south, however, when several occurrences were discovered in the late 1980s in south-central Kern County, just north of Wheeler Ridge.

The total population of Bakersfield cactus was not estimated historically. Historical photographs showing extensive stands of Bakersfield cactus are believed to have been taken southwest of Sand Ridge near the eastern margin of the Kern Lake bed. When the known sites were last inventoried, fewer than 20,000 clumps of Bakersfield cactus were estimated to remain. Only four areas had populations of 1,000 clumps or more: Comanche Point, Kern Bluff, north of Wheeler Ridge, and Sand Ridge. The metapopulations reported to incorporate the greatest morphological diversity included those in the Bena and Caliente Hills, Kern Canyon, and Sand Ridge.

Threats

The primary reason for the decline of Bakersfield cactus was habitat loss. The formerly extensive tracts of Bakersfield cactus near Edison and Lamont were destroyed by the conversion of habitat to row crops and citrus groves; much of the conversion occurred prior to 1931. In the late twentieth century, residential development eliminated numerous occurrences in northeast Bakersfield. Petroleum production has contributed to habitat loss and fragmentation, particularly in the vicinity of Oildale. Populations near Hart Park, the Kern Bluffs, Oil-dale, Fairfax Road, and parts of Sand Ridge have been degraded by off-road vehicle activity, trash dumping, and sand and gravel mining. Overgrazing may have damaged plants near Hart Park, Mettler, and Caliente, and flooding decimated populations along Caliente Creek and the Kern River. Air pollution is also suspected to have contributed to the decline of Bakersfield cactus.

At the end of the twentieth century, all these causes of decline continued to threaten existing populations. Almost all the known sites were on private land, much of which had commercial value. Residential development constituted the most serious threat. Conversion for either agricultural or residential use was possible, and inundation was a potential intermittent problem for populations in floodplains. The largest concentration of clumps in the Wheeler Ridge metapopulation was situated adjacent to an overflow drain for the Aqueduct, resulting in a risk of flooding if an earthquake occurred anywhere along the drain's length. Even the two protected populations were adjacent to agricultural land and faced impacts from pesticide drift. Both off-road vehicle use and mining continued to degrade the populations mentioned earlier.

Direct competition from introduced, annual grasses is believed to threaten the survival of mature Bakersfield cactus plants and to hinder the establishment of new plants. Indirect effects from exotic grasses also may threaten Bakersfield cactus in several ways. First, the dense herbaceous growth may promote a greater fire frequency and intensity than would have occurred with the sparse native vegetation typical in historical times. Second, dense grass cover may harbor insects that damage cactus. Third, the moist microclimate created by dense herbaceous growth may promote growth of decay organisms and cause pads to rot in years of above-average precipitation.

A lack of genetic diversity may threaten some populations of Bakersfield cactus. Contributing factors to this problem include the small size of many populations, a lack of gene flow between populations, and infrequent sexual reproduction. Populations low in genetic variation are more vulnerable to diseases and parasites and to chance events, including environmental fluctuations, catastrophes, and genetic drift.

Conservation and Recovery

The Nature Conservancy began preservation efforts for Bakersfield cactus more than 25 years ago by purchasing a portion of Sand Ridge. The Nature Conservancy doubled the size of the Sand Ridge Nature Preserve to 275 acres (110 hectares), and the preserve was transferred to the Center for Natural Lands Management in 1998. Prescribed burns will be used to control exotic grass competition. Several colonies of Bakersfield cactus were acquired for conservation purposes in the late 1990s.

Salvage efforts have been undertaken by local members of the California Native Plant Society, who transplanted Bakersfield cactus clumps from sites slated for destruction to Sand Ridge Nature Preserve and the California Living Museum in Bakersfield. Prior to construction of the East Hills Mall in Bakersfield, a few of the cactus clumps growing on the site were removed, then were replanted in a display bed when the mall was completed. Transplanted individuals have not been monitored at any of the sites to determine survival rates or reproductive success.

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
Sacramento Fish and Wildlife Office
Federal Building
2800 Cottage Way, Room W-2605
Sacramento, California 95825-1846
Telephone: (916) 414-6600
Fax: (916) 460-4619

References

Benson, L. 1969. The Native Cacti of California. Stanford University Press, Palo Alto, Calif.

Heady, H. F. 1977. "Valley Grassland." In Terrestrial Vegetation of California, edited by M. G. Barbour and J. Major. Wiley, New York.

Hoover, R. F. 1970. The Vascular Plants of San Luis Obispo County, California. University of California Press, Berkeley.

Wester, L. 1981. "Composition of Native Grasslands in the San Joaquin Valley, California." Madrono 28: 231-241.