Otay Tarplant

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Otay Tarplant

Hemizonia conjugens

ListedOctober 13, 1998
FamilyCompositae (Asteraceae)
DescriptionGlandular, aromatic annual with deep green or gray-green leaves covered with soft shaggy hairs and yellow flowers.
HabitatClay soils on slopes and mesas.
ThreatsUrbanization, agricultural conversion, invasion of non-native plants, off-road vehicle use, increased erosion, grazing, trampling by humans.


Otay tarplant, Hemizonia conjugens, a glandular, aromatic annual in the aster family (Asteraceae or Compositae), has a branching stem 2-9.8 in (5-25 cm) in height and deep green or gray-green leaves covered with soft, shaggy hairs. The yellow flower heads are composed of eight to 10 ray flowers and 13-21 disk flowers with hairless or sparingly downy petals. The phyllaries (bracts, or modified leaves, below the flower head) are keeled with short-stalked glands and large, stalkless, flat glands near the margins. Otay tarplant occurs within the range of H. fasciculata and H. paniculata. Otay tarplant can be distinguished from these species in having eight to 20 ray flowers.

Otay tarplant was first described in 1958 based on a specimen collected in 1903 from river bottom land in the Otay Valley area of San Diego.


Otay tarplant distribution is highly correlated with the distribution of clay soils or clay subsoils. It is typically found in clay soils on slopes and mesas and is associated with native grasslands, mixed grasslands (i.e., native grassland interspersed with non-native grass species such as ripgut grass [ Bromus diandrus ], foxtail chess [ B. rubens ], and hare barley) and open, grass coastal sage scrub.


Three of the 25 historic localities of Otay tarplant in the United States have been eliminated. It is likely, however, that other unreported populations have also been eliminated as about 70% of the suitable habitat for this species within its known range has been developed or is under cultivation. Otay tarplant currently has a limited distribution near Otay Mesa in southern San Diego County, California; there is one known population near the U.S. border in Baja California, Mexico.

About 30,300 acres (12,262 hectares) of land with clay soils or clay subsoils are situated within the general range of Otay tarplant in San Diego County. Clay soils are dense and favor grassland development. It is likely that much of this area was once vegetated with native grassland and open and grassy coastal sage scrub, which provided suitable habitat for Otay tarplant. About 37% of this area has been urbanized and another 37% has been cultivated. Although the cultivated lands could be restored to natural habitat capable of supporting Otay tarplant, these areas do not currently support this species and are not likely to support the species in the foreseeable future based on proposed land use. Thus, only 26% of the historically available area is still available to Otay tarplant.

Otay tarplant, like many annual species, can vary significantly in numbers of individuals from one year to the next due to a variety of factors, including rainfall, timing of rainfall, and temperature. In the 22 extant populations in California, there may be as many as 300,000 individuals under favorable conditions; however, the number of individuals in any given year is probably considerably less. Without knowledge of the species' demography, seedbank and seedbank dynamics, estimations of effective population size are impossible. Until its rediscovery in Baja California in 1977, this species was considered potentially extinct in California as a result of extensive development within its range.

Of the 22 extant populations of Otay tarplant in California, 12 have more than 1,000 individuals. The largest population complex, Horseshoe Bend-Gobblers Knob (Rancho San Miguel) supports about 200,000 individuals, more than 65% of all known plants. The five largest populations (Horseshoe Bend-Gobblers Knob, Rice Canyon, Poggi Canyon, Proctor Valley, and Dennery Canyon) support about 94% of all reported individuals. Of the 17 remaining populations, seven are reported to support 1,000-6,000 individuals each, and 10 support fewer than 1,000 individuals each. All populations of this species in the United States are on private lands.


Habitat destruction or modification adversely affects species native to this area by reducing population densities and contributing to habitat fragmentation. Rapid urbanization and agricultural conversion in Orange and San Diego Counties has already eliminated or reduced populations of this species. The trend of habitat loss and fragmentation is expected to continue as the population of southern California expands. This species is also adversely affected by the invasion of non-native plants, off-road vehicle use, increased erosion, grazing, and trampling by humans.

Otay tarplant appears to tolerate mild levels of disturbance such as light grazing. Such mild disturbances create sites necessary for germination; however, the species is otherwise threatened by activities such as development and intensive agriculture.

Conservation and Recovery

About 70% of the potentially suitable habitat for this species has been cleared for agriculture and urbanization. About 40% of all remaining individuals will be eliminated by currently approved and proposed development projects. These impacts have been considered by the U. S. Fish and Wildlife Service through development of the Multiple Species Conservation Program. Of the remaining populations after implementation of these various developments, about 90% will be situated adjacent to, or within the immediate vicinity of, urban development and recreation areas. These plants will be threatened by the secondary effects of encroaching development (e.g., non-native plant species replacement, isolation, and fragmentation). Management provided through the Multiple Species Conservation Program and on San Diego National Wildlife Refuge lands, however, will help alleviate these effects for projects subject to the Multiple Species Conservation Program.

The four largest populations (Horseshoe Bend, Rice Canyon, Dennery Canyon, and Proctor Valley) of Otay tarplant support 90% of all known individuals. At Horseshoe Bend, the largest population (about 65% of all individuals) will be impacted by a residential-commercial development project (Rancho San Miguel), utilities, and State Route 125. These impacts will result in loss of about 60% of the individuals and most of the occupied habitat in the Rancho San Miguel complex. The remaining portion of the Horseshoe Bend population, which constitutes about 35% of the known individuals of the species, will be conserved as part of the Multiple Species Conservation Program. Direct impacts to the Rice Canyon population (about 15% of all individuals) have been for the most part avoided. The remaining population, however, is isolated and in proximity to urban development. It is likely that this population will decline significantly in the foreseeable future. A third major population is located in the vicinity of Dennery Canyon. The majority of this population will be conserved in open space. A significant portion of the potential habitat within the population, however, was impacted by grading in the spring of 1997 for a residential-commercial project (Cal Terraces). This project resulted in preservation of 3 acres (1.2 hectares) out of 17.5 acres (7.08 hectares) of suitable habitat on the project site. The fourth largest population (Proctor Valley) is partially within an approved development.

Several populations of Otay tarplant have also been affected by off-road vehicle activity on Otay Mesa. For example, about 30 acres (12.1 hectares) of suitable and occupied habitat at Dennery Canyon have been severely impacted by off-road vehicle activities. Implementation of the Multiple Species Conservation Program requires that these effects be alleviated.

Several other major populations of Otay tarplant will be largely conserved (Wolf Canyon, Otay Valley, Old Salt Creek, Jamacha Hills); however, these populations will be adjacent, or in proximity, to recreation or future development. In addition, populations that are conserved through the development process may be affected by federal and state activities not subject to the Multiple Species Conservation Program, including state transportation projects (California Department of Transportation), border fencing, off-road vehicle activity, and new facilities (Immigration and Naturalization Service), and airport expansion (Federal Aviation Administration). One alternative for State Route 125 may affect as much as 57 acres (23 hectares) of Otay tarplant habitat. State Route 905 passes through suitable habitat and expansion of this highway will likely reduce the extent of this habitat. At least five populations of Otay tarplant on Otay Mesa are at risk from U.S. Immigration and Naturalization Service Border Patrol activities due to the proximity of the U.S.-Mexican border. Off-road vehicle activity relating to Border Patrol activities has impacted and potentially significantly reduced one major population (Spring Canyon). These activities also impact considerable suitable but currently unoccupied habitat on private land on Otay Mesa. Another population may be impacted by a proposed Border Patrol field station on Otay Mesa. To some degree those populations covered under the Multiple Species Conservation Program will still be subject to the effects of habitat fragmentation, off-road vehicle activity, and disturbance described previously in this rule.

The majority of the known populations of San Diego thorn mint (Acanthomintha ilicifolia ), Dudleya stolonifera, and Otay tarplant occur on privately owned land. Actions on private lands that may significantly affect biological resources require review under the California Environmental Quality Act. Local lead agencies empowered to uphold and enforce the California Environmental Quality Act have made determinations that have affected, or will adversely affect, these species and their habitats.

In 1991, the State of California established the Natural Communities Conservation Planning program to address conservation needs of natural ecosystems throughout the state. The focus of the current planning program is the coastal sage scrub community in southern California, although other vegetation communities are being addressed in an ecosystem approach. Otay tarplant is currently covered under the Multiple Species Conservation Program and the Central/Coastal Subregional Natural Communities Conservation Planning/Habitat Conservation Plan (Central/Coastal Natural Communities Conservation Planning) of Orange County, California, and is being considered for inclusion as a covered species under the Multiple Habitat Conservation Plan.


U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N. E. 11th Ave.
Portland, Oregon 97232-4181


U. S. Fish and Wildlife Service. 13 October 1998. "Determination of Endangered or Threatened Status for Four Plants from Southwestern California and Baja California, Mexico." Federal Register 63 (197): 54937-54956.