Endangered Species Recovery Program

D. Hoovers Woolly-Star (Eriastrum hooveri)

• 1. Description and Taxonomy
  • Taxonomy
  • Description
  • Identification
• 2. Historical and Current Distribution
  • Historical Distribution
  • Current Distribution
• 3. Life History and Habitat
  • Reproduction and Demography
  • Habitat and Community Associations
• 4. Reasons for Decline and Threats to Survival
  • Reasons for Decline
  • Threats to Survival
• 5. Conservation Efforts
• 6. Recovery Strategy

1. Description and Taxonomy

Taxonomy.-- Hoovers woolly-star was named originally by Jepson (1943) as Huegelia hooveri. In a later taxonomic revision, Mason (1945) assigned the currently-accepted name of Eriastrum hooveri to the species. Both the scientific and common names honor Robert F. Hoover, who collected the type specimen in 1937 in Kern County, 11 kilometers (7 miles) south of Shafter (Mason 1945). Hoovers woolly-star is an inconspicuous member of the phlox family (Polemoniaceae).

Description.-- The wiry stems of this species may or may not branch and vary in height from 1 to 20 centimeters (0.4 to 8 inches) at flowering (Figure 11). The leaves are thread-like and may have two narrow lobes near the base. Hoovers woolly-star has tiny (less than 5 millimeters; less than 0.2 inch long), white to pale blue flowers that are nearly hidden in tufts of woolly hair. The stamens (male reproductive parts) are shorter than the corolla (Abrams 1951, Munz and Keck 1959, Patterson 1993, Taylor and Davilla 1986, Lewis 1992).

Identification.-- Many-flowered eriastrum (Eriastrum pluriflorum) frequently occurs with Hoovers woolly-star; the former has dark blue flowers that are 16 millimeters (0.6 inch) or greater in length, stamens that protrude from the corolla, and leaves with up to 10 lobes. Small-flowered Eriastrum species that occur within the same range are distinguished from Hoovers woolly-star by flower color and stamen length (Abrams 1951, Munz and Keck 1959, Patterson 1993, Taylor and Davilla in litt. 1986, Lewis 1992).

Figure 11. Illustration of Hoover's woolly-star (from Abrams, Vol. 3, 1951, with permission).

2. Historical and Current Distribution

Historical Distribution.-- Prior to 1986, Hoovers woolly-star was known from 19 sites in 4 counties, based on herbarium collections and written observations. The majority of the occurrences were on the San Joaquin and Cuyama Valley floors, and the others were from the low mountains at the west side of the San Joaquin Valley (Figure 12). In Kern County, Hoovers woolly-star was known from the vicinities of Lokern, Oildale, Semitropic, Shafter, and the Temblor Range. In Fresno County, known occurrences were concentrated near Kerman, Mendota, and Raisin City, except for one site each in the Jacalitos and Panoche Hills. The Cuyama Valley records consisted of one collection each from Santa Barbara and San Luis Obispo Counties (Taylor and Davilla 1986).

Current Distribution.-- Hoovers woolly-star since has been discovered in Kings and San Benito Counties and at numerous additional sites in the four original counties, particularly in foothill areas. Most of the occurrences are concentrated in 4 metapopulations. In descending order by estimated number of individuals, these metapopulations are (1) the Kettleman Hills in Fresno and Kings Counties, (2) Carrizo Plain - Elkhorn Plain - Temblor Range - Caliente Mountains - Cuyama Valley - Sierra Madre Mountains in San Luis Obispo, Santa Barbara, and extreme western Kern Counties, (3) Lokern - Elk Hills - Buena Vista Hills - Coles Levee - Taft -Maricopa in Kern County, and (4) Antelope Plain - Lost Hills - Semitropic in Kern County. Small, isolated populations occur in scattered areas including the Alkali Sink Ecological Reserve and the Guijarral, Jacalitos, Panoche, and Tumey Hills in Fresno County; Buttonwillow, Devils Den, Lamont, Midway Valley, and Rosedale in Kern County; and the Panoche Hills in San Benito County (Lewis 1992, 1994, CDFG 1995, Holmstead 1993, Danielsen et al. 1994, EG&G Energy Measurements 1995a,b). According to Skinner and Pavlik (1994), the species also occurs in Tulare County.

Figure 12. Distribution of Hoover's woolly-star (Eriastrum hooveri).

3. Life History and Habitat

Reproduction and Demography.--Hoovers woolly- star is an annual, but the seeds germinate later in the growing season than do those of many of the associated annual plants. Seedlings may emerge from January or February until mid-April Taylor and Davilla 1986, E. Cypher unpubl. data). The typical flowering period for Hoovers woolly-star extends from March into June (Munz and Keck 1959, Skinner and Pavlik 1994, Lewis 1992, Cypher 1994a). Pollination ecology has not been investigated. However, other members of the genus Eriastrum are pollinated by native bees (superfamily Apoidae) and beeflies (family Bombyliidae) (Grant and Grant 1965). The tiny seeds probably are dispersed by wind or by tumbling of dead stems (Taylor and Davilla 1986). Unlike many other annuals, dead stems of Hoovers woolly-star may persist until the next growing season (Lewis 1992).

Within metapopulations, Hoovers woolly-star typically occurs as scattered groups of plants, with each group occupying an area of less than 0.4 hectare (1 acre) (Lewis 1994). Densities are highly variable among sites and among years. In 1993, average densities reported for Hoovers woolly-star in occupied habitat were 3.6 per square meter (0.3 per square foot ) at Elk Hills (EG&G Energy Measurements unpubl. data), 8.4 per square meter (0.8 per square foot ) in Lokern, and 10.3 per square meter (0.9 per square foot ) in the Kettleman Hills (Cypher 1994a). However, metapopulation densities would be considerably smaller due to the presence of unoccupied stretches between the groups of plants. Densities of Hoovers woolly-star fluctuate from year to year and are highest in years of above-average precipitation (Holmstead 1993). At Elk Hills, densities in natural colonies were 5 to 15 times greater in 1993, a year of above-average rainfall, than in 1991, which was a year of average rainfall (EG&G Energy Measurements 1995a,b).

Habitat and Community Associations.-- Hoovers woolly-star seems to be much more adaptable than other endemic plants of the San Joaquin Valley. Optimal habitats for Hoovers woolly-star are characterized by stabilized silty to sandy soils, a low cover of competing herbaceous vegetation, and the presence of cryptogamic crust (a layer of moss, lichen, and algae). However, this species also has been found on loamy soils, in areas of dense vegetation, and in areas lacking cryptogamic crust (Taylor and Davilla 1986, Cypher 1994a, Lewis 1994, EG&G Energy Measurements 1995a,b). Hoovers woolly-star may reinvade disturbed soil surfaces such as well pads and dirt roads within 1 year after the disturbance ceases if seed sources remain in the vicinity (Holmstead 1993, Danielsen et al. 1994, EG&G Energy Measurements unpubl. data, R. Lewis pers. comm.). In fact, this species may benefit from light to moderate soil disturbance in areas that are densely vegetated by exotic plants (Holmstead and Anderson 1993, EG&G Energy Measurements unpubl. data).

Populations of Hoovers woolly-star occur in alkali sinks, washes, on both north- and south-facing slopes, and on ridgetops. This species occurs in a wide variety of plant communities. Most are characterized by shrubs such as common saltbush, seepweed, and matchweed (Gutierrezia californica), but shrub cover in occupied habitats typically is less than 20 percent. Herbaceous plant species frequently found in association with Hoovers woolly-star include red brome, goldfields, many-flowered eriastrum, and red-stemmed filaree. Populations of Hoovers woolly-star have been reported at elevations ranging from 50 to 915 meters (165 to 3,000 feet) (CDFG 1995, Taylor and Davilla 1986, Holmstead 1993, Cypher 1994a, Danielsen et al. 1994, Lewis 1992, 1994, EG&G Energy Measurements 1995a,b).

4. Reasons for Decline and Threats to Survival

Reasons for Decline.--Valley-floor populations of Hoovers woolly-star have been destroyed primarily by farming operations and secondarily by urban development (Taylor and Davilla 1986, E. Cypher pers. observ.).

Threats to Survival.-- Occurrences of Hoovers woolly-star in the vicinity of Buttonwillow, Lost Hills, Rosedale and sites along Interstate Highway 5 are threatened by commercial development. Agricultural conversion continues to threaten several popultions on the Valley floor. Flooding, as a result of high precipitation, groundwater recharge programs, agricultural wastewater diversion, or waterfowl management, could destroy populations in low-lying areas (Skinner and Pavlik 1994, Taylor and Davilla 1986). Dense growth of associated vegetation, such as in areas where exotic grasses dominate or where fire has been suppressed, may create unsuitable conditions for growth of Hoovers woolly-star (J. Hinshaw pers. comm.). Hoovers woolly-star remains primarily in hilly areas, many of which are oil fields; petroleum production does not pose a threat in most cases but could be detrimental if large areas of occupied habitat were disturbed. The acquisition of Elk Hills by Occidental Petroleum may lead to greater surface disturbance if rates of exploration and production are increased.

5. Conservation Efforts

Hoovers woolly-star was federally listed as threatened in 1990 (USFWS 1990; Table 1). Field surveys sponsored by USBLM, California Energy Commission, U.S. Department of Energy, California Department of Water Resources, and USFWS resulted in the discovery of many new occurrences of Hoovers woolly-star between 1986 and 1997 (Anderson et al. 1991, Taylor and Davilla 1986, Lewis 1992, 1994, Stebbins et al. 1992, Holmstead 1993, EG&G Energy Measurements 1995a,b, Enterprise Advisory Services 1997, 1998). Through a consultation with USFWS, the U.S. Department of Energy conducted periodic monitoring of six representative Hoovers woolly-star sites at Elk Hills through 1997 (EG&G Energy Measurements 1995a, 1995b, 1996, Enterprise Advisory Services 1997, 1998). Occidental Petroleum, the current owner of the Elk Hills oilfield, plans to set aside a conservation area containing Hoovers woolly-star, among other rare species (B. Cypher pers. comm.). In addition, U.S. Department of Energy has sponsored several research projects on the ecology of Hoovers woolly-star, its response to oilfield activity, and the conditions under which it will recolonize disturbed areas (Holmstead 1993, Holmstead and Anderson 1993, EG&G Energy Measurements 1995a,b, J. Hinshaw pers. comm.). Preliminary studies on the demography of Hoovers woolly-star and its response to grazing were conducted in 1993 with funding provided by USBLM, CDFG, and Endangered Species Recovery Program (Cypher 1994a). Hoovers woolly-star also has benefited from the acquisition of conservation lands for listed animals. It is known to occur on the Alkali Sink Ecological Reserve, Buttonwillow Preserve, Carrizo Plain Natural Area, Coles Levee Ecosystem Preserve, Lokern Natural Area, and Semitropic Ridge Preserve. In 1990, Mobil Oil Corporation constructed exclosures around Hoovers woolly-star on their lands in Lost Hills (Lewis 1994).

6. Recovery Strategy

Recovery of Hoovers woolly-star can be accomplished using public lands and other areas already dedicated for conservation. As with the other listed plants, the goal is to protect populations throughout the species range and representing a variety of topographic positions and community types. Considering that habitat conversion is ongoing in valley-floor areas and that oil production could increase on public lands, the continued existence of populations cannot be assumed unless a specific commitment is made to protect them from incompatible uses. Some amount of unoccupied suitable habitat is important to allow population fluctuations among years, and a buffer zone is important to minimize external influences. Thus, a minimum block size of 16 hectares (40 acres) is recommended, with an average density of 625 Hoovers woolly-star plants per hectare (250 per acre). Monitoring must continue at representative sites within each metapopulation to determine trends. Management strategies and recovery needs should be reassessed if population densities at the monitoring sites decline over 3 or more successive years of above-average rainfall that are separated by 1 or more years of below-average rainfall.