Status and Distribution: The California gnatcatcher (CAGN) was listed as an threatened species by the USFWS on March 30, 1993 (USFWS,1993). Critical habitat has recently been proposed for this species, however none has been designated to date.
The species is restricted to low (approximately 3 ft. high) stands of coastal and inland sage scrub occurring on arid hillsides, mesas and washes of southern California below 2000 feet elevation (Atwood, 1990). Atwood (1990) found that 94% of a sample of gnatcatcher localities for coastal southern California, including Los Angeles, Orange and San Diego Counties, fell at or below 250 meters (820 feet) in elevation. Inland populations in Riverside County occurred at higher elevations, with 80% of the localities falling between 120-250 meters (400 and 820 feet).
The total CAGN population is believed to be about 2,000 to 5,000 pairs (CDFG 1998) of which about 15 pairs exist on the CNF (0.75% or less of known population). The Cleveland NF population, in the upper portion of the San Diego River watershed, is the only known occurrence on National Forest System lands. Prior to the 1993 Eagle Fire there were about 30 pairs of CAGN on the Forest.
Recent sightings at Sycamore Flat (near Lytle Creek), at the confluence of Lytle Creek and Cajon Wash, and on the Etiwanda Fan (Davis et al. 1998) indicate a possible CAGN population along the lower foothills of the eastern San Gabriel Mountains that may extend onto the San Bernardino and Angeles National Forests. Historically California Gnatcatchers were common in these areas (G.Cardiff, pers. comm.). There is also a slight possibility that gnatcatchers may occur on the lower western slopes of the San Jacinto Mountains.
Recovery Plan: None.
Suitable Habitat Definition: California Gnatcatchers are obligate, permanent residents of coastal sage scrub. They may make limited use of adjoining habitats, especially during the winter when territories are typically larger than during the breeding season (Atwood, 1990; Rotenberry and Scott, 1998). Woody perennial plant cover in California Gnatcatcher territories typically exceeds 40 or 50 per cent (Beyers and Wirtz 1997).
Coastal sage scrub consists primarily of soft subshrubs (to about 1m or 3 feet), many of which are drought-deciduous, with an admixture of evergreen shrubs. It generally occurs below 900 meters (3000 feet). Four major types of coastal sage scrub are defined by Westman (1986): Diablan, Venturan, Riversidean, and Diegan. Diablan Sage Scrub occurs in the central coastal portion of California, and is not within the historic range of the California Gnatcatcher. Riversidean, and Diegan Sage Scrubs occur within the current range of the gnatcatcher, and within the Mountain and Foothill Assessment area, and will be generally described below (after Holland, 1986).
Riversidean Sage Scrub occurs in western Riverside County and in parts of inland Los Angeles and San Diego Counties. Holland (1986) describes this as the most xeric form of Coastal Sage Scrub in southern California. Stands are usually fairly open. Characteristic species include Artemisia californica, Encelia farinosa, Eriogonum fasciculatum, Isomeris arboreus, Lotus scoparius, Salvia apiana and Salvia mellifera.
Alluvial fan sage scrub is occupied by California Gnatcatchers in some areas, e.g. Lytle Creek area on San Bernardino National Forest. Its composition is typically intermediate between Riversidean sage scrub and Diegan sage scrub.
Diegan Coastal Sage Scrub occurs in coastal Orange County, coastal San Diego County, and Baja California. Dominant shrubs are Artemisia californica and Eriogonum fasciculatum, with a lesser component of Salvia apiana and Malosma laurina. It typically occurs in open stands.
On the CNF, all known CAGN populations are found in Diegan coastal sage scrub (inland form) and in a coastal sage scrub/chaparral mix dominated by chamise and California sage. Gnatcatchers are found at elevations ranging from 300 to 700 meters (900 to 2200 feet).
CAGN are apparently restricted to lower elevations by an inability to tolerate areas where the January mean minimum temperature is less than 2.5 degrees Centigrade (36 degrees F); this climatic constraint appears to set the eastern limit for the distribution of the species (Mock 1998). For this reason most areas that are at elevations greater than 450 meters (1400 feet) do not support substantial populations of California Gnatcatchers (Mock 1998).
In a study of the size of habitat patches used by California gnatcatchers, Atwood et al. (1998) found that habitat patches of less than 1 hectare (2.5 acres) were used at least once over the course of a 5 year study. Only patches that were 20 hectares (50 acres) or greater in size were used 4 or 5 years during the 5 year study, implying that large patches of coastal sage scrub are needed to sustain gnatcatcher populations over time.
Population Levels & Trends on National Forest Service lands: On the Palomar District, a population of 30 pairs of CAGNs was discovered in 1992-93 on NFS lands in the upper San Diego River valley. This population was reduced to about 15 pairs after a May 1993 wildfire that burned over 1000 acres of coastal sage scrub. Some areas were reburned in a 1996 wildfire. Most of the burned area has still not been recolonized although vegetation has recovered (S. McKelvey, pers. comm.).
Two CAGNs were located in the northern portion of Pamo Valley in 1992. No gnatcatchers have been located in this area since the Second Fire in 1994. On the Trabuco District, potential CAGN habitat is present along the eastern and western margins of the district including portions of the Black Star and Verdugo range allotments. Two pairs of gnatcatchers occurred near San Juan fire station prior to the 1993 Ortega wildfire, but have not been detected since the fire.
All known Forest populations have decreased as a consequence of wildfires. The frequency of fire starts is increasing and human-caused fires could result in the loss of all CAGN populations on Forest lands.
Threats: Major threats to the CAGN include loss of habitat to development and too-frequent fire, as well as predation and nest parasitism by other birds and animals. Fire is the most serious threat to Forest populations and this threat is increasing as urban areas expand and public use of the Forests increases.
Protection of Occurrences / Degree of Risk to Occurrences on NFS lands: The population at the San Diego River is within a proposed Research Natural Area. Forty-two acres of occupied habitat is in a protected land classification. It is also adjacent to the rapidly developing community of Ramona and is subject to considerable recreational use, which contributes to fire starts. It is unlikely that this use will decrease. Although the San Diego River population is currently near a number of California Gnatcatcher populations on private lands, much of the habitat on private land is being lost to development. This will isolate the San Diego River population and will decrease the probability that the area will be recolonized after wildfires.
Degree to which NFS lands can contribute to recovery: Due to the minimal amounts of known or potential habitat on Forest lands, the small populations present on NFS lands, and the vulnerability of Forest populations to extirpation due to wildfires, NFS lands may contribute minimally to the recovery of this species.
Although the potential for major new populations is low, surveys are needed to more fully determine the distribution of California gnatcatchers in the Cleveland, San Bernardino, and Angeles National Forests. Information on occurrences at low elevations in the eastern San Gabriel Mountains would be particularly useful.
Proximity to roads or trails: No known significance. Miner et al. (1998) studied the gnatcatcher population at Crystal Cove State Park. In a sample of 89 nests, they found no significant difference in nest success when comparing nests that were less than 3 meters from heavily used trails and roads with nests that were farther away from roads and trails.
Fire Effects: Habitat is not suitable for a few years after fire. Too-frequent fires (intervals of less than 5-10 years) increase probability of type conversion to non-native grasslands, especially in drier inland areas. The gnatcatcher population on the upper San Diego River has been negatively affected by recurring fires in the early 1990s. High fire frequencies and invasions of exotic grasses are serious problems in gnatcatcher habitat. Coastal sage scrub is a highly flammable vegetation type and maintaining mature stands in fire-prone areas near the urban interface is a significant management challenge (Beyers and Wirtz 1997).
O'Leary and Westman (1988) studied four ungrazed coastal sage scrub sites in the Santa Monica Mountains (Venturan Sage Scrub) and four recently grazed inland Riversidean sage scrub sites in southwest San Bernardino Co. and Riverside County. They followed sites for four years after fire. Three of the coastal sites exhibited rapid recovery of vegetation to preburn levels. They attributed this to "substantial resprouting of subshrubs and shrubs" and resprouting and seeding of native grasses. The fourth coastal site became dominated by the non-native Red Brome (Bromus madritensis ssp. rubens). This site was adjacent to a grazed area.
At the four inland sites, O'Leary and Westman (1988) first observed a lack of shrub resprouting and a dominant layer of forbs and vines. After a few years, sites became dominated by annual exotic grasses, especially Red Brome (Bromus madritensis ssp. rubens). They speculated that the more recent grazing and higher air pollution levels experienced at the inland sites had reduced the abundance of native perennial grasses and increased the abundance of non-native annuals. Only one species of shrub resprouted on the inland sites, causing shrub recovery to be much slower there than in coastal areas and allowing herbaceous vegetation to dominate sites for a longer time.
Westman (1981a) found that high fire intensities suppress resprouting of coastal sage shrubs, allowing the herb layer to become dominant. Less intense fires favor resprouting and lead to suppression of the herb layer. Complicating interpretation, however, is an apparent difference in sprouting behavior between coastal and inland populations of the same shrub species (Myers and Ellstrand 1986, Westmand and O'Leary 1986). Westman et al. (1981b) also found limited recovery of Artemisia californica and other shrubs through first-year seedling recruitment after intense fire; they found that resprouting individuals apparently produced seeds that contributed to seedling recruitment in the next growing season. They postulated that seeds had not survived the fire or were viable for only one year; however they did not study seed viability or presence of seed in the soil. Malanson and O'Leary (1982) also found that shrubs that resprouted and produced seed after a fire were more important seed sources than pre-fire seed caches in the soil.
Zedler et al (1983) studied the response of a sage scrub stand to two sequential burns within a one year period. An area that burned in August 1979 was planted with annual ryegrass, and partially reburned in July 1980. (A prior burn had occurred in the 1950's). No resprouting was observed in Artemisia californica and Eriogonum fasciculatum after the 1979 burn; some resprouting of Salvia apiana occurred. All plants reproduced from seed with Artemisia californica being most abundant. After the 1980 fire, Eriogonum fasciculatum was absent from the twice-burned area. Salvia apiana and Artemisia californica were still present, with some 1-year old A.californica resprouting. Zedler et al. (1983) also suggest that the lack of resprouting at their study site "may be a local expression of a general tendency for resprouting to be less successful and obligate seeding to be more prevalent with increasing aridity in the entire chaparral zone", as postulated by Keeley (1977).
Post-fire use by Gnatcatchers:
Diegan Sage Scrub at Rancho San Diego was observed to support gnatcatchers during the non-breeding season within 5-10 years after fire (Mock & Bolger, 1992). They also observed that areas being utilized by breeding gnatcatchers had not burned for at least 15-20 years. Beyers and Wirtz (1997) found that California Gnatcatchers did not use burned sites until aerial shrub cover approached or exceeded 50 per cent.
Survey Considerations: The breeding season of the CAGN begins in late February and extends through July. Initial nesting attempts probably occur during mid March to early April. Current FWS protocol requires 6 surveys at least one week apart if surveys occur between March 15 and June 30. If surveys are conducted between July 1 and March 14, a total of nine surveys must be conducted at least 2 weeks apart.
- Atwood, J.L., S.H. Tsai, C.A. Reynolds, and M.R. Fugagli. 1998. Distribution and population size of California Gnatcatchers on the Palos Verdes Peninsula, 1993-1997. Western Birds 29 (4): 340-350.
- Atwood, J.L. 1990. Status review of the California Gnatcatcher (Polioptila californica). Unpublished technical report, Manomet Bird Observatory, Manomet, MA. 79 pp.
- Beyers, J.L. and W.O. Wirtz. 1997. Vegetative characteristics of coastal sage scrub sites used by California gnatcatchers: Implications for management in a fire-prone ecosystem. Pp. 81-89 in Greenlee, J.M. (ed.), Proceedings: First conference on fire effects on rare and endangered species and habitats, Coeur d'Alene, Idaho, November 1995. International Association of Wildland Fire, Fairfield, WA.
- California Department of Fish and Game. 1998. California Natural Diversity Database. CDFG, Sacramento CA.
- Cardiff, G. 1998. Personal communication. San Bernardino County Museum curator.
- Davis, L.H., R.L. McKernan, and J. S. Burns. 1998. History and status of the California Gnatcatcher in San Bernardino County, California. Western Birds 29(4):361-365.
- Holland, R.F.1986. Preliminary description of the terrestrial natural communities of California. Non-game Heritage Program, California Department of Fish and Game. Sacramento, CA.
- Keeley, J.E. 1977. Fire-dependent reproductive strategies in Arctostaphylos and Ceanothus. pp 391-396 in Proceedings of the symposium on the environmental consequences of fire and fuel management in Mediterranean ecosystems. U.S. Forest Service, Washington, DC.
- Miner, K.L, A. Wolf, and R. Hirsch. 1998. Use of restored coastal sage scrub by California Gnatcatchers in a park setting. Western Birds 29 (4): 439-446.
- McKelvey, S. Personal communication. Palomar District Wildlife Biologist. Cleveland National Forest, Ramona, CA.
- Mock, P.J. 1998. Energetic constraints to the distribution and abundance of the California Gnatcatcher. Western Birds 29 (4): 413-420.
- Mock, P.J. and D. Bolger. 1992. Ecology of the California Gnatatcher at Rancho San Diego. Unpublished report prepared for Home Capital Development Corp.54 pp.
- Mock, P.J., B.L. Jones, M. Grishaver, J. Konecny, and D. King.1990. Home range size and habitat preferences of the California gnatcatcher in San Diego County. Unpublished abstract, American Ornithologists Union Annual Meeting, Los Angeles, CA.
- Myers, M.A. and N.C. Ellstrand. 1986. Post-fire succession at an inland (Riversidean) site of coastal sage scrub: variation in community response. Pp. 129-132 in DeVries, J.J. (ed.), Proceedings of the chaparral ecosystems research conference, May 16-17, 1985, Santa Barbara, California. California Water Resources Center, Davis, CA.
- O'Leary, J.F.and W.E. Westman. 1988. Regional disturbance effects on herb succession patterns in coastal sage scrub. J. Biogeography 15:775-786.
- Rotenberry, J.T. and T.A. Scott. 1998. Biology of the California Gnatcatcher: Filling in the gaps. Western Birds 29 (4): 237-241.
- US Fish and Wildlife Service. 1993. Endangered and threatened wildlife and plants:
- Final rule determining threatened status for the California Gnatcatcher. Federal Register.
- Westman, W.E. 1981a. Diversity relations and succession in Californian coastal sage scrub. Ecology 62(1): 170-184.
- Westman, W.E., J.F. O'Leary, G.P.Malanson. 1981. The effects of fire intensity, aspect, and substrate on post-fire growth of California coastal sage scrub. Pp. 151-179 in Margaris, N.S. and H.A. Mooney (eds.), Components of Productivity of Mediterranean-Climate Regions- Basic and Applied Aspects. W.Junk Publishers, The Hague/Boston/London.
- Westman, W.E. and J.F. O'Leary. 1986. Measures of resilience: the response of coastal sage scrub to fire. Vegetatio 65: 179-189.
- Zedler, P.H., C.R. Gautier, G.S. McMaster. 1983. Vegetation change in response to extreme events: the effect of a short interval between fires in California chaparral and coastal scrub. Ecology 64(4): 809-818.