A Maximum Estimate of the California Gnatcatcher's Population Size in the United States

WESTERN BIRDS Volume 23, Number 1, 1992 A MAXIMUM ESTIMATE OF THE CALIFORNIA GNATCATCHER'S POPULATION SIZE IN THE UNITED STATES JONATHAN L. ATWOOD, Manomet Bird Observatory,P.O. Box 1770, Manomet, Massachusetts 02345 The CaliforniaGnatcatcher, Polioptila californica, was recentlyrecog- nizedas a speciesdistinct from the widespreadBlack-tailed Gnatcatcher, P. melanura, of the southwesterndesert regionsof the United States and Mexico (Atwood 1988, American Ornithologists'Union 1989). Although CaliforniaGnatcatchers are distributedthroughout much of BajaCalifornia, the northernmostsubspecies, P. c. californica, now occursonly in remnant fragmentsof coastalsage scrub habitat from LosAngeles County, Califor- nia, southto El Rosario,Baja California(Atwood 1991). Atwood (1980) speculatedthat the numberof CaliforniaGnatcatchers remainingin the UnitedStates was "no more than 1,000 to 1,500 pairs," from estimatesof 30 pairs in VenturaCounty, 130 pairs in Los Angeles County,50 pairsin San BernardinoCounty, 325 pairsin OrangeCounty, 400 pairsin RiversideCounty, and 400 pairsin San Diego County.These valueswere derivedfrom reportsof variousobservers, limited field work in differentportions of the species'range, and visualestimates of habitat availabilityin differentareas. Despite the preliminarynature of theseresults, the patternof continuinghabitat loss evident at that time indicated"imme- diate concern for the survival"of P. c. californica in the United States (Atwood 1980). Extensive destruction of suitable California Gnatcatcher habitat has con- tinuedsince 1980 (Rea and Weaver 1990). Of 56 southernCalifornia sites where Atwood (1980) recordedcoastal sage scrub vegetation and Califor- nia Gnatcatchers,18 (32%) had been completelydestroyed by urban developmentby 1990, and 15 (27%) had been partially degradedby current constructionprojects (J. Atwood and K. Shields,unpubl. data). Similarhabitat loss, primarily associated with agriculturaldevelopment, has alsoincreased in northwesternBaja California;Best (1983) foundthat the San Quintin KangarooRat (Dipodomysgravipes), which historicallyoc- curredalong Baja California'sPacific coast from San Telmo southto El Western Birds 23:1-9, 1992 1 CALIFORNIA GNATCATCHER'S POPULATION SIZE Rosario,"has been virtually eliminated from mostof its nativerange during the past decade[owing] to extensivecultivation." Because of continuing habitatloss throughout the gnatcatcher'sUnited Statesand Mexicanrange, federaland Californiaendangered species status was recently proposed for the CaliforniaGnatcatcher's northern subspecies. As an effortto providea baselinefor futurestudies, in thispaper I estimatethe maximumpopulation of P.c. californica that mightcurrently exist in the United States. METHODS Determining numbersof a small passerinewhose populationdensity variesthroughout a relativelyextensive geographic range is difficult.Be- causeCalifornia Gnatcatchers are confinedto coastalsage scrub vegetation (Atwood 1980), populationestimates could, in theory, be derived from measurementsof the availableacreage of this habitat coupledwith the species'average territory size. However, many areasof vegetationthat are correctlyclassified as coastalsage scrub are not occupiedby gnatcatchers, suggestingthat population estimatesbased on vegetationmapping will invariablybe inflated.Direct censusing of gnatcatchersthroughout southern Californiahas also proven impossible, because public access to many large tracts of private land has been denied. Althoughsome censusdata have beencollected by the southernCalifornia development industry, little of this informationhas been released to the public,and independentverification of theseresults has beenprevented. In this analysis,I use a more generalapproach to estimatecrudely the maximum number of California Gnatcatchersremaining in the United States. Atwood and Bolsinger(in press)showed that in Los Angeles, Orange, and San Diego counties,93% of recent CaliforniaGnatcatcher observationswere made at elevationsunder 250 m; in RiversideCounty, most recent records (75%) were made at elevations under 500 m. Here I use these elevationsto approximatethe maximum range of California Gnatcatchersin each of thesecounties. Using the geographicinformation systemCAMRIS (ComputerAided Mappingand ResourceInventory Sys- tem; R. Glenn Ford, EcologicalConsulting, Inc., Portland,Oregon) and elevation contours digitized from U.S. Geological Survey (USGS) 1:100,000 scaletopographic maps, I calculatedthe extent of land lying below 250 m elevationin Los Angeles,Orange, and westernSan Diego counties,and below500 m in westernRiverside County. Next, againusing thesetopographic maps, I examinedall 1-minuteblocks of latitude-longi- tude below these elevations,and entered into CAMRIS those blocksthat were predominatelydesignated as "BuiltUp" or as "Orchard,Vineyard." Finally, I calculatedthe remainingamount of undevelopedlowland that might support California Gnatcatchersin each county. Although P.c. californica historicallyoccurred in Ventura and San Bernardinocounties, Atwood(1980) foundthat the specieshas been largelyor entirelyextir- pated from theseareas; consequently, these counties were excludedfrom analysis.Similarly, I excludedfrom area calculationsthose lowland portions of Los AngelesCounty located near the SantaMonica Mountains, where CaliforniaGnatcatchers have never been reliably reported (Atwood 1980). CALIFORNIA GNATCATCHER'S POPULATION SIZE USGS maps (1:100,000 scale)used in this analysisincluded (followed by publicationdate and, in brackets,dates of aerial photographyused in the most recent revisions):Borrego Valley, 1982 [1976-77]; El Cajon, 1979 [1975]; Long Beach, 1981 [1979]; Los Angeles,1979 [1976]; Oceanside, 1982 [1972-74, 1979]; Palm Springs,1984 [1975, 1979]; San Bernar- dino, 1982 [1975, 1978]; San Diego, 1979 [1972, 1975-77]; SantaAna, 1983 [1978-79]; Santa Barbara, 1982 [1977-78]. Focusedsurveys conducted during 1990 by the U.S. Fish and Wildlife Service(L. Salata, unpubl. data) and U.S. Navy (T. Burr, unpubl. data) locatedapproximately 175-200 pairsof CaliforniaGnatcatchers on Camp PendletonMarine Corps Base,San Diego County,and 25-50 pairson the adjacentFallbrook Naval Weapons Station. On the basisof topographic map designationsof "BuiltUp" and "Orchard"areas, approximately 405 km2 of undevelopedland lie below250 m elevationon thesetwo military bases.These acreagemeasurements and populationestimates suggest that CaliforniaGnatcatchers occur in undevelopedlowlands of coastalsouthern Californiaat densitiesranging from 0.49 to 0.62 pairsper squarekilome- ter. Note that these values do not refer to population densitieswithin particularareas of coastalsage scrub that are occupiedby gnatcatchers,or to densitieswithin pure standsof occupiedand unoccupiedcoastal sage scrub.Rather, the "habitat"definition used here is simply"low-elevation, undeveloped"land. I usedthese densitiesin conjunctionwith the measurementsof undevelopedacreage in each countyto estimatethe population sizeof CaliforniaGnatcatchers remaining in the United Statesduring the late 1970s. Assumptionsinherent in this analysissuggest that current (1991) populationlevels cannot significantly exceed these values. Unpublisheddocuments cited in the text have been depositedin the JosselynVan Tyne MemorialLibrary of the WilsonOrnithological Society, Universityof Michigan,Museum of Zoology,Ann Arbor, Michigan48109. RESULTS AND DISCUSSION Figure1 showsthe distributionof undevelopedlowland within the United Statesrange of the CaliforniaGnatcatcher during the mid- to late 1970s, when the USGS maps on which my analysisis based were compiled. Multiplyingthe acreageof thisarea by the densitiesderived from surveyson Camp Pendletonand Fallbrookyields populationestimates of 1811 to 2291 pairs(Table 1). These resultsare similarto Atwood's(1980) population estimate(1000-1500 pairs),which was basedon differenttypes of informationthat were obtainedshortly after the time period considered here. This analysisassumes that no CaliforniaGnatcatchers occur above the elevationallimits used to define the species'potential distribution. In fact, small numbers of P c. californica do occur above 250 m elevation in Orange and San Diego counties,and above 500 m elevationin Riverside County(Atwood and Bolsinger,in press).Use of elevationas an indexof gnatcatcherdistribution thus tends to underestimatethe total number of birdsremaining in the United States.However, this inaccuracyis more than offsetby four other assumptionsthat inflate estimatesof the population 3 CALIFORNIA GNATCATCHER'S POPULATION SIZE size, and suggestthat fewer than approximately2000 pairs currently remain north of Mexico. First, the amount of land currentlydeveloped in southernCalifornia is certainlygreater than indicatedon maps preparedduring the mid- to late 1970s. For example,the RiversideCounty PlanningDepartment (1990) noted that "from 1980 throughJanuary 1, 1985, the county'spopulation grewfrom 663,166 to 800,949 people,an increaseof 137,783 or 20.8%. From January1, 1985, to January1, 1990, the populationgrew... to 1,110,021 people,an increaseof... 38.6%." Comparisonsof 1980 and LOSANGELES CO.-- RIVERSIDECO. ORANGE SANDIEGO Go. SOUTHERN CALIFORNIA mid-late 1970's 0 10 20 30 40 50 ' ..... Ki'lomet;r9 ' ' I UNDEVELOPEDLOWLAND 117 30 Figure1. Extentof undevelopedlowland in coastalsouthern California during the mid-to late 1970s. Basedon (a)designations of "BuiltUp" and "Orchard,Vineyard" areasshown on USGS 1:100,000 scaletopographic maps and (b)the 250-m (Los Angeles,Orange,

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