Use of Refugia by California Gnatcatchers Displaced by Habitat Loss

USE OF REFUGIA BY CALIFORNIA GNATCATCHERS DISPLACED BY HABITAT LOSS

JONATHAN L. ATWOOD, Manomet Center for Conservation Sciences, P.O. Box 1770, Manomet, Massachusetts 02345 DAVID R. BONTRAGER, 80645 Lost Creek Rd., Dexter, Oregon 97431 AMY L. GOROSPE, Department of BiologicalSciences, California State University, Long Beach, California 90840

Because the California Gnatcatcher (Polioptila californica) is one of the focal speciesfor conservationof coastalsage scrubbeing plannedunder the state of California's Natural Community Conservation Planning (NCCP) program (Atwood and Noss 1994, Reid and Murphy 1995), the species' response to large-scale loss of habitat is an important factor that may influence both the design of habitat reserves and development of future habitat-managementstrategies. For example, can birds that are displacedby habitat loss be expected to move successfullyinto adjacent areas of undis- turbed habitat? Or is suitable gnatcatcherhabitat generally "saturated"in terms of the number of breedingpairs it is able to support, implyingthat birds displacedby destruction of coastal sage scrub will not successfully establishthemselves in neighboringrefugia? An opportunityto examine these questionswas presentedin late October 1993, when the Laguna fire bumed approximately5261 ha (13,000 acres) of coastalsage scrub, chaparral,and grasslandin the San Joaquin Hills of coastalOrange County (Bontrageret al. 1995). Pre-fire surveysconducted by Jones and Stokes Associatesand Ed Almanza and Associatesin 1992 suggestthat habitat occupiedby approximately127 pairs of gnatcatchers was destroyedor severelydegraded by the burn (Bontrageret al. 1995). Becausethe burn took place soon after the 1993 breedingseason, when the population is augmented by recently fledged juveniles, as many as 500 individualgnatcatchers may have been affected.Surveys during spring 1994 found only 12 pairs within the burned area, 9% of the pre-fire (1992) estimate(Bontrager et al. 1995). During surveysimmediately after the fire in November and December 1993, several experienced observers felt that many of the gnatcatchersseen in nearby unburnedhabitat were behavingas thoughthey might have been displacedfrom their territoriesas a resultof the fire (Bontrageret al. 1995). Additional gnatcatchersnear the San Joaquin Hills were displaced in January and February 1994, when habitat that supportedapproximately five to ten breedingpairs in 1993 was destroyed during constructionof a segment of the San Joaquin Hills Transportation Corridor north of Newport Coast Drive (E. Woehler pers. comm.). In this study we compare the 1993-1995 population estimates of breeding California Gnatcatchers in coastal Orange County around the peripheryof the Laguna fire with estimatesfrom controlsites unaffectedby the burn. In particular,we ask two questions:(1) did breedingpopulations in habitat refugia show an increaseduring 1994 that might be attributed to displacementof birds by the Laguna fire, and (2) did any such increased densities of birds in the habitat refugia persist into the 1995 breeding season?

406 WesternBirds 29:406-412, 1998 USE OF REFUGIA BY DISPLACED CALIFORNIA GNATCATCHERS

METHODS

In the San Joaquin Hills within 2 km of the Laguna fire's perimeter we identified five unburned refugia for which baseline (1993) population data were available (Figure 1; Table 1). Each site, which included variable amounts of coastalsage scrub, was isolatedfrom other patchesof scrub by a distanceof at least 1 km. Under this operationaldefinition, the sampling units referred to here differ slightly from those used by Bontrager et ai.

lOOO2000

NLL

ß CAGNLOCATIONS (PRE-FIRE) [] STUDYAREAS (UNBURNED CSS) • UNBURNED CSS •: BURNEDCSS

Figure 1. Habitat refugiasurrounding the 1993 Laguna fire, includingpre-fire (1992) distributionof coastal sage scrub and breeding pairs of the California Gnatcatcher within the burn area. TR, Turtle Rock, SH, Sycamore Hills, NLL, North Laguna Laurel; UCI, UC Irvine EcologicalPreserve; CC, Crystal Cove State Park.

407 USE OF REFUGIA BY DISPLACED CALIFORNIA GNATCATCHERS

'talkie 1 Refugiaand ControlSites for Study of the LagunaFire

Location Dominantvegetation a Area (ha)b

Refugia CrystalCove State Park Sagebrushscrub; quail brushscrub 101 North LagunaLaurel Sagebrushscrub; sagebrush-black sage scrub; blacksage scrub; scrub-chaparral ecotone 80 SycamoreHills Sagebrush-buckwheatscrub; sagebrush-black sage scrub;black sage scrub; scrub-chaparral ecotone 151 UCI EcologicalPreserve Sagebrush-buckwheatscrub; southern cactus scrub; sagebrush-grassland;ruderal 25 Turtle Rock Sagebrush-buckwheatscrub; sagebrush scrub; coyotebrush scrub; mixed sage scrub; ruderal; southern cactus scrub 159 Control sites Agua Amarga Canyon Sagebrushscrub; sagebrush-buckwheat scrub; southern cactus scrub 82 U.S Navy Fuel Depot Sagebrushscrub; ruderal 57 "Location 2 TM Sagebrush-buckwheatscrub 100 "Moratorium" zone Ruderal;sagebrush scrub; sagebrush-buckwheat scrub; southerncactus scrub; purple sage scrub; toyon-sumac chaparral;sage scrub-grasslandecotone 551 Ocean Trails Southerncactus scrub; sagebrush-buckwheat scrub; ruderal;sagebrush scrub 114 Point Vicente Sagebrushscrub; southern cactus scrub; ruderal 71 aVegetation categories based on Jones and Stokes Associates (1993). bAreaestimates approximate, and not limitedto areasof coastalsage scrub vegetation. tUnnamed location in coastal Orange County; see Erickson and Miner (1998)..

(1995). Similarly, we identifiedsix control sites on the Palos Verdes Penin- sula, Los Angeles County (Atwood et al. 1998), and in coastal Orange County (Erickson and Miner 1998). Because these localities were far removedfrom the San JoaquinHills and isolatedby extensiveareas of urban development,we used them to evaluatepopulation changes in the habitat refugiathat might have been associatedwith the Laguna fire. Year-to-year consistencyof survey methods and personnel was main- tained within each study location. The estimatednumber of gnatcatcher pairs was basedon cumulativeobservations made throughouteach breeding season,often includingthe mapped locationsof uniquelycolor-banded birds and simultaneouslyactive nests.

RESULTS

The numbersof breedingpairs of CaliforniaGnatcatchers at five habitat refugiaand six controlsites are providedin Table2. Estimatesin the refugia for 1994 were significantlydifferent from those for 1993 (P-- 0.043, Wilcoxon signed-ranktest); 1993 and 1994 estimatesfrom the control sites did not differ significantly(P: 0.256). In 1994, all five refugia showed populationincreases ranging from 25% (Turtle Rock) to 105% (CrystalCove

408 USE OF REFUGIA BY DISPLACED CALIFORNIA GNATCATCHERS

'l'al•le 9• Number of Breeding Pairs of California Gnat- catchersat Refugia and Control Sites

Population estimate

Location 1993 1994 1995

Refugia North Laguna Laurel 6 8 5 SycamoreHills 10 16 18 UC Irvine EcologicalPreserve a 4 7 6 CrystalCove State Park 20 41 21 Turtle Rockb 51 64 43 Control sites Agua Amarga Canyon 8 6 4 U.S. Navy Fuel Depot 5 5 0 "Location 2 TM 20 29 16 "Moratorium" zone 25 27 15 Ocean Trails 7 9 3 Point Vicente 3 3 1

aFide E. Woehler. Excludes (from all three years) one pair present during 1993 and 1994 but which disappearedbefore the 1995 breedingseason, apparentlyas a result of constructionassociated with the San JoaquinHills Transportation Corridor in December 1994. blncludessites referred to by Bontrageret al. (1995) as Ridgeline,Sand Canyon Reservoir,Turtle Rock fragments,and Turtle Rock Reservoir. tUnnamed location in coastal Orange County; see Erickson and Miner (1998).

State Park) (mean increase 59.6%, standard deviation 32.4). Among the control sites, from 1993 to 1994, the population increased at three, decreasedat one, and did not change at two (Figure 2). Populationsat the controlsites declinedsignificanfiy from 1994 to 1995 (P = 0.027; Wilcoxon signed-ranktest). Among the refugia there was no statisticallysignificant change from 1994 to 1995 (P = 0.138; Wilcoxon signed-rank test), although numbers decreasedat four of the five sites, includinga decline of 49% at Crystal Cove State Park (Figure 2).

DISCUSSION

Increased numbers of California Gnatcatchers were found in unburned areas of coastalsage scrub immediately after a fire that bumed over 5200 ha of coastal sage scrub, grassland,and chaparral (Bontrager et al. 1995). During the 1994 breedingseason these unburnedrefugia supported signifi- canfly more breeding pairs than before the fire. Populationsat the control sites distant from the Laguna fire did not increasesignificanfiy from 1993 to 1994. In 1995, gnatcatcherpopulations declined at all of the controlsites and at four of the five refugia.The single refugiumwhere the populationincreased

409 USE OF REFUGIA BY DISPLACED CALIFORNIA GNATCATCHERS •oo%t CONTROLS REFUGIA 50%ø%-1 I _ ! ,[I

• -50% - z

•- -100%- z ,1993 -1994 1.1.1 [q]1994 -1995 1.1.1 o_ -150% I I I I I I I I AA FD LOC2 MZ 01- PV NLL SH UCl CC TR

Figure2. Gnatcatcherpopulation changes in the San JoaquinHills habitatrefugia and control sites, 1993-1995. Abbreviationsfor habitat refugia as in Figure 1. Control sites: AA, Agua Amarga Canyon; FD, U.S. Navy Fuel Depot; LOC2, unnamed location in coastal Orange Co. (Ericksonand Miner 1998); MZ, "Moratorium" zone; OT, Ocean Trails; PV, Point Vicente.

from 1994 to 1995 (Sycamore Hills) may have received immigrants displaced by adjacent constructionthat began in June 1994 and destroyed occupied gnatcatcherhabitat (D. Bontrager unpubl. data). At the time of the Laguna fire (October 1993), we hypothesizedthat gnatcatcherdensities in unburned refugia located near the fire's perimeter might increaseduring the 1994 breedingseason, but, if pre-fire occupancy of these areas was close to saturationlevel, populationswithin the refugia would revert approximatelyto their pre-fire levels by spring 1995. The observationsreported here are consistentwith the first part of this hypoth- esis, but we remain unsure if density-dependentfactors contributedto the declinesseen from 1994 to 1995. We believe that the evidenceis strong that the increases observed in the refugia during 1994 were related to displacementof birds by the Laguna fire. Though the population declines seen from 1994 to 1995 are consistentwith our predictionthat density- dependentfactors might have acted to restore gnatcatcherbreeding densities in the refugia to their approximate pre-fire levels, the concurrent population declines at control sites confoundthis interpretation. If winter weather in 1994-1995 causedregional populationdeclines (Erickson and Miner 1998), then any density-dependenteffects in the habitatrefugia might well have been obscured. In experimentalstudies of the effectof forestfragmentation in the Amazon basin of Brazil, Bierregaard and Lovejoy (1989) documentedshort-term increasesin densitiesof forestbirds in habitatrefugia adjacent to large-scale deforestation;after approximately200 days, these elevated densities de- creasedto levels lower than those encounteredprior to the displacement.

410 USE OF REFUGIA BY DISPLACED CALIFORNIA GNATCATCHERS

Darveau et al. (1995) found increaseddensities of forest birds along riparian corridorsadjacent to recent clearcuts.Hagan et al. (1996) observedincreased densitiesbut reducedpairing successof Ovenbirds($eiurus aurocapillus)in forest fragmentsadjacent to areas of recent lumberingand proposedthat "an abrupt fluctuationof this magnitudecaused by displacedindividuals from lost habitat might impose behavioralpressures that destabilizesome fragment[ed] bird populations."If correct, this hypothesissuggests that substantialsynchro- nous loss of habitat might temporarilyelevate the gnatcatcher'spopulation levelswithin habitat refugiato a point where increasedterritorial interactions could reduce reproductive success.Thus, even though actual counts of breeding pairs might show short-term increases,density-dependent effects might ultimatelycause populationdeclines. Especially with a relativelyshort- lived specieslike the CaliforniaGnatcatcher which may, on average,have a life-timebreeding expectancy of only one or two years (Atwood et al. 1998, Bontrager unpubl. data), such effects could have important ramificationsfor the dynamicsof local populations. These hypothesessuggest that extensivewildfires may damagegnatcatcher populationsin unburnedrefugia as well as within the bum. Becausefire is clearlyone of the most importantfactors to considerin maintainingviable tracts of coastal sage scrub, understandingthe relationships among burn extent, use of habitat refugia, and recolonizationof recoveringburned areas by sensitivespecies such as the California Gnatcatcheris important. Until these topicsare better studied,we suggestthat frequentsmall controlledbums within reservesof coastalsage scrubhabitat are less likely to have long-term adverse effects on gnatcatcherpopulations than occasionallarge fires. Populationtrends in the CaliforniaGnatcatcher are likely to be influenced by a variety of factorsranging from the effectsof weather on reproductive successand survivorshipto increasedcompetition for resourcescaused by the immigrationof birds displacedby habitat loss. Our understandingof how these and other factors interact to determine gnatcatcherpopulation dy- namics remains limited, yet the issue has critical implicationsfor both the design and effective managementof viable habitat reserves.Consequenfiy, continuationof in-depth, long-term studies at various sites throughoutthe species' range in southern California remains an important objective for recovery.

SUMMARY

We analyzedannual estimatesof the number of breedingpairs of Califor- nia Gnatcatchersfrom 1993 to 1995 in five habitat refugialocated adjacent to the October1993 Lagunafire in coastalOrange County and at six control sites distantfrom the burn. Observationsimmediately after the fire suggested that many of the over 500 gnatcatcherswhose habitat was burned were displaced rather than killed outright. During the 1994 breeding season counts in all habitat refugia were higher than during the previous year; among the control sites, there was no trend. These results suggest that gnatcatchersdisplaced by habitat loss may, at least in the short term, pack more denselyinto remainingareas of intact coastalsage scrub. Populations at both refugia and control sites decreasedfrom 1994 to 1995, possiblyas

411 USE OF REFUGIA BY DISPLACED CALIFORNIA GNATCATCHERS a resultof density-independentfactors such as weather;we couldnot assess whether increaseddensities of breedinggnatcatchers in unburnedrefugia would have persistedover more extendedperiods of time.

ACKNOWLEDGMENTS

This studyis a productof the SuperparkProject. Financialsupport for various aspectsof thiswork was provided by the U.S. Navy,Southern California Edison, U.S. Fish and WildlifeService, Ed Almanza and Associates,the NationalFish and Wildlife Foundation,LSA Associates,the Palos VerdesPeninsula Land Conservancy,the trusteesof Manomet Center for ConservationSciences, and an anonymousdonor. Dana Kamadaand SusanSheakley contributed valuable field observations. Access to studyareas was made possible by The IrvineCompany, the CaliforniaDepartment of Parksand Recreation,the County of Orange, and the citiesof Irvine and Laguna Beach.Eric Woehler, Richard Erickson, Karen Miner, and Maria Andros provided use of their unpublisheddata. RaymondSauvajot and Ed Almanza providedhelpful commentson an early draft of the manuscript.Thanks to RichardJ. and Donna O'Neill who generouslyprovided a home away from home for Dave Bontrager. BarbaraAtwood and CharlotteBontrager gave encouragement during their spouses' travels to southern California.

LITERATURE CITED

Atwood,J. L., and Noss,R. F. 1994. Gnatcatchersand development: A "trainwreck" avoided? Illahee 10:123-132. Atwood,J. L., Tsai,S. H., Reynolds,C. R., andFugagli, M. R. 1998. Distributionand populationsize of the CaliforniaGnatcatcher on the PalosVerdes Peninsula, 1993-1997. W. Birds 29:340-350. Bierregaard,R. O., Jr., and Lovejoy,T. E. 1989. Effectsof forestfragmentation on Amazonianunderstow bird communities.Acta Amaz6nica19:215-241. Bontrager,D. R., Erickson,R. A., andHamilton, R. A. 1995. Impactsof the October 1993 Laguna Canyon fire on CaliforniaGnatcatchers and CactusWrens, in Brushfiresin CaliforniaWildlands: Ecology and ResourceManagement (J. E. Keeleyand T. Scott,eds.), pp. 69-76. Int. Assoc.Wildland Fire, Fairfield,VA. Darveau,M., Beauchesne,P., Belanger,L., Huot, J., and Larue, P. 1995. Riparian forest stripsas habitat for breedingbirds in boreal forest. J. Wildlife Mgmt. 59:67-78. Erickson,R. A., and Miner, K. L.. 1998. Six years of synchronousCalifornia Gnatcatcherpopulation fluctuations at two locationsin coastalOrange County, California. W. Birds 29:333-339. Hagan, J. M., Vander Haegen, W. M., and McKinley, P.S. 1996. The early developmentof forestfragmentation effects on birds.Cons. Biol. 10:188-202. Jones and StokesAssoc., Inc. 1993. Methods used to surveythe vegetationof OrangeCounty parks and open spaceareas and the IrvineCompany property. Preparedfor Countyof Orange, EnvironmentalManagement Agency, Santa Ana (availablefrom WilsonOrnithol. Soc., Mus. Zool., Univ. Mich., Ann Arbor, MI 48109-1079). Reid, T. S., and Murphy, D. D. 1995. Providinga regional context for local conservationaction. BioScience Suppl. 1995 (Scienceand BiodiversityPolicy): 84-90.

Accepted 6 July 1998

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