The Auk 117(3):663-686, 2000 DIVERSITY OF BIRDS ALONG AN ELEVATIONAL GRADIENT IN THE CORDILLERA CENTRAL, COSTA RICA JOHN G. BLAKE• AND BETTEA. LOISELLE Departmentof Biologyand International Center for TropicalEcology, University of Missouri-St.Louis, St. Louis, Missouri 63121, USA ABSTRACT.--Speciesdiversity and communitycomposition of birds changerapidly along elevationalgradients in CostaRica. Suchchanges are of interestecologically and illustrate the value of protectingcontinuous gradients of forest.We used mist nets and point counts to samplebirds alongan elevationalgradient on the northeasternCaribbean slope of the Cordillera Central in Costa Rica. Sites included mature tropical wet forest (50 m); tropical wet, cool transition forest (500 m); tropical premontanerain forest (1,000 m); and tropical lower montane rain forest (1,500 and 2,000 m). We recorded 261 speciesfrom 40 families, including 168 speciescaptured in mist nets (7,312 captures)and 226 detectedduring point counts(17,071 observations). The sampleincluded 40 threatenedspecies, 56 elevationalmi- grants,and 22 latitudinalmigrants. Species richness (based on rarefaction analyses) changed little from 50 to 1,000rn but was lower at 1,500 and 2,000 m. Mist nets and point countsoften provided similar views of community structure among sites based on relative importance of differencecategories of species(e.g. migrant status,trophic status). Nonetheless, impor- tant differencesexisted in numbersand typesof speciesrepresented by the two methods. Ninety-threespecies were detectedon point countsonly and 35 were capturedonly. Ten families,including ecologically important ones such as Psittacidae and Cotingidae, were not representedby captures.Elevational migrants and threatenedspecies occurred throughout the gradient,illustrating the needto protectforest at all elevations.A comparablestudy from the Cordillera de Tilaran (Younget al. 1998)demonstrated similar patternsof specieschange alongan elevationalgradient. Comparisons with that studyillustrated that point countsare a valuablecomplement to mist-netstudies. Both studiesindicated the diversenature of the avifaunaalong elevationalgradients in CostaRica. Received8 December1998, accepted7 De- cember 1999. COSTARICA has a well-deservedreputation ulated later studieson the roles of biotic (com- for conservationand biologicaldiversity (Jan- petition, resourceabundance, vegetation struc- zen 1983,Gamez and Ugalde 1988).Yet, outside ture) and abiotic(rainfall temperature)factors of protectedareas, much of the country has on speciesdistribution patterns and commu- been deforested,making national parks and nity structurein tropical forests(Beehler 1981, other reservesespecially important for conser- Loiselle and Blake 1991). Declinesin bird-spe- vationof birds and otherorganisms. Costa Rica cies richness with elevation are common, but alsohas been the site of many studieson birds important differencesexist in the patternsof (at least 340 publicationsfrom 1979 to 1995; changeamong functional groups (i.e. foraging Winker 1998).Two areashave been the focusof guilds, migrant status) of birds (e.g. Stiles many of thesestudies: Monteverde Cloud For- 1983).Declines in speciesrichness have been at- est Reserveand surrounding areas,and La Sel- tributed to declinesin forestarea at higher el- va BiologicalStation and adjacentBraulio Car- evations, declines in abundance and size dis- rillo National Park. Patterns of diversity of tribution of invertebrates, competition, and plants and animalsalong elevational gradients changesin environmentalconditions (Terborgh have been examined in both regions(Harts- 1971, Beehler1981, Janes1994). Local migra- horn 1983,Stiles 1983). tions of birds along elevationalgradients also Early work by Orians (1969) and Terborgh are an important factorstructuring bird assem- (1971) on elevationaldistribution patterns of blages and are a critical considerationin con- birds in CostaRica and Peru,respectively, stim- servationefforts (Stiles1988, Loiselleand Blake 1991, Winker et al. 1997). Mature tropical forestextends from near sea E-mail: [email protected] level at La Selva south for about 35 km to more 663 664 BLAKEAND LOISELLE [Auk, Vol. 117 than 2,900 m atop VolcanBarva on the Carib- STUDY AREA bean slopeof the CordilleraCentral. It is the We conductedresearch at La SelvaBiological Sta- last remaininggradient of continuousforest in tion, located in the lowlands of northeastern Costa Central America to extend over such an eleva- Rica (10ø25'N,84ø01'W) and adjacentBraulio Carril- tional range (Norman 1985).Life zonesrange lo National Park. La Selva encompassesapproxi- from lowland tropical wet forest in La Selva mately 1,500ha, of which about67% is old-growth and lower elevationswithin the park to mon- forest.Braulio Carrillo (ca.45,000 ha) bordersLa Sel- tane rain forestat the volcanotops (Hartshorn va to the south; more than 75% of the corridor con- and Peralta 1988). The fact that forest is pro- nectingLa Selvato the main forestblock of Braulio Carrillo is old-growth forest. Approximate eleva- tected along a continuouselevational gradient tions at our main study sites were 50 m at La Selva makes it particularly important for conserva- and 500 m, 1,000m, 1,500m, and 2,000m in the park tion (Stiles and Clark 1989), especially given (Fig. 1). Forestlife zones (Holdridge 1967) included that many speciesof birds make regular ele- tropical wet (50 m); tropical wet, cool transition (500 vational migrationsalong this gradient (Stiles m); tropical premontanerain (1,000m); and tropical 1988)and that mostlowland forest in the re- lower montanerain (1,500m and 2,000m; Hartshorn and Peralta 1988). Approximate distancesbetween gion hasbeen cleared(Butterfield 1994). study sites were 10 km (50 to 500 m), 6 km (500 to We have conductedstudies on birds along 1,000 m), 5 km (1,000 to 1,500 m), and 7 km (1,500 to most of this gradient (from ca. 40 to 2,000 m), 2,000 m). We typically refer to forest types by ele- providing a unique opportunity to evaluate vationbut notethat they correspondto differentand changesin bird diversityand turnoverin spe- relativelydistinct life zones(except the sitesat 1,500 ciescomposition among elevations. Our studies m and 2,000 m). All siteswere locatedin old-growthforest. Cano- also provide an opportunity to compare pat- py heightswere approximately30 to 40 m at 50 m terns of diversitybetween the Monteverdere- elevation, 35 to 40 m at 500 m, 30 to 35 m at 1,000 m, gion of Cordillera de Tilaran, recently de- 25 to 30 m at 1,500m, and 20 m at 2,000m (Hartshorn scribedby Younget al. (1998),and the La Sel- and Peralta1988). Numbers of tree species(trees >10 va/Volcan Barva region of the Cordillera Cen- cm dbh in 1-haplots; Lieberman et al. 1996)were 115 tral (ca. 80 to 85 km apart). The studies at speciesat 100 m; 131 speciesat 500 m; 100 speciesat 1,000 m; 74 speciesat 1,500m; and 55 speciesat 2,000 Monteverdespanned five life zones and illus- m. The compositionof tree specieschanged contin- trated the importanceof the regionfor conser- uously along the gradient with no discretebreaks vation and the spatial complexityof bird dis- (Lieberman et al. 1996). tribution patterns (i.e. distinctivenessof the La Selvareceives approximately 4,000 mm of rain avifaunain different life zones).Missing from annually.The dry seasontypically lasts from late that study, however,were data from lowland Januaryor early Februaryto March or April, with a sites and from birds not readily captured in second, less-pronounceddry seasonin September and October.Although few climaticdata are avail- mist nets (e.g.many canopybirds). Our studies able from higher elevationsalong the La Selva-Vol- from La Selva and Braulio Carrillo include data can Barva transect,rainfall probablyis highestbe- from lowland habitats and are based on sam- tween 1,000 and 1,500 m. Hartshorn and Peralta ples from mist nets and point counts.These (1988) reported mean annual rainfall ranging from two methodstypically sampledifferent but of- 3,268 mm at 2,260 m elevation to 5,096 at 970 m in ten complementarycomponents of the avifau- areas adjacentto the transect along which we worked.The seasonalpattern of rainfall in Braulio na. The majorobjectives of this paperare to: (1) Carrillois similarto that at La Selva,but thedry'sea- describeavian diversity and turnover along a sonis shorterand lesspronounced. Rain or mist and 2,000-m elevationalgradient in Costa Rica, (2) cloudsoccur almost daily at high elevations. determinestructural changes in the avifauna that reflect variationin ecologicalfunctions of METHODS birds in forests at different elevations,and (3) discuss differences in diversity among life Mist nets.--Wesampled birds with mist nets and point counts(see below). Mist-net studiesstarted in zoneslocated within differentmontane regions La Selva in 1985 and continued until 1994. Studies in of Costa Rica. In addition, we compareresults Braulio Carrillo started in 1985 and continued obtained from mist nets with those obtained through1989. Most data were collectedduring De- from point counts. cemberto April (late wet season,dry season,to early July2000] ElevationalGradient in CostaRica 665 LA SELVA 50m ,•km 500m-- .-C•OSTA 1000 1500 Cacho 2000 Negro PARQUE ,, Bar'va NACIONAL BRAULIO CARRILLO FIG. 1. Study sitesin La SelvaBiological Station and Braulio Carrillo National Park, CostaRica. Elevations representtropical wet forest(50 m); tropicalwet, cooltransition forest (500 m); tropicalpremontane forest (1,000 m); and tropical lower montane forest (1,500