Bird-Habitat Relationships in a Vegetational Gradient in the Andes of Central Chile1
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The Condor 99:719-727 0 The Cooper Ornithological Society 1997 BIRD-HABITAT RELATIONSHIPS IN A VEGETATIONAL GRADIENT IN THE ANDES OF CENTRAL CHILE1 CRISTIAN E ESTADES~ Departamentode Manejo de RecursosForestales, Universidadde Chile, Casilla 9206, Santiago, Chile Abstract. An unusualpattern has been describedin Patagonianforests, where bird spe- cies diversity seems to be inversely correlated with vegetation complexity. To test the oc- currence of this pattern in the northern Nothofagusforests of central Chile, a study was conducted at the Ruble National Reserve during the austral summers of 1994 and 1995. Twenty-nine plots were set along a vegetational gradient. In each plot, the density of all diurnal bird specieswas estimated, and the vegetationcomposition and structurewere mea- sured. The highest and the lowest bird speciesdiversity were found in semideciduousscrub habitat and Poaceae steppes,respectively. In nongrasslandsites, no vegetation variable ex- plained bird speciesrichness or diversity. Total foliage volume and foliage height diversity were significantly correlatedwith bird speciesdiversity in the nonforest sites but not in the forested areas. However, foliage height diversity significantly explained the variation of bird density and bird species richness in the forest sites. The presence of the tree Nothofagus obliqua was positively correlated with bird diversity and insectivore density. The patterns observed at Nuble lie between the traditional positive relationship between bird diversity and foliage diversity and the inverse relationshipfound in Patagonia.I proposethat particular floristic features of the vegetation rather than structureitself might explain better the diver- sity of bird speciesin Patagonianforests. Key words: bird communities,Chile, Patagonianforests, diversity,vegetation structure, Nothofagus. INTRODUCTION METHODS Temperate forest birds have been poorly studied The study was conducted at the Ruble National in Chile, and few investigations have described Reserve in central Chile (Fig. 1). This area in- the composition and structure of entire bird cludes two ecological subregions: “Bosque Cad- communities (Jaksic and Feinsinger 1991). ucifolio Montano” and “Cordilleras de la Ar- Moreover, all these studies have been conducted aucanfa” (Gajardo 1994). Most of the 720 km* in evergreen or semideciduous lowland forests, Reserve is covered with upland steppesof Fes- leaving the bird assemblages of the deciduous tuca and Chusquea, deciduous scrub of Notho- beech forests in the Andes cordillera of central fagus antarctica or N. pumilio, and deciduous and southern Chile virtually unknown. Vuilleu- forests of N, obliqua and N. pumilio (Faiindez mier (1972) described an unexpected pattern in et al. 1994). Mean annual precipitation within bird communities in northern Patagonia in Ar- the Reserve ranges from 2.3 to 3.3 m; mean gentina. He found fewer speciesin beech forests minimum temperature in July ranges from than in the structurally simpler scrub-steppehab- -1.2”C to 0.9”C; and mean maximum tempera- itats. In the same area, Ralph (1985) found a ture in January ranges from 14.8”C to 22.1”C significant inverse relationship between foliage (Santibtiez and Uribe 1993). complexity and bird diversity in nongrassland Bird censuses were carried out during Feb- sites. ruary 1994 and January 1995, corresponding to The aim of the present study is to describe the late breeding season. Twenty-nine census habitat associations of bird communities in a sites were established along a vegetational gra- forest-scrub-steppegradient in the Andes of cen- dient from steppe to forest. When possible (20 tral Chile and to determine whether they exhibit sites), variable line transects (Bumham et al. the patterns found by Ralph (1985). 1980) were used to estimate bird density. At sites where walking was difficult (nine sites) variable circular plots were used (Buckland 1Received 5 September 1996. Accepted 22 April 1987). I assumed that these two methods pro- 1997. 2 Current address:Department of Wildlife Ecology, duced similar estimatesof a population’s density University of Wisconsin, Madison, WI 53706. (Ralph 1985). Observations were carried out be- 17191 720 CRISTIAN F. ESTADES + censusplots National 5 10 km FIGURE? 1. Geographic location of the study area, Ruble National Reserve, in central Chile. tween Ok00 and 12:O0. Detections were record- standardized as the number of specieson a 2-ha ed at intervals of 10 m, with a maximum detec- area. tion distance of 80 m. Flying birds were not re- At every site, foliage structure and plant spe- corded unless they showed clear evidence of us- cies composition were described in four hori- ing the plot (e.g., swallows hunting insects). zontal layers (o-0.2, 0.21-2, 2.1-8, and 8.1-16 Censuses were conducted by two observers, m). In each layer, total foliage volume (TPV,,,,) each contributing roughly 50% of the observa- was estimated as the proportion of the layer oc- tions within each plot. Transects or points were cupied by plant leaves, twigs, and branches.This surveyed 4-10 times in 1994 and 6 times in proportion was multiplied by the layer’s height 1995. Data for each bird species at each census to obtain its TFV,,,, (m3 per 0.01 ha). The rel- site were pooled to estimate population density. ative species frequency at each layer was mul- Densities from 1994 and 1995 were averaged tiplied by the TFV,,,, to obtain foliage volume and their variances were calculated. for individual species. The site’s total foliage Bird species diversity (BSD) at each site was volume (TFV) and individual species’ foliage calculated using the Shannon-Wiener (H’) di- volume were calculated by the addition of the versity index (MacArthur and MacArthur 1961). four TFV,aye,values. Vegetation measures were Bird speciesrichness (BSR) at each site was es- estimated in both 1994 and 1995 and then av- timated using the jackknife procedure (Heltshe eraged. Plant species diversity (PSD) was esti- and Forrester 1983). To eliminate the effect of mated for each layer and for the entire site using sample size, richness estimates were corrected the proportion of the TFVlaye,and the proportion using a logarithmic transformation and were of the TFV represented by all the species, re- BIRD-HABITAT RELATIONSHIPS IN CENTRAL CHILE 721 spectively. With these values, an H’ index of di- vation (1,200-1,400 m) deciduous scrub. In versity was calculated. In order to determine an open valley bottoms. Sparse scrub with dense index of foliage height diversity (FHD), the grass (mainly Festuca) cover. Scrub 2-5 m tall three upper layers were divided into two equal (five sites). sub-layers whose TFV,,,, were calculated. This 8. Chusquea argentina steppe (CaS). Mid-to subdivision was done because the four original high-elevation (1,300-1,900 m) steppe. Occurs layers were considered inappropriate to describe in many topographic situations. Composed the foliage profile precisely. Finally, the TFV,,,,j mainly of C. argentina and Festuca scabrius- TFV ratios for the seven resulting layers were culu. Lower than 0.5 m tall (three sites). used to estimate the H’ index. 9. Poaceae steppe(PS). Mid to high-elevation As a way of identifying different kinds of (1,300-2,000 m) steppe. Located in windy habitats for birds, the census sites were grouped zones. Main species are grasses of the genus initially in terms of their vegetation structureand Agrostis, Festuca, Poa, and Hordeurn. In some composition using hierarchical clustering (Digby zones there is a sparse shrub cover of mainly and Kempton 1987). However, to make the re- Chiliotrichum rosmarinifolia (two sites). sulting classification more consistent with field The use of these habitats by bird specieswas observations of bird assemblages,two sites were described in terms of their mean absolute den- arbitrarily reassigned to another type of habitat. sities. A measure of habitat breadth (B) was cal- Finally, nine vegetation types (bird habitats) culated for all speciesusing the relation B(x) = were recognized at the studied area. These are: eZpiLOg@used‘), by MacArthur (1964), where pi is 1. Nothofugus pumilio forest (NpF). High-el- the density of speciesx in habitat i divided by evation (1,300-1,700 m) deciduousbeech forest, the sum of densities for the species at all the on wind-protected slopes. Trees lo-14 m tall. habitats studied. Birds were assigned to trophic Sparse understory (two sites). guilds following Johnson (1967) and Jaksic and 2. N. obliqua forest (NoF). Low-elevation Feinsinger (1991) and using the author’s field ( 1,000-l ,200 m) deciduousbeech forest, located experience. Bird species names follow Araya in protected valleys and slopes. Trees 8-12 m and Chester (1993), and plant names follow tall. Sparse to dense understory (six sites). Marticorena and Quezada (1985). 3. N. obliqua-pumilio forest (NopF). Mid-el- Linear regression analysis and Pearson cor- evation (1,200-l ,400 m) highly stratified decid- relations were performed in order to relate birds uous beech forest. Located mainly on south-fac- with vegetation variables and individual plant ing slopes in protected valleys. Dense understo- species. For this last analysis, only species ry of Chusquea argentina. Trees 14-16 m tall (plants and birds) recorded at five or more sites (five sites). were considered. 4. Austrocedrus chilensis-N. obliqua forest RESULTS (AcNoF). Low-elevation (l,OOO-1,200 m) mixed cedar-beech forest. Occurs on rocky BIRDS slopes. Very sparse overstory and understory. Forty-six bird species were recorded during the Trees 8-12 m tall (one site). transect and point surveys (Table 1). The most 5. Semideciduous scrub (SS). Low-elevation abundant specieswere the White-crested Elaenia (l,OOO-1,200 m) scrub. Located on dry slopes. (1.37 ha-l), Black-chinned Siskin (1.30 ha-‘), Composed mainly of Colletia hystrix, C. argen- Thorn-tailed Rayadito (0.86 ha-‘), Common tina, Diostea juncea, and Fabiana imbricata, Diuca-Finch (0.78 ha-i), and House Wren (0.77 with a few scatteredbeech trees.