Composition and seasonal changes of the bird community in ... - BioOne

J. Field Ornithol. 74(1):59–65, 2003

Composition and seasonal changes of the bird community in the west pampa grasslands of Argentina J. P. Isacch,1,4 M. S. Bo,1 N. O. Maceira,2 M. R. Demarıá,2 and S. Peluc3 Departamento de Biologıá, Fac. Cs. Exactas y Nat., Univ. Nacional de Mar del Plata, Funes 3250, 7600 Mar del Plata, Argentina 2 Instituto Nacional de Tecnologıá Agropecuaria (INTA) EEA, Villa Mercedes, Prov. San Luis, Argentina 3 INTA EEA San Juan, Argentina 1

Received 23 October 2001; accepted 1 April 2002 ABSTRACT. We studied the species composition and seasonality of the bird community in the semi-arid west pampa grasslands of Argentina. The study was carried out in natural grasslands with and without a history of cattle grazing (‘‘mixed grassland’’ and ‘‘sorgastral,’’ respectively). We counted birds using the strip-transect method. We recorded a total of 22 bird species including only 12 grassland-dependent species. The more representative species of these grasslands were Chaco Pipit (Anthus chacoensis), Long-tailed Meadowlark (Sturnella loyca), and Grassland Sparrow (Ammodramus humeralis). Richness and abundance of birds were higher in mixed grasslands than in sorgastral in spring and summer, but not in winter. Inside habitats, richness and abundance were higher in spring and summer than in winter. We observed a seasonal change in guild dominance, with granivores dominant in winter, although bird density decreased during winter for all guilds. Despite the low richness recorded, the presence of certain rare species, such as the Chaco Pipit and the near threatened Greater Rhea (Rhea americana), add conservation value to west pampa grasslands. SINOPSIS.

Composicio ´ n y cambios estacionales de la comunidad de aves en las pampas occidentales

de Argentina

Se estudio´ la composicioń de especies y la estacionalidad, de la comunidad de aves del pastizal de la pampa occidental (pastizales semia´ridos). El estudio se llevo´ a cabo en pastizales naturales (sorgastrales y pastizales mixtos) del a´rea central de la Provincia de San Luis, Argentina. Se muestrearon las poblaciones de aves usando transectas de ancho fijo. Se registraron un total de 22 especies de aves en pastizales naturales de la pampa occidental (solo 12 especies de pastizal). Las especies ma´s representativas de este ha´bitat fueron Anthus chacoensis, Sturnella loyca y Ammodramus humeralis. La riqueza y abundancia de aves fueron ma´s altas en pastizales mixtos que en sorgastrales en primavera-verano, pero no hubo diferencias en el invierno. Para cada ha´bitat, la riqueza y abundancia fueron ma´s altas en primavera-verano que en invierno. Se observo´ un cambio estacional en la dominancia a nivel de gremios tro´ficos, debido a una dominancia de granı´voros en invierno, a pesar de que la densidad de todos los gremios disminuyo´ en invierno. A pesar de la baja riqueza registrada, la presencia de Anthus chacoensis (estatus discutido), y Rhea americana (cercana a la amenaza), suman un valor adicional para la conservacioń de los pastizales de la pampa occidental. Key words: Argentina, grassland birds, seasonality, semiarid grassland, western pampa

The pampas region of South America’s temperate grasslands has experienced tremendous agricultural development, mainly during the past century (Soriano et al. 1991). Agroecosystems have replaced native grasslands in most of the region (Leoń et al. 1984). The central Argentinean semi-arid grasslands (San Luis and La Pampa Province; the ‘‘west pampa’’ sensu Soriano et al. 1991) constitute the western limit of the pampas region (Leoń and Anderson 1983). Extensive areas of natural grasslands are still conserved in this zone, because semi-arid con4 Corresponding author. Email: jpisacch@mdp. edu.ar

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ditions and soil fragility constrain agricultural development (Leoń et al. 1984). The main habitat of the west pampa is the ‘‘sorgastral’’ grassland, dominated by Sorghastrum pellitum (Anderson et al. 1978). When these grasslands are grazed by cattle, S. pellitum is progressively replaced in dominance by Elyonurus muticus (Anderson et al. 1978; Aguilera et al. 1998), and the community becomes a ‘‘mixed grassland.’’ Both plant species are tussock perennial C4 grasses; therefore, grassland physiognomy remains similar despite the changes in plant composition. Other natural grasslands found in the zone are dominated by the native species Stipa tenuissima, Piptochaetium napostaense, and Schizachyrium condensatum, and probably reflect

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historical impacts from plowing or extreme grazing and secondary succession. However, in this study we will use the designation ‘‘mixed grasslands’’ for those grasslands that, for agricultural management reasons, have moved away from the more ancient ‘‘sorgastral’’ community. Most studies of bird communities in pampa grasslands are restricted to the wet zone (flooding pampa; Comparatore et al. 1996; Isacch and Martıńez 2001). Bird communities in other arid and semi-arid areas of southern South America have also been studied but always in areas with shrubby (Monte Desert; Marone 1991, 1992; Lo´pez de Casenave and Marone 1996; Marone et al. 1997) or woody vegetation (Chaco Forest; Capurro and Bucher 1982, 1986; Lo´pez de Casenave et al. 1998). There are no published studies on western pampa bird communities except for records of bird presence (Nellar Romanella 1993). The presence of pristine or scarcely modified natural grasslands in the western pampa allowed us to study bird community composition and its seasonal dynamics in this ecosystem. The knowledge of these basic patterns in natural grasslands are neccesary to assess the effects of habitat transformation on bird communities (Aguilera et al. 1999). Here, we compare the richness, abundance, trophic guild densities, and species densities between seasons and habitats in the sorgastral and mixed grassland bird communities. METHODS Study area. Our study was carried out in central San Luis Province, Argentina, in several ranches located on a strip of approximately 50 3 35 km in size (Fig. 1). According to Penã Zubiate et al. (1998), the strip occupies sandy flats with anthropic dunes, sandy flats with permanent lagoons, and central sandy flats. These areas present important constraints on agriculture, determined by drought and sandy soils, poor in organic matter, and with low waterretention capacity (Penã Zubiate et al. 1998). The main activity is cattle ranching, although in the eastern part of the region some grasslands have been replaced with sown pastures and crops (Penã Zubiate et al. 1998). Pristine grasslands (‘‘sorgastral,’’ dominated by Sorghastrum pellitum) are situated mainly to the west, in the central sandy flats. Mixed grass-

J. Field Ornithol. Winter 2003

lands (dominated by Elyonurus muticus and other perennial grasses) occur throughout the study area, but are dominant in the central and eastern areas, where the human impact is higher. Mixed grasslands have higher plant diversity than sorgastral, especially because species evenness increases when S. pellitum dominance decreases through selective grazing (Anderson et al. 1978). In addition, cattle grazing and agriculture have promoted the invasion of two tree species into the grassland, chanãr (Geoffroea decorticans) and caldeń (Prosopis caldenia), both coming from the adjacent Espinal Phytogeographic Province (thorn-forest of Leoń and Anderson 1983). As a consequence, natural grasslands under grazing present a greater structural heterogeneity than grasslands with pristine or nearly pristine conditions. San Luis Province has a pronounced rainy period in spring and summer and an arid one in fall and winter. The study zone is included between the 400–500 mm average annual historical isohyet. The water balance indicates the existence of a permanent water deficit during the year (Veneciano et al. 2000). The mean temperatures varies approximately between 88C (July) and 238C (January), with a frost-free period of 142 days (Veneciano et al. 2000). Bird censuses. We sampled bird populations using the strip transect method (Emlen 1971) during spring and summer (November 1999 and early March 2000) and winter (July– August 2000). We made 26 counts during winter and 30 during spring-summer. We made 36 counts in mixed grasslands and 20 in sorgastral, because the former habitat is more common than the latter in the region. Strip transects were 500 3 60 m. Transect longitude and rate of traverse were similar to Conner and Dickson (1980). Each transect was run after sunrise for a maximum of 4 h and during the last 3 h of daylight. We did not count birds under extreme weather conditions (windy and/or rainy days) because of the adverse effect on bird activity (Conner and Dickson 1980). In each transect we recorded species, number of individuals, and the site where the birds were perched (grassland, tree and/or both). We did not count birds obviously in transit (e.g., migrating), but we did include those that were searching for food in flight. Data analysis. Differences between means were tested using a t-test or a non-parametric

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Bird Communities in the Western Pampa

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Fig. 1. Pampa region in Argentina and location of the study area (box). Dotted line indicates the limit of the Western Pampa subregion (after Soriano et al. 1991).

Mann-Whitney Test when the assumptions for the parametric test were violated even after data transformation (Zar 1996). With these tests we evaluated the null hypothesis of no differences between richness, abundance, and trophic guild densities in different seasons, and different habitats individually. Censuses in different seasons were performed in the same ranches and paddocks, but not on the same transects, which assured independence between samples, and allowed us to make statistical comparisons between periods. Bird species were classified into trophic guilds of herbivore, carnivore, insectivore, gra-

nivore, and omnivore (granivore-insectivore), according to Capurro and Bucher (1986), Canevari et al. (1991) and Marone (1992). RESULTS

We recorded a total of 22 bird species in the western pampa grasslands, 12 of which were grassland birds and the other 10 associated with trees or generalist habitats (Table 1). Grassland species were defined as those feeding and breeding mostly in grassland habitats. The more abundant species were Anthus chacoensis (Chaco Pipit, 30% of total individuals),

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Table 1. Mean bird species density (individuals/transect) recorded in natural grasslands of the west pampa of Argentina. The species are grouped by grassland dependence. Data are separated according to season and habitat. Migratory status and trophic guild are indicated for each species. Mixed grassland

Sorgastral Spring and summer A: grassland-dependent species Rhea americana Nothura darwini Nothura maculosa Rynchotus rufescens Anthus correndera Anthus chacoensis Circus cinereus Asio flammeus Ammodramus humeralis Sicalis luteola Sturnella loyca Leistes supercilliaris

Spring and summer

Winter

Migratory statusa

Trophic guildb

0c 0 0 0 0 0 0 0 0 0 0c 0

0.06 0.22 0c 0.11 0.29 3.57 0c 0 2.17 1.72 2.56 1.11

0c 0.06 0c 0 0 0 0 0.06 0.56 0 0.11 0

Pr Pr Pr Sr Sr Sr O O Pr Pr Pr Sr

H O O O I I C C G G O O

species 0 0 0 0.13 0 0 0 0 0 0

0.17 0.11 0.06 0.33 0.06 0.17 0.06 0.33 0.17 0.11

0 0 0 0c 0 0.11 0 0 0 0

Pr Pr Pr Pr O Pr O O O O

C C C I I G G G O O

Winter

0 0c 0 0c 0 3.63 0 0 0.08 0 1.83 0

B: woody-dependent or habitat-independent Polyborus plancus 0 0.42 Milvago chimango 0c Falco femoralis 0c Annumbius annumbi 0 Xolmis irupero 0 Zonotrichia capensis 0c Diuca diuca 0 Sicalis flaveola 0 Molothrus bonaeriensis 0 Molothrus badius a

Pr, permanent resident; Sr, summer resident; O, occasional (less than three records out of 57 censuses). H, herbivore, G, granivore; I, insectivore; O, omnivore; C, carnivore. c Species recorded only off of transect. b

Sturnella loyca (Long-tailed Meadowlark, 23%) and Ammodramus humeralis (Grassland Sparrow, 16%). The best represented trophic guilds were insectivores (represented mainly by the Chaco Pipit), granivore (the Grassland Sparrow dominant) and omnivores (the Long-tailed Meadowlark dominant; Table 1). All species except Asio flammeus (Short-eared Owl) were recorded in spring and summer, whereas only 8 of 22 total species were recorded during winter (Table 1). The grassland birds recorded only during spring and summer were pipits (Anthus chacoensis and A. correndera, Correndera Pipit), Sicalis luteola (Grassland YellowFinch) and Leistes supercilliaris (White-browed Blackbird). The species recorded in both seasons decreased in density and/or were recorded only off of transects during winter (Table 1). Richness and abundance of birds were differ-

ent between habitats (mixed grassland and sorgastral) during the spring and summer period, but they were not different during winter (Table 2). Richness and abundance of birds was seasonal in sorgastral and mixed grassland (Table 2). At the trophic guild level, granivore density was different between habitats only during spring and summer, with higher values in mixed grassland than sorgastral (Table 2). Only mixed grassland had different granivore densities between seasons, being higher during spring and summer (Table 2). Insectivore and omnivore densities were not different between habitats, but both were seasonal, with higher values during spring and summer. DISCUSSION The grasslands of the western pampa have features that explain their low avian species

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Table 2. Richness (mean 6 SD), abundance (mean 6 SD), and trophic guild density (individuals/transect) recorded in natural grasslands of the west pampa of Argentina. Data are separated according to season and habitat. Two sample testsa Between habitats Sorgastral Spring and summer

Winter

Mixed grassland Spring and summer

Winter

Richness 2.1 6 0.8 0.1 6 0.4 4.4 6 2.6 0.7 6 1.0 Abundance 5.4 6 4.2 0.1 6 0.4 12.9 6 10.3 0.9 6 1.4 0.1 0 4.5 0.7 Granivore density 3.1 0.1 4.2 0 Insectivore density 1.8 0 2.8 0.2 Omnivore density a

Between seasons Mixed Sorgastral grassland ,0.001 ,0.001 NS ,0.001 ,0.01

,0.0001 ,0.0001 ,0.001 ,0.0001 ,0.01

Spring and Winsummer ter 0.01 ,0.001 ,0.01 NS NS

NS NS NS NS NS

t-test or Mann-Whitney test.

richness (Table 1): a relatively simple vegetational structure (grasslands dominated by one or a few perennial grass species), scarce precipitation, year-round water deficit, and wide temperature fluctuation. Wiens (1973) pointed out that grassland sites have a low and relatively constant number of breeding species and that desert sites have low bird densities. Cueto and Lo´pez de Casenave (1999) indicate that simple habitat factors (e.g., vegetation strata) are important in determining geographic patterns of bird species richness, although species richness is also influenced by climatic conditions such as precipitation, relative humidity, and temperature variability. If we compare species richness of grasslands birds in the western pampa with richness in other relatively simple Argentinean habitats at similar latitudes, more humid sites (e.g., flooding pampa grasslands, Comparatore et al. 1996) and those with higher vegetational complexity (such as Monte Desert, Marone 1992) support a higher number of species than western pampa grasslands. The Chaco Pipit reaches a relatively high abundance in the western pampa grasslands. Little information is available about the distribution of this species in Argentina (Straneck 1987). The species has been cited as typical of the eastern Chaco Ornithogeographic Province (Nores 1987), but the results of our study extends its known distribution considerably. Further work on its distribution and conservation status is needed; it was considered as near threatened by Collar et al. (1994) but not included as threatened by BirdLife (2000).

The higher species richness found during spring and summer in mixed grassland compared to sorgastral could be explained by the former’s higher structural heterogeneity and higher plant species diversity. These traits are a consequence of grazing, which diminishes the dominance of Sorghastrum pellitum and allows an increase in associated herbaceous species. Simultaneously, grazing and plowing favor the invasion of trees into the mixed grasslands, giving them structural heterogeneity. However, we do not think that human disturbance necessarily enhanced bird richness in the semi-arid pampa. Both sorgastral and mixed grasslands are natural states of the grassland mosaic, maintained historically by factors such as fire and herbivory by native mammals (Soriano et al. 1991). Human activity has simply shifted the balance from a dominance of sorgastral to a dominance of mixed grassland. Seasonal variation in bird assemblages may be related to low winter temperatures (Herrera 1981) or to alternation between rainy and dry periods (Capurro and Bucher 1986; Poulin et al. 1992). In the western pampa, both factors are important. In addition, more species generally considered non-migratory disappeared during winter in the western pampas grasslands, indicating geographic movements to avoid this unfavorable climatic period. We observed a seasonal change in guild dominance, although the density of all guilds decreased during winter. During the spring and summer period, we observed similar densities of granivores, insectivores, and omnivores, but

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during winter granivores were more abundant. A similar pattern was recorded in other arid habitats of South America such as the Monte Desert (Marone 1992) and Chaco Forest (Capurro and Bucher 1986). Arid and semi-arid ecosystems present a high availability of seeds during most of the year (Brown et al. 1979). In seasonal and unpredictable habitats, seeds represent plants’ escape strategy from unfavorable conditions (MacMahon and Wagner 1985). In the western pampa grasslands, most grasses are warm-season species that disperse seeds at the end of summer. As a result, seed availability is maximum during the autumn and winter (J. Lo´pez de Casenave, pers. comm.). This could explain the higher number of granivores and some omnivores during winter, compared with insectivores. For conservation, it is necessary to create protected areas in the western pampa grasslands (there are none at present), and to control the increases in agriculture (Aguilera et al. 1999). Preliminary studies employing satellite imagery show that in a region of approximately 4000 km2, which includes the study area, nearly 1000 km2 of natural grasslands were replaced by sown pastures or agriculture between 1992–99 (Demarıá et al. 2001). The presence of the threatened Chaco Pipit and the near threatened Greater Rhea (Rhea americana; BirdLife 2000), in this habitat add additional value to the western pampa grasslands. ACKNOWLEDGMENTS

We are grateful to the following owners and personnel of the ranches of San Luis Province where we collected the data: Los Pozos, Marıá Teresa, Marianela, El Charaboń, San Nicola´s, Don Hernań, La Travesıá Fernańdez, and La Travesıá Battaglia. We thank J. Lo´pez de Casenave, J. Wiens, C. R. Brown, and an anonymous reviewer for comments that improved the manuscript. We thank M. Nellar, P. Laterra, M. Aguilera, D. Steinacker, and R. Sager for discussion. We appreciate help with the English from Daniel Brooks of the Association of Field Ornithologists program of editorial assistance. This work was financed by the Agencia Nacional de Promocioń Cientı´fica y Tecnolo´gica de Argentina (PICT 08-03994), the Instituto Nacional de Tecnologıá Agropecuaria, Universidad Nacional de Mar del Plata, and Comisioń de Investigaciones Cientı´ficas de la Provincia de Buenos Aires.

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