Javier Lopez de Casenave

Edge-interior differences in vegetation structure and composition in a Chaco semi-arid forest, Argentina

Abstract The differences in species composition and horizontal and vertical vegetation structure between a natural forest edge and the adjacent forest interior were analyzed in an old-growth Chaco forest of Argentina. Total basal area in the shrub stratum was slightly greater at the edge, whereas total basal area in the arboreal strata was slightly greater in the forest interior. The abundance of trees and thick stems were significantly greater in the interior, and total stem density, species richness and abundance of thin stems were significantly greater at the forest edge. Compared with the interior, the edge showed greater cover at low heights in the shrub layer and lower cover in the canopy. Edge-interior differences are similar to those reported for both tropical and temperate forests, and appear to be related mainly to the environmental conditions at edges and perhaps to vertebrate-related dispersion mechanisms associated with edges. The similarity of this forest edge with other modified areas elsewhere in the Chaco forest and the relationship with fragmentation processes are discussed.

Activity Patterns of Harvester Ants Pogonomyrmex pronotalis and Pogonomyrmex rastratus in the Central Monte Desert, Argentina

Seed-eating ants could have a significant effect on plant communities in deserts and semiarid zones. This effect is mediated through spatial and temporal foraging patterns, and seed selection within patches. Foraging patterns of harvester ants in South American deserts are almost unknown. The purpose of this work is to determine the temporal variations in the activity levels of Pogonomyrmex pronotalis and P. rastratus in the central Monte desert, and how these patterns may be related to abiotic factors, particularly to soil temperature. Activity levels and soil surface temperature were recorded at hourly intervals in five colonies for each species during the activity season (October, December, February, and April) in both 1999–2000 and 2000–2001 periods (except for October 1999). Surface ant activity starts in October, increases between December and February, and then ceases by April. Surface ant activity is diurnal throughout the season and usually has a unique peak during midday in October and April, and two peaks in the morning and the afternoon from December to February. The proportion of the activity budget devoted to nests maintenance activity was similar for both species. Activity levels of foraging workers tended to be higher in P. pronotalis than in P. rastratus.P. pronotalis is active between 20 and 59°C, with higher levels of activity between 35 and 45°C, whereas P. rastratus shows activity between 18 and 58°C, with higher levels of activity between 30 and 40°C. Our results suggest that temporal changes in surface activity respond mainly to soil temperature fluctuations. However, at intermediate temperatures (those probably encompassing the thermal tolerance range of these ant species), temperature appears not to be a good predictor of daily and seasonal activity fluctuations.

SEED PREFERENCES IN SPARROW SPECIES OF THE MONTE DESERT, ARGENTINA: IMPLICATIONS FOR SEED-GRANIVORE INTERACTIONS

Abstract We assessed seed preference in four species of sparrows that are common to the central Monte desert of Argentina. Our hypotheses are that (1) sparrows always prefer grass over forb seeds and (2) different species of sparrows show the same preferences when consuming seeds. We presented seeds from eight species of plants (four grasses and four forbs) in both choice and nonchoice experiments. In general, sparrow species preferred grass over forb seeds, though one of them (Rufous-collared Sparrow [Zonotrichia capensis]) included one forb species (Parthenium hysterophorus) among its preferred seed items. Among grass seeds, sparrows avoided or showed less preference for the tiny seeds of Sporobolus cryptandrus, which suggests that seed weight may affect seed choice among grasses. Avoidance of most forb species, by contrast, could not be associated with seed size, probably because of the presence of unpalatable or toxic compounds in forb seeds. Experimental evidence suggests that the main feeding pressure of sparrows is clearly directed to grass seeds, which coincides with reported information on bird diet. Results support assertions that sparrows can cause a negative effect on the composition and abundance of grass seeds in the soil seed-bank, a necessary condition for birds in playing a significant role on grass population dynamics in the central Monte desert of Argentina. Preferencias por Semillas de los Emberízidos del Desierto del Monte, Argentina: Implicancias para la Interacción Semillas-granívoros

Bird assemblages of protected and exploited coastal woodlands in east-central Argentina

Abstract We surveyed vegetation and bird assemblages in protected and exploited woodlands in the warm temperate, coastal woodlands of Buenos Aires Province, Argentina. Exploited woodland experienced selective logging of Celtis tala until at least 1960; presently it is used for sheltering domestic livestock. Vertical structure and floristic composition were simpler in exploited than in protected woodland. Likewise, avian density and species richness were lower in exploited woodland. Viewing the avifaunas from a guild perspective, we found the insectivore guild and frugivore-insectivore guild differed substantially between protected and exploited woodlands. The bird densities of these guilds were higher in protected woodland, and the species richness of the insectivore guild was lower in the exploited woodland. Apparently the insectivore guild responded primarily to structural differences, whereas the frugivore-insectivore guild may have responded more to differences in floristic composition. The granivore guild also differed between the two woodlands, but primarily because of changes in the density of the Rufous-collared Sparrow (Zonotrichia capensis). This bird species was more abundant in the protected woodland.

Seed Preferences of Three Harvester Ants of the Genus Pogonomyrmex (Hymenoptera: Formicidae) in the Monte Desert: Are They Reflected in the Diet?

ABSTRACT Harvester ants play an important ecological role as seed consumers in arid areas. We performed choice experiments to study preferences of Pogonomyrmex rastratus (Mayr), Pogonomyrmex mendozanus (Cuezzo & Claver), and Pogonomyrmex inermis (Forel) (Hymenoptera: Formicidae) for seeds of six native species (three grasses, two forbs, and a shrub) in the central Monte desert, Argentina. We hypothesized that ant diet composition would reflect ant preferences. Thus, based on previous ant diet studies, we expected that 1) the three species would prefer grass to nongrass seeds, 2) P. inermis would have a lower preference for nongrass seeds than P. rastratus and P. mendozanus and 3) relative abundance of seeds in the diet would be positively associated with relative preference. In general, relative abundance of seeds in the diet was positively associated with relative preferences. Ants preferred grass seeds, but P. inermis did not have a lower preference for nongrass seeds. We also analyzed the relationship between preferences for seeds of the six species and their size and morphology, and we found higher preferences for seeds of intermediate size but no relationship with seed morphology. The overall match between seed preferences and diet composition could increase the chances of ants affecting the abundance and composition of some seed resources in the Monte desert, with important community implications.

Use and Importance of Crop and Field-Margin Habitats for Birds in a Neotropical Agricultural Ecosystem

Abstract. The Neotropical Region has experienced large habitat transformations as a result of intensified agriculture. These changes have affected the populations of many species of birds in the Espinal ecoregion of Argentina. However, relationships between birds and agriculture in the neotropics are poorly known. We assessed the effects of crops and field margins in an area of agriculture typical for the Espinal ecoregion on the structure of the bird community, and we assessed the value of this habitat for species of conservation concern. Birds and vegetation were sampled in and along the margins of fields of soybean (the most widespread crop) and alfalfa. Twenty-five of the 41 species recorded—including all the species of conservation concern—were found almost exclusively in field margins; only five species occurred almost exclusively in within the fields. All other species were found in both margins and fields. Density, richness, and diversity were much greater on the margins than in the fields. Bird density in soy fields was lower than in alfalfa. Bird richness and diversity in the two crops, however, did not differ. Density in field margins increased with the cover and height of trees and shrubs. Most of the field-margin species were woodland-border species, but the species of conservation concern were all associated with grassy field margins. Because further declines in the populations of these threatened species are expected, their conservation in agricultural areas would benefit from a broad policy of habitat-conservation plans for field margins such as those practiced in North America and Europe.

Influence of seed size on feeding preferences and diet composition of three sympatric harvester ants in the central Monte Desert, Argentina

Selective seed consumption by harvester ants may affect seed abundance and composition and, ultimately, plant communities. We evaluated the influence of seed size on preferences and diet of Pogonomyrmex mendozanus, P. rastratus, and P. inermis in the central Monte Desert, Argentina. In choice experiments with Pappophorum spp. seeds of different sizes, P. mendozanus and P. rastratus preferred large seeds, maximizing energy reward. P. inermis showed a less-marked preference for large seeds, which was probably due to morphological constraints imposed by its small body size. Under natural conditions, none of the three species selected larger Pappophorum spp. seeds probably because of high travel and handling costs. Seeds of intermediate size predominated in the diet of the three species but a slight size match was detected as P. mendozanus carried larger seeds than P. rastratus and this than P. inermis, matching body-size differences. Thus, ants probably maximize energy reward but face morphological restrictions and higher costs when carrying and holding large seeds. While seeds of intermediate size are the most vulnerable ones to ant predation, small seeds are favored, as they are abundant in the soil seed bank and lowly predated.

NEOTROPICAL AUSTRAL MIGRANT LANDBIRDS: POPULATION TRENDS AND HABITAT USE IN THE CENTRAL MONTE DESERT, ARGENTINA

Abstract We studied density changes of two groups of Neotropical austral migrant landbirds—the South American temperate-tropical (SATT) and cold-temperate (SACT) migratory systems—in the main habitat types of the central Monte desert of Argentina (open Prosopis flexuosa woodland and Larrea cuneifolia shrubland) over 10 years. Five species, all tyrant flycatchers (Tyrannidae) made up SATT, whereas only two of the seven species of SACT were tyrannids. Densities of both SATT and SACT were higher in open woodland than in shrubland. SATT density did not differ among years, but SACT density did, having lower values in 1994 in both habitats. In subsequent years, SACT densities increased but did not reach values similar to those previous to 1994. The decline in 1994 coincided with a two-year drought period that began in 1993, but lower density in the following years did not appear to be related to climatic conditions in the study area, suggesting a low capacity of SACT species to recover population abundance after periods of stress. In contrast, SATT density was not associated with local climatic conditions, possibly because several SATT species used the study area only as a stopover site. During the breeding season, birds of both migratory systems disproportionately use the open woodland, which offers more sites to nest and feed than does the shrubland. Although most SATT and SACT species are abundant and not currently of conservation concern, human activities in the central Monte desert promote the structural simplification of the habitat, which could threaten future populations of Neotropical austral migrant landbirds in this ecosystem.

Selection of Foraging Sites by Desert Granivorous Birds: Vegetation Structure, Seed Availability, Species-Specific Foraging Tactics, and Spatial Scale

Abstract Seed availability in the algarrobal of the Monte desert, Argentina, as in other deserts, is highly heterogeneous at small scales and is associated with vegetation structure. Granivores are expected to use space selectively (within the capacities of their foraging techniques), resulting in a heterogeneous effect on the seed bank. First, we describe the foraging repertoire of granivorous birds in the algarrobal to develop predictions about their expected use of space. Although the granivory guild as a whole tracked the temporal availability of seeds, species within the guild differed in foraging behavior and seasonal changes. We assessed selection of space by foraging birds through a two-scale bird-centered analysis, comparing the distributions of used and available sites. Granivorous birds, considered as a guild, used the whole range of available microsites, though as a result of an aggregation of contrasting partial patterns. Microsites with more cover of shrubs, grasses, and litter were preferred for predispersal consumption, which is consistent with the frequent technique of attacking grasses from low woody perches. By contrast, use of microsites for postdispersal consumption did not differ from random, which suggests that there are no safe microsites for seeds in this habitat. At a larger scale not particularly related to heterogeneity in food availability, a selective pattern was clearer: birds avoided mesosites with low shrub and litter cover, far from trees. In conclusion, patterns are not straightforward and depend on considerations of spatiotemporal scale and species-specific characteristics. Even when predictions seemed clear-cut and were tested with proper scale and target-group, granivorous birds did not select space according to environmental clues of food abundance.

FORAGING BEHAVIOR AND MICROHABITAT USE OF BIRDS INHABITING COASTAL WOODLANDS IN EASTCENTRAL ARGENTINA

Abstract We examined foraging behavior and microhabitat use of four passerine bird species inhabiting an old growth coastal woodland in Buenos Aires Province, Argentina. Based on foraging maneuvers, we identified two groups: (1) nonaerial foragers formed by Tropical Parulas (Parula pitiayumi) and Masked Gnatcatchers (Polioptila dumicola) that hopped on branches and twigs while searching and gleaning prey from the nearby foliage, and (2) aerial foragers formed by White-crested Tyrannulets (Serpophaga subcristata) and Small-billed Elaenias (Elaenia parvirostris) that searched for and detected prey while perched, and captured it from foliage or in the air. Nonaerial foragers found prey primarily in the canopy while aerial foragers captured prey at all heights in the woodland. Tree species selection was similar among the four species. All bird species disproportionately foraged in Scutia buxifolia and avoided the use of Ligustrum lucidum trees. Our results indicate that these bird species differed in microhabitat selection in old growth coastal woodland, and that nonaerial foragers were more sensitive to foliage architecture and foliage height distribution than aerial foragers. These results illustrate the importance of woodland logging to bird densities, and provide basic information for effective management. RESUMEN.—Nosotros estudiamos el comportamiento de alimentación y el uso de microhábitats de cuatro especies passeriformes que habitan bosques costeros maduros de la Provincia de Buenos Aires, Argentina. Considerando las maniobras de alimentación encontramos dos grupos: (1) las recolectoras, formado por Pitiayumí (Parula pitiayumi) y Tacuarita Azul (Polioptila dumicola), las cuales saltando por las ramas y ramitas buscan y recolectan presas desde el follaje cercano, y (2) las cazadoras por revoloteo, formado por Piojito Común (Serpophaga subcristata) y Fiofío Pico Corto (Elaenia parvirostris), que buscan y detectan presas desde una percha y, usando tácticas aéreas, capturan las presas desde el follaje o en el aire. Las especies recolectoras capturan sus presas principalmente en el dosel del bosque, mientras que las cazadoras por revoloteo se alimentan en todas las alturas. El uso de las especies arbóreas del bosque fue similar para las cuatro especies de aves. Todas seleccionaron alimentarse en árboles de Scutia buxifolia y evitaron usar los de Ligustrum lucidum. En general, las especies de aves difieren en su selección de los microhábitats en el bosque costero maduro; las especies recolectoras fueron más afectadas por la arquitectura del follaje y la abundancia de cobertura en altura que las especies cazadoras por revoloteo. Los resultados obtenidos ayudan a comprender los efectos del talado del bosque sobre la densidad de aves, y proveen información básica para su efectivo manejo.