David N. Bilenca

Distributional Changes of Landbird Species in Agroecosystems of Central Argentina

Abstract. We present the first quantitative comparative study between current (2006–2008) distributions of landbird and those recorded prior to the massive planting of transgenic soy beans in the Pampas of central Argentina (Narosky and Di Giacomo 1993). We surveyed landbirds along transects covering 23 counties of Buenos Aires province. To allow a comparison between our observations and those of Narosky and Di Giacomo, we used the number of counties in which each species was recorded as an estimate of current and past distributions. We found grassland and wetland specialists in significantly fewer counties than did Narosky and Di Giacomo (P < 0.05), while habitat generalists and woodland specialists showed no significant change. The earlier study, however, covered a longer period of time, which could explain the reduction of wetland dwellers as a result of variation in the supply of temporary water bodies. The decreased area of occupancy of grassland specialists, on the other hand, may reflect the reduction of native grasslands due to increased agricultural cultivation, particularly in the Rolling Pampa, where agricultural expansion has left few remnant grasslands and we did not detect three formerly common grassland specialists, Hymenops perspicillatus, Embernagra platensis, and Pseudoleistes virescens. These findings emphasize the need for conservation actions to support populations of grassland bird in the Pampas.

Environmental factors associated with rodent infestations in Argentine poultry farms

Abstract 1. Seasonal variations in rodent infestations were studied at poultry farms in central Argentina as well as the environmental factors associated with rodent infestations at poultry farms. 2. During the summer and winter of 1998 rodent infestation was monitored at 10 poultry farms by means of tracking stations, and 21 environmental variables were selected that a priori could be related with the extent of rodent infestation. 3. There was a higher rodent infestation in winter than in summer. 4. Species detected by rodent sightings at the poultry farms included Akodon azarae, Calomys spp. (C. laucha and possibly C. musculinus) and Mus musculus. 5. In summer, rodent infestation was positively associated with the perimeter of the farm, whereas there was an inverse relationship with the maintenance conditions of the sheds. In winter, rodent infestation was positively associated with the percentage of the borders of the farm covered with vegetation, and with the amount of plant cover, especially the amount of plant cover which was above 20 cm in height. 6. There was no significant relationship between rodent infestation and the time lag since the last application of rodenticide (first-generation anticoagulant) at the farms. 7. These results suggest that strategies for controlling rodents based solely on chemical methods may not be good enough for efficient rodent control. Farms which are properly managed, with a better control of vegetation growth at both the perimeter and within the internal area, in addition to a higher maintenance of sheds have appreciably lower rodent infestations.

Prevalence of infection with hantavirus in rodent populations of central Argentina

We studied hantavirus seroprevalence and virus variability in rodent populations in Diego Gaynor, northwest of Buenos Aires province, Argentina. Rodent samplings were conducted in railroads and cropfield borders in March and July 1999, September and December 2000, and March 2001. Antibody detection was performed by an enzyme link immunosorbent assay (ELISA), using the recombinant nucleoprotein of Andes (AND) virus as antigen. Tissue samples were taken from positive antibody individuals in order to confirm the presence of hantavirus genomic material and to identify virus genotypes. Akodon azarae was the most abundant species, followed by Oligoryzomys flavescens, while Calomys laucha and C. musculinus were rarely caught. We found a rate of seroprevalence of 9.3% for a total sample of 291 A. azarae and 13.5% for 37 O. flavescens. After molecular analyses of hantavirus, we confirmed the presence of hantavirus genomic material in 16 individuals with ELISA (+) results and two individuals with ELISA (-). Four amplimers for each species were sequenced and compared to the corresponding sequences of representative hantaviruses. We identified the AND Cent Lec from three O. flavescens, and the Pergamino virus from four A. azarae and from one O. flavescens. A. azarae males had higher seroprevalence than females, and heavier individuals showed higher seroprevalence than lighter ones. We did not find seroprevalence differences according to sex in O. flavescens, although this result may have been produced by the low sample size. The lowest seroprevalence was found in a period of high rodent density, when juveniles prevailed in the population. We found higher seroprevalences than those detected in previous studies for other localities of central Argentina where cases of hantavirus pulmonary syndrome (HPS) have been reported. The presence of AND Cent Lec virus in rodent populations of the study area, which is responsible of HPS cases in central Argentina, suggests that human populations are at risk of HPS disease, although there were not reported cases of this disease until today.

Seasonal abundance and distribution among habitats of small rodents on poultry farms. A contribution for their control

Abstract Small rodent communities and their distribution among habitats (sheds, perimeters and interphases) were seasonally sampled over 2 years on 30 poultry farms. Interphases were present when sheds were close to perimeters (in 16 of 30 farms). The exotic Mus musculus and four sylvan species: Calomys laucha, Akodon azarae, Oligoryzomys flavescens and Calomys musculinus were captured. Mus musculus was more abundant in sheds and A. azarae in interphases and perimeters. The other species did not differ among habitats. Mus musculus abundance did not vary through the year, whereas the abundance of the other species was greater in autumn – winter. Sheds of farms with interphases were more probably infested with sylvan species than sheds of farms without interphases. Different kinds of management are needed to control both groups of species: not building sheds close to perimeters or keeping vegetation low in interphases for sylvan species, and periodical control in sheds and on chicken food for M. musculus.

Landbird Assemblages in Different Agricultural Landscapes: A Case Study in the Pampas of Central Argentina

Abstract. A combination of elements (e.g., crops and pasture lands, strips of roadside vegetation, woodlots, temporary wetlands) increases the heterogeneity of rural landscapes. Agricultural landscapes range from homogeneous, dominated by a single element (i.e., pure cropland or pure pastoral farming) to heterogeneous, dominated by two or more elements (i.e., mixed farming). In this study we characterized landbird assemblages in various agricultural landscapes (cropland, pastoral, and mixed landscapes) in the Pampas of central Argentina, surveying along transects to quantify species richness, composition, and abundance. Mixed landscapes were more heterogeneous than pastoral areas and cropland. Species richness, particularly of generalists, was greater in mixed landscapes. Richness of grassland specialists (species of increased conservation concern in the Pampas) was lower in croplands than in pastoral and mixed landscapes. An indicator-species analysis supported the association of some grassland specialists (Rhea americana, Asthenes hudsoni, Embernagra platensis, Pseudoleistes virescens) with pastoral landscapes. Our results suggest that maintaining landscape heterogeneity throughout the Pampas (i.e., mixed landscapes) is important for preserving species richness of birds. But several threatened grassland specialists inhabiting the Pampas, such as Rhea americana and Asthenes hudsoni, are found exclusively in pastoral landscapes, so the long-term survival of these species will depend on the conservation of this landscape type.

A contribution from Barn Owl pellets analysis to known micromammalian distributions in Buenos Aires province, Argentina

No abstract available

Impactos de la transformación agropecuaria sobre la biodiversidad en la provincia de Buenos Aires

Impacts of agricultural transformation on biodiversity in the province of Buenos Aires, Argentina. Agricultural practices have a series of profound transformations on natural systems. Similarly to what have happened in other regions, the introduction of agro-ecosystems in the Pampas of the province of Buenos Aires in Central Argentina has substantially modified its structure and functioning, with consequent changes for regional biodiversity. In this paper we summarize the main findings of the studies conducted by our research group regarding the impact of agricultural practices on biodiversity in Buenos Aires province during the last 2-3 decades, namely: 1) agriculturization has resulted in detectable changes in the abundance and distribu- tion of many vertebrate species, particularly in retractions of species of birds and mammals associated with grass- lands, highlighting the need to maintain pastoral landscapes with large areas of natural grasslands, as several endangered species are found exclusively in such landscapes, 2) in highly fragmented agricultural landscapes, maintenance of natural vegetation along roadsides, embankments and fences helps to retain a significant frac- tion of wildlife in agroecosystems, 3) the coexistence of Pampas deer (Ozotoceros bezoarticus) and cattle is possible under rotational grazing systems and / or under low stocking rate schemes and 4) the presence of trees is a limit- ing factor of the abundance of pest birds (pigeons and parrots) in the province of Buenos Aires, which raises the possibility of a proper management of these species depending on the density and type of trees to be planted.

Seasonal variations in small mammal-landscape associations in temperate agroecosystems: a study case in Buenos Aires province, central Argentina

Abstract We studied the associations between small mammal assemblages and patterns of land use and landscape structure, and their seasonal variations in temperate agroecosystems. We collected barn owl, Tyto alba, pellets from nest sites located in the Pampean region included in Buenos Aires province (36 sites in winter and 29 in summer). We used Google Earth and SAC-C imagery classification to describe land use and landscape structure around pellet collection sites. In winter, we found a significant relation between the relative abundances of small mammal species and landscape variables. Landscape accounted for 33.2% of variance in species data. Calomys spp. were relatively more abundant in sites with higher winter crops cover, while Akodon azarae and Oligoryzomys flavescens increased their relative abundances in sites with more grassland cover. The commensal rodents Mus musculus and Rattus spp. were relatively more abundant in sites with higher urban cover, or near households and grain storage facilities. We found no significant small mammal-landscape association in summer. Our results are in agreement with previous studies showing that land use patterns may influence small mammal assemblages. Moreover, our results also show that seasonality plays a major role modulating the intensity of these small mammal-landscape associations in temperate agroecosystems.

Characteristics of Dispersing Pampean Grassland Mice (Akodon azarae) in Agroecosystems of Central Argentina

We studied the characteristics of dispersing Akodon azarae in agroecosystems of central Argentina. The landscape of the study area consists of cropfields surrounded by weedy margins, which provide good quality habitat for A. azarae. We sampled six times between June 1991 and May 1993. We defined a central area (center) where all A. azarae were removed, while capture-mark-release trapping was conducted in the adjacent areas (laterals). Individuals were characterized as residents if they did not change their home range between trapping sessions, or colonizing dispersers if they changed their home range and colonized the centers. We compared dispersers and residents with respect to sex ratio, proportion of reproductively active individuals, body length and weight. In general, residents were larger than dispersers but they did not differ with respect to reproductive activity and sex ratio. Differences were greater for females in the reproductive period and for males in the nonreproductive period, suggesting that during reproductive period smaller females of lower competitive ability may disperse to obtain a territory and reproduce, while male dispersal in the nonreproductive period would be non adaptive because the smaller male dispersers have low chance of survival in an impoverished habitat with high rodent density.

Spatial segregation among pampas deer and exotic ungulates: a comparative analysis at site and landscape scales

The pampas deer (Ozotoceros bezoarticus) is native to South America and is endangered in Argentina. In Buenos Aires province, Argentina, the last population of pampas deer is harbored in the Bahía Samborombón Wildlife Refuge (BSWR), which includes protected areas and numerous cattle ranches. This population has been declining and changing progressively in its distribution since 1985, and negative interactions between pampas deer and introduced ungulates (livestock and feral pigs) have been proposed among the main causes of its decline. We examined the abundance, distribution, and interactions among pampas deer, cattle, and feral pigs at site and landscape scales in the BSWR. At the site scale, coexistence between pampas deer and cattle at the same paddock was only possible at moderate stocking rates (0.2–0.4 AU/ha), and no deer were recorded at paddocks when stocking rates were above 0.6 AU/ha. Pampas deer and feral pigs co-occurred more frequently, but deer switched their behavior and increased levels of surveillance in response to proximity of feral pigs. At the landscape scale, a clear spatial segregation and differential distribution was detected among pampas deer, cattle, and feral pigs along the BSWR. Our results suggest that the current distribution of pampas deer at the study area is a consequence of avoiding contact with both livestock and feral pigs at the site scale. Thus, negative interactions recorded between pampas deer and introduced ungulates at the site scale were expressed at the landscape scale. Both in situ and ex situ conservation efforts are urgently needed to conserve pampas deer.