Ricardo A. Ojeda

Discovery of tetraploidy in a mammal

Polyploidy, or having more than a pair of each type of chromosome, is considered to be unlikely in mammals because it would disrupt the mechanism of dosage compensation that normally inactivates one X chromosome in females. Also, any imbalance in chromosome number should affect the normal developmental processes and therefore constitute an evolutionary end, as in triploid humans.

DESERTIFICATION IN CENTRAL ARGENTINA: CHANGES IN ECOSYSTEM CARBON AND NITROGEN FROM IMAGING SPECTROSCOPY

Many arid and semiarid (dryland) regions are subject to desertification from intensive land-use pressures such as cattle ranching. However, the lack of quantitative approaches required to assess desertification has slowed our understanding of how vege- tation and soils are changing in dryland regions. Using airborne high-fidelity imaging spectroscopy and field measurements, we developed the first regional assessment of veg- etation structural and soil biogeochemical properties in the Monte Desert biome, Argentina. We evaluated the long-term impacts of grazing on vegetation cover and soil carbon (C) and nitrogen (N) storage, which are core indicators of biogeochemical status and change in drylands. A comparative analysis was carried out on vegetation and soil properties in four major Monte Desert plant communities, as well as within the U.N. Nacunan Man-and- Biosphere Reserve and adjacent areas subjected to long-term grazing. The four dominant plant communities differed substantially in vegetation cover, leaf area index, foliar N concentration, and soil organic C and N stocks. Imaging spectroscopy with Monte Carlo spectral mixture analysis provided accurate estimates of fractional pho- tosynthetic vegetation (PV), nonphotosynthetic vegetation (NPV), and bare soil cover at ,5-m spatial resolution throughout a 763-km 2 region. Hotspots of grazing management (e.g., ranch centers, water sources) have undergone some woody vegetation encroachment, but the spatial patterns were localized and variable. More clearly, a widespread NPV de- crease and bare soil increase was common outside of the reserve. Soil organic C and N stocks were highly correlated with PV 1 NPV cover fractions. Soil organic C and N storage was 25-80% lower in areas subjected to long-term grazing, as compared to protected ecosystems within the Nacunan Reserve. Long-term grazing has depleted stores of C and N in soils and dramatically altered vegetation structure in the Monte Desert. A persistent lack of NPV due to grazing has likely impaired C and N cycles considered central to the biogeochemical functioning of the region. This study demonstrates a novel approach to remotely measure both surface and soil properties most indicative of progressive land degradation and desertification in dryland regions.

The exotic mammals of Argentina

Exotic mammals in South America represent about 20% of world mammal introductions. The aim of our paper is to provide a global assessment of the exotic mammals of Argentina, their pathways, impacts, and a synthesis of their attributes as potential invasive species. We reviewed and compiled data from a diversity of sources and databases on alien mammals occurring in feral state exclusively. We recorded 18 species of exotic mammals for Argentina. The majority of introductions occurred between the 18th and 19th centuries and their ports of entry were located in temperate ecosystems, between 34° and 55° SL. Most of their entry pathways were associated with human activities (e.g. sport hunting, food and fur industry). The exotic mammals occupy ecoregions similar to their original distributions, but most of them have experienced a range expansion to novel habitats. The fauna of exotic mammals of Argentina represents a good opportunity to understand the dynamics of the invasion process as they represent a diversity of ecological groups and environmental contexts.

Assessing mammal responses to perturbations in temperate aridlands of Argentina

Abstract Loss of species richness and reduced abundance have been the main responses to perturbations of small- and medium-sized mammals in arid and semi-arid areas. Nonetheless, some mammal species whose habitat requirements are met in disturbed patches may benefit from structural changes in the environment brought about by perturbations. In desert areas of Argentina different mammal responses are attributable to both a lower complexity of disturbed patches and a decrease in sheltered places. Therefore, species associated with high plant cover either decrease in number or become locally extinct at sites affected by fire or grazing, whereas species using open areas are favored by these events. The negative effects historically assigned to different disturbances may be mitigated by adopting a hierarchical approach that considers patch diversity in different successional stages as well as the multiple opportunities of patch occupation by mammal species.

Faunal Commercialization and Conservation in South America

Data on legal exportation of wildlife from Argentina show that enormous numbers of animals are taken each year in the wildlife trade; 5.4 million vertebrates representing 25 species are exported annually. Greatest export value (75%) is obtained from introduced species. The wildlife trade is a multimillion-dollar market in Argentina, and very likely one of the principal industries on the continent. Commercial exploitation may be significant in depleting the populations of South American wildlife species. (Accepted for publication 4 April 1984) dedicated professionals who merit full support by Argentinas government in their attempts to protect a threatened segment of their countrys heritage.

How Desert Rodents Overcome Halophytic Plant Defenses

onvergent evolution among desert rodents has received a great deal of attention in recent years, with research centering on the question of how phylogenetically unrelated species evolving on different continents have developed similar morphological, behavioral, ecological, or physiological characteristics in response to similar selective pressures posed by the desert environment (Mares 1975, 1993a, 1993b, Schluter and Ricklefs 1993). The heat and aridity of deserts pose severe challenges for both plants and animals. Deserts generally support reduced levels of plant biomass compared with more mesic habitats (Hadley and Szarek 1981, Rosenzweig

Water conservation in the South American desert mouse opossum, Thylamys pusilla (Didelphimorphia, Didelphidae).

The aim of this paper was to analyze the renal performance of the insectivorous desert mouse opossum, Thylamys pusilla, a marsupial that inhabits temperate drylands of the Neotropics. We measured the urine concentrating ability of this marsupial in the field and under three different experimental conditions in the laboratory. (1) Ad libitum treatment: animals were fed ad lib. on apples, sausages and cat food; (2) restriction treatment: foods high in nitrogen were supplied ad lib., while only apple, a food with high water to nitrogen load, was restricted, and (3) fasting: 2 days of food deprivation. Osmolarity values recorded in the field were similar to those obtained under the restriction treatment. The maximum urinary concentration found (9015 mOsm/l) is the highest reported for a marsupial and more in keeping with similarly sized desert rodents. Similar urine concentrations in T. pusilla and desert rodents suggest that xeric environments, rather than phylogeny, play a major role in water conservation. The kidney of T. pusilla possesses a long papilla and shows high renal indices (M/C: 9.37; RMT: 11.76). Our results question the idea that urine concentration is not high in desert-dwelling insectivores. Although their dietary water intake is potentially high, this may not always be sufficient to eliminate the nitrogenous waste resulting from insectivory. Thus, T. pusilla uses its ability to conserve water to overcome periods of food shortage and high nitrogen load.

Mammals of the Monte Desert: from regional to local assemblages

Abstract Much of South America consists of diverse arid and semiarid regions characterized by high mammal endemism as a result of a complex interplay between place and lineage histories. In this review we summarize and highlight several biogeographical and ecological features of the small mammals of South America drylands, with special focus on the Monte Desert biome. We provide information on population characteristics, community structure, food and habitat use, and responses to disturbances. Major findings at different scales include the distinctiveness and high species turnover across South American drylands and Monte Desert ecoregions; synchronous population fluctuations with high variability between years; herbivory and omnivory as dominant trophic strategies; community structure organized through habitat and food segregation; and the importance of a landscape mosaic of grazed and ungrazed areas for maintenance of small and medium-sized mammal diversity.

Diversity of mammals in the tropical–temperate Neotropics: hotspots on a regional scale

The tropical–temperate interface of the southern Neotropics harbours an interdigitating array of biomes (Puna, Monte, Chaco, Yungas). This topographic and climatically complex region needs urgent conservation efforts, as it is being transformed by human activities at an accelerating pace. We analyse georeferenced field records of mammal species in northwestern Argentina (provinces: Catamarca, Jujuy, Salta, Tucuman) in order to define biodiversity hotspots on the basis of 0.5°× 0.5° grid cells within northwestern Argentina according to total richness of mammal species, richness of megaspecies (species above 10 kg), and endemic species (species restricted to Argentina or neighbouring countries with shared biomes). The mammal fauna of northwestern Argentina is fairly well known (176 species). The biomes differ considerably in species richness (Puna low, Yungas high) and species composition. We found no significant difference between endemic and non-endemic species regarding cell occupancy or body size. Cell occupancy was not correlated to body size. Across grids, species richness, number of megaspecies as well as richness of endemics are all correlated to sampling effort. More than 50% of the species in the region are restricted to one or two biomes. Overall, the species turn-over between biomes in northwestern Argentina is high. Using a simple algorithm we identified 10 grid cells which covered 90% of the total number of recorded species, and contrast them with the protected areas. While the Puna and Yungas biomes are rather well protected, the arid and semiarid Monte and Chaco are in need of urgent attention in biodiversity conservation.

Elevation patterns in rodent diversity in the dry Andes: disentangling the role of environmental factors

Abstract Biodiversity patterns are the result of the interaction of numerous contemporary factors and historical opportunities for allopatric speciation. Several hypotheses regarding climatic features and topographic characteristics have been reported as determinants of species richness along elevation gradients. However, how these factors interact to shape small mammal species richness along the dry Andes ecosystem is not well understood. The objectives of this paper were to analyze patterns of species diversity along the central dry Andes, and to evaluate how climatic and topographic factors explain diversity patterns. Our results showed a positive and monotonic relationship between small mammal species richness and altitude, whereas abundance patterns showed a midelevation peak. Climate and topography were the most important predictor variables explaining small mammal species richness and abundance patterns in the Andes. This study underlines the role of the Andes in promoting and sustaining biodiversity, as well as the need to encourage conservation planning in mountain ecosystems. Resumen Los patrones de biodiversidad son el resultado de la interacción entre numerosos factores contemporáneos y eventos históricos que propician la especiación alopátrica. Numerosos factores climáticos y topográficos han sido mencionados como determinantes para los patrones altitudinales de riqueza de especies. Sin embargo es poco conocido aún como estos factores interactúan para modelar la riqueza de especies a lo largo de la Cordillera de los Andes. Los objetivos de este trabajo fueron analizar los patrones de diversidad de especies a lo largo de los Andes templados, y evaluar como los factores climáticos y topográficos explican los patrones de riqueza y abundancia de especies. Nuestros resultados muestran una relación monotónica y positiva entre riqueza de especies y altura. Mientras el patrón de abundancia es unimodal. Para ambos patrones el clima y la topografía son las variables predictoras más importantes. Este estudio resalta el rol de los Andes en promover y sostener la biodiversidad, y acentúa la necesidad de fomentar planes de conservación en ecosistemas montanos.