Ricardo Baldi

Ecological correlates of group-size variation in a resource-defense ungulate, the sedentary guanaco.

For large herbivores, predation-risk, habitat structure and population density are often reported as major determinants of group size variation within and between species. However, whether the underlying causes of these relationships imply an ecological adaptation or are the result of a purely mechanistic process in which fusion and fragmentation events only depend on the rate of group meeting, is still under debate. The aim of this study was to model guanaco family and bachelor group sizes in contrasting ecological settings in order to test hypotheses regarding the adaptive significance of group-size variation. We surveyed guanaco group sizes within three wildlife reserves located in eastern Patagonia where guanacos occupy a mosaic of grasslands and shrublands. Two of these reserves have been free from predators for decades while in the third, pumas often prey on guanacos. All locations have experienced important changes in guanaco abundance throughout the study offering the opportunity to test for density effects. We found that bachelor group size increased with increasing density, as expected by the mechanistic approach, but was independent of habitat structure or predation risk. In contrast, the smaller and territorial family groups were larger in the predator-exposed than in the predator-free locations, and were larger in open grasslands than in shrublands. However, the influence of population density on these social units was very weak. Therefore, family group data supported the adaptive significance of group-size variation but did not support the mechanistic idea. Yet, the magnitude of the effects was small and between-population variation in family group size after controlling for habitat and predation was negligible, suggesting that plasticity of these social units is considerably low. Our results showed that different social units might respond differentially to local ecological conditions, supporting two contrasting hypotheses in a single species, and highlight the importance of taking into account the proximate interests and constraints to which group members may be exposed to when deriving predictions about group-size variation.

Ecological drivers of guanaco recruitment: variable carrying capacity and density dependence.

Ungulates living in predator-free reserves offer the opportunity to study the influence of food limitation on population dynamics without the potentially confounding effects of top-down regulation or livestock competition. We assessed the influence of relative forage availability and population density on guanaco recruitment in two predator-free reserves in eastern Patagonia, with contrasting scenarios of population density. We also explored the relative contribution of the observed recruitment to population growth using a deterministic linear model to test the assumption that the studied populations were closed units. The observed densities increased twice as fast as our theoretical populations, indicating that marked immigration has taken place during the recovery phase experienced by both populations, thus we rejected the closed-population assumption. Regarding the factors driving variation in recruitment, in the low- to medium-density setting, we found a positive linear relationship between recruitment and surrogates of annual primary production, whereas no density dependence was detected. In contrast, in the high-density scenario, both annual primary production and population density showed marked effects, indicating a positive relationship between recruitment and per capita food availability above a food-limitation threshold. Our results support the idea that environmental carrying capacity fluctuates in response to climatic variation, and that these fluctuations have relevant consequences for herbivore dynamics, such as amplifying density dependence in drier years. We conclude that including the coupling between environmental variability in resources and density dependence is crucial to model ungulate population dynamics; to overlook temporal changes in carrying capacity may even mask density dependence as well as other important processes.

Abundance Estimates of the Lesser Rhea Rhea pennata pennata in the Argentine Patagonia: Conservation Implications

The southern subspecies of the lesser rhea is distributed through the Argentine Patagonia and southern Chile. Habitat-loss and poaching were identified as the main threats to lesser rhea populations by the IUCN, which classified the species as ‘Least concern’ in 2014. Although Rhea pennata pennata is among the most conspicuous wildlife across the large Patagonian rangelands, the available estimates of abundance are scarce, mostly restricted to reports between 12 and 30 years old, and resulted from different surveying methods. Our purpose in this work was to obtain estimates of R. p. pennata abundance across different sites in the Argentine Patagonia, and to explore the main factors affecting the spatial variation in abundance. We surveyed over 4,000 km of line transects across six sites and obtained abundance estimates using the Distance sampling analysis. Also, we fitted restricted mixed models to evaluate the effects of different factors on the variation in R. p. pennata encounter rates among sites. We found that the abundance of R. p. pennata was very low either in the encounter rate (taking until 113 km of survey to detect a group of rheas depending on the site) or population density (between 0.0063 and 0.28 individuals.km-2). The occurrence of sheep ranching affected negatively the variation in the abundance of R. p. pennata among sites, whereas the availability of grassland habitat increased the chance of finding groups during the surveys. Line-transect surveys following the Distance sampling procedures are adequate to estimate encounter rates, although the low number of observations requires repeated surveys per site to obtain reliable estimates of animal density. Extended and sustained survey efforts and the implementation of a monitoring program are crucial to assess population trends of lesser rheas. Law enforcement to control poaching, increase in public awareness and an action plan to reduce threats are all necessary steps to conserve this emblematic species in Patagonia.

Location of breeding warrens as indicators of habitat use by maras (Dolichotis patagonum) in Península Valdés, Argentina

Abstract We characterized the habitat use by maras (Dolichotis patagonum) on a microhabitat scale in the area surrounding the warren, assessing the conditioning effect of the warren over space use and exploitation of other resources. We evaluated the relationships between the probability and intensity of use, habitat configuration and distance to the warren, counting feces along transects departing from each warren. Our results showed that the location of breeding warrens was positively associated with the habitat use by maras on a microhabitat scale. The core area of the annual activity of maras was concentrated around the warren and there was no evidence of alternative areas of activity. According to the fitted models, maras used microhabitats with a high proportion of bare soil and close to infrastructure elements. The spatial autocorrelation components indicated that intensively used patches are small and disperse. The patterns of habitat use observed in this study suggest that maras use multipurpose areas including the breeding site and resources needed throughout the year. These patterns suggest that warrens are good all year-round indicators of mara habitat use and spatial ecology.

Analysis of the spatial variation in the abundance of lesser rheas using density surface models

Abstract Context. The study of the spatial variation in abundance of wild populations and the identification of factors explaining the observed patterns are key both to understand aspects of basic ecology and the effects of human activities. This is usually difficult to evaluate for low-density and widely distributed species, such as the lesser rhea (Rhea pennata pennata), an endemic bird from South America. Recent advances in spatial modelling such as the density surface models (DSM) combine distance-sampling procedures with modelling techniques to produce maps of spatial variation in abundance, and its relationship with predictive variables. Aims. We aimed to analyse the spatial distribution and abundance of lesser rhea, and the variables that affect its abundance in Península Valdés (PV) Argentine Patagonia. Methods. We conducted 338.4u2009km of ground surveys of lesser rheas in PV during the end of the Austral summer of 2015. Spatial models were constructed using DSM. Ecological and human-related variables were included in the models to account for variation in the abundance of animals at 4-km2 spatial resolution. Key results. We estimated an overall density of 0.44 birds km–2 (CVu2009=u200932%) for the prediction area of 3320u2009km2. High values of normalised difference vegetation index, a correlate of plant productivity, were associated with increased numbers of lesser rheas. The location of ranch buildings, indicators of human presence, had a strong negative effect on lesser rheas, although their abundance increased at high sheep stocking rates. Conclusions. As reported by previous studies in different sites, the abundance of lesser rheas in our study area was low. The use of DSM allowed a detailed examination of the spatial variation, as well as the variables involved and the uncertainty of the prediction. Implications. The use of DSM techniques can be a useful tool for conservation planning and monitoring. Spatial, high-resolution data combined with knowledge on the factors affecting the number of animals are crucial to target specific conservation actions and monitor their results, and should allow government agencies to make better decisions concerning conservation-oriented management.