Many wild species are affected by human activities occurring at broad spatial scales. For instance, in South America, habitat loss threatens Greater Rhea (Rhea americana) populations, making it important to model and map their habitat to better target conservation efforts. Spatially explicit habitat modeling is a powerful approach to understand and predict species occurrence and abundance. One problem with this approach is that commonly used land cover classifications do not capture the variability within a given land cover class that might constitute important habitat attribute information. Texture measures derived from remote sensing images quantify the variability in habitat features among and within habitat types; hence they are potentially a powerful tool to assess species-habitat relationships. Our goal was to explore the utility of texture measures for habitat modeling and to develop a habitat suitability map for Greater Rheas at the home range level in grasslands of Argentina. Greater Rhea group size obtained from aerial surveys was regressed against distance to roads, houses, and water, and land cover class abundance (dicotyledons, crops, grassland, forest, and bare soil), normalized difference vegetation index (NDVI), and selected first- and second-order texture measures derived from Landsat Thematic Mapper (TM) imagery. Among univariate models, Rhea group size was most strongly positively correlated with texture variables derived from near infrared reflectance measurement (TM band 4). The best multiple regression models explained 78% of the variability in Greater Rhea group size. Our results suggest that texture variables captured habitat heterogeneity that the conventional land cover classification did not detect. We used Greater Rhea group size as an indicator of habitat suitability; we categorized model output into different habitat quality classes. Only 16% of the study area represented high-quality habitat for Greater Rheas (group size > or =15). Our results stress the potential of image texture to capture within-habitat variability in habitat assessments, and the necessity to preserve the remaining natural habitat for Greater Rheas.

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This work reports the first results of a 3-year study (1998 -2001) on habitat use and preferences by wild lesser rheas (Rhea pennata pennata) in the ecotone Monte-Patagonian steppe of Argentina. Ponds and four different habitat alternatives for lesser rheas were determined using satellite images and different structural vegetation characteristics: steppe, shrubland, shrub steppe, and mallines. Lesser rheas (adults, juveniles and chicks) used all habitats available to feed, although they showed preference for mallines, open areas that offer good visibility and abundant food resource. Rheas used shrub steppe and steppe for nesting, but they did not show preference for either habitat type. Nest site had a higher percentage of vegetation cover than control sites. Lesser rheas apparently preferred concealed sites for nesting since they offer protection from severe climate conditions and from predators. Our results suggest that habitat preference by lesser rheas counterbalances profitability of feeding to the corresponding cost of predation.

In Argentina this species is typically associated with the Pampas grasslands, the most human-modified habitat in the country (Bertonatti & Corcuera, 2000). As a result, greater rheas occur in agroecosystems in which grassy plains and open brush areas have been replaced with crops and cultivated pastures (Martella et al., 1996, Pereira et al., 2003). In addition to this habitat modification, egg gathering, floods and hunting have also affected greater rhea populations, and the species is locally extinct in many areas (Bucher & Nores, 1988). The greater rhea is categorized as Lower Risk: near-threatened on the IUCN Red List (2003). Because alfalfa Medicago sativa and wild dicotyledons are the most important items in the diet of wild greater rheas in agricultural areas of central Argentina, with alfalfa consumed particularly during winter when it is not available to domestic livestock, agricultural landscapes devoted to traditional cattle production would probably be able to support wild populations of greater rheas (Martella et al., 1996). It is therefore necessary to develop detailed studies on habitat use of this species. To maintain viable greater rhea populations it is also important to evaluate the habitat use of captive-bred greater rheas released into the wild. Our work had the following specific objectives: (1) to study habitat use and preferences of captive-born greater rheas, (2) to study habitat use and preferences of wild greater rheas to determine if captivity and radio-tagging affect habitat use, and (3) to study nest-site selection.

In this work we report results of radiotracking studies on the movements and home range sizes of two near-threatened species, the greater rhea (Rhea americana) and the lesser rhea (Pterocnemia pennata pennata) in relation to different land use regimes. We radiomonitored greater and lesser rheas for 3 years in their respective habitats: the Pampas and the Patagonia regions. We chose two study areas in each habitat with similar agricultural activities and different hunting control. We did not find significant differences in movements and home range size between study areas of each species. This suggests that disturbance caused by human presence in the areas did not affect rhea spacing behaviors. Moreover, lesser rheas showed larger home range and movements than greater rheas, showing that the home range size is not an immutable property of body mass, and that abundance and distribution of food appears to be the main factor that influences the movements and home range size of these birds.

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