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Isoleg theory predicts that coexistence between two competing species can occur when the subordinate, generalist species uses alternative patches of resources as the density of the dominant specialist species increases. These models provide a framework to determine if a native species is subordinate to an introduced one, thus requires conservation considerations. We investigated habitat interactions between a wild neotropical camelid and domestic exotic Ungulates in the Andean Puna semi-desert of South America. The Puna or Altiplano is an arid environment located in high altitudes, with cold and dry weather, and two main types of habitats, steppes and swamp areas. There was a high overlap of diets between species. Vicunas were generalists in the use of habitat, but invested foraging effort in swamp habitats, while livestock were taken to swamp habitats by local people. Vicunas and livestock were spatially segregated. Vicunas used the preferred habitat less than expected from their foraging preference. We concluded that vicunas and livestock coexist because vicunas occupy sub-optimal habitats whilst livestock concentrate in the richest ones. Vicunas can deal with poor habitats because they have several adaptations to live in deserts. (C) 2008 Elsevier Ltd. All rights reserved.