At present, it is believed that the population of guanacos has increased in the "Isla Grande de Tierra del Fuego", arising a conflict with livestock and forestry activities. The aim of this study was to evaluate the use of the guanaco's food resources, taking into account the presence of different types of domestic livestock, in order to provide a tool which will enable the evaluation of environmental management projects. We conducted the study in different areas, considering the presence/absence of domestic livestock and of guanaco. The diet was analyzed by identifying botanical remains present in the pretreated feces. The relative frequencies of ingested taxa were obtained and analyzed according to their functional groups, being the soft, herbaceous and graminoid grasses the most consumed, where the soft grasses were the most frequently ingested. Tree species only appear in diets of guanacos, in a low frequency, compared to other forms of life. In the case of soft grasses, forbs and Graminoides, the diets differ according to ingested species and intake frequency. These results allow us to establish that the guanaco selects the items to be consumed, changing its diet based on the presence of other herbivores, and has a trophic overlap with domestic livestock, mainly with sheep.

The botanical composition of the diet of sheep in two contrasting environments of the fueguian steppe was compared by means of a microscopic analysis of feces, during the spring and summer 2006. In both areas of study, grasses showed to be the most important group in the diet of both locations and of both seasons, followed by native dicotyledonous. In addition, it was found that in the abandoned pasture of Cullen ranch, the intake of Hieracium pilosella, an invasive species, is high. Management strategies should be targeted to the conservation of grasses which are the life forms that are most susceptible to over grazing disturbance. Besides, these results contribute to the stage of advancement of the invasive species H. pilosella in the fueguian steppe and to the knowledge of the incorporation of this species in the diet of livestock, and possibly in the diets of native herbivores.