Habitat loss is one of the predominant problems faced by species. In this thesis I focused on woody vegetation encroachment in the savannahs of Bolivia's Gran Chaco. I explored the effects of this vegetation succession on the habitat of the Chacoan guanaco Lama guanicoe to illustrate the case of a large, grassland dependant mammal in a changing environment. Using vegetation maps I described the advance of woody encroachment on Chacoan savannahs and identified the dominant shrub Pithecellobium chacoense thus colonising savannah expanses. The regeneration and growth of P. chacoense is not inhibited by fire. My results showed that the current distribution of guanacos across the mosaic of vegetation is restricted to the most open areas, which confirms that woody encroachment is a factor promoting loss of habitat for Chacoan guanacos, and thus constitutes a direct threat to the species. The prevalence of woody vegetation translates into increased cover, which is likely to increase the risk from predation by jaguar and puma, both of which are present in the study area. Results from direct observations showed that the main factor influencing guanaco vigilance was habitat cover, whether guanacos were alone or in groups, suggesting that predator avoidance is a key factor shaping guanacos' preference for open areas. Given the presence of both guanacos and cattle in the increasingly scarce open grasslands of my study area I hypothesized that these herbivores would be competitors for food. However, micro-histological analysis showed that guanaco and cattle in fact differ broadly in their diets. As a range-restricted, large mammal living in relict population, the guanaco is an excellent ambassador for prioritising conservation actions in the Gran Chaco region, namely mitigating savannah loss by means of participatory range management initiatives.