We investigate the dynamic effect that the Tandroy's unsustainable practices have on the forest. The Tandroy people lives in Androy, a region located in the southern part of Madagascar. They are mainly an agricultural and cattle hearding society whose subsistence relies on the slash-and-burn farming agriculture (\textit{hatsake)} and the burning of cactus which are given as fodder to the livestock (\textit{ororaketa})\textit{.} These activities generate ecological pressure\ on the surrounding dry forest and socioeconomic risks related to the lack of sustainability of these practices in the long run. In this paper we address the notion of sustainability and confront it with the Tandroy's current productive and economic system. By means of viability theory, we characterize the actions and scenarios that are compatible with a sustainable use of the forest in the region.