This study focuses on the transitional steps leading from a hunter-gatherer economy to reindeer pastoralism among the Sami of northern Sweden. Changes in land use and settlement patterns are interpreted from an ecological perspective taking into account the ecosystem dynamics of the alpine zone. During the initial phase of pastoralism, land-use strategies included the requisitioning of previously unused ecological niches. Exploitation of forest resources near the tree line catalyzed an ecosystem degradation process that eventually necessitated a shift in land use towards more flexible and sustainable strategies. During the period AD 800–1500 a combination of economic, social, and ecological processes resulted in a splitting up of village communities into small mobile units and more sustainable forms of land use.