Research on common-pool resources in the last 30 years has hinged on concepts such as rules and property rights for understanding how access and use of natural resources is managed by communities and other actors. However, a small body of literature on mobile pastoralism maintains that resource governance might not always be based on resource-related rules, but instead on negotiation and general norms of reciprocity. Situations conventionally labelled as ‘open access’ might therefore not always be as unregulated and unmanaged as they seem. Here, we examine what the absence of rules for resource access and use means in practice, and how resource users adapt such a governance system to increasing scarcity of pasture land. We conducted interviews, group discussions and participatory mapping exercises in two neighbouring areas, Hamar and Bashada, in the lower Omo area in southern Ethiopia. Both groups are culturally closely related to each other, but showed important differences in their ability and willingness to change their institutions to adapt to resource scarcity. In both Hamar and Bashada, access to grazing was generally non-exclusive. Instead, we found a complex mosaic of ways in which access to grazing was practiced and sanctioned, characterised largely by negotiations and interplay between individual actors rather than by firm rules. Both groups were confronted with increasingly erratic rainfalls and insufficient availability of pasture. Strikingly, while the Bashada had recently established a strictly enforced set-aside area to provide grazing for the end of the dry season, the Hamar rejected such ideas and sought grazing in protected areas, which eventually led to conflict between herders and authorities. Reasons for these diverging strategies might be connected to subtle differences in the degree to which decision-making is individualised and social coordination accepted. These seem to have important implications for community adaptability to changing environmental and societal conditions.