Exploitation and degradation of urban green areas reduce their capacity to sustain ecosystem services. In protecting and managing these areas, research has increasingly focused on actors in civil society. Here, we analyzed an urban movement of 62 civil-society organizations-from user groups, such as boating clubs and allotment gardens, to culture and nature conservation groups-that have protected the Stockholm National Urban Park. We particularly focused on the social network structure of the movement, i.e., the patterns of interaction between movement organizations. The results reveal a core-periphery structure where core and semi-core organizations have deliberately built political connections to authorities, whereas the periphery gathers all user groups involved in day-to-day activities in the park. We show how the core-periphery structure has facilitated collective action to protect the park, but we also suggest that the same social network structure might simultaneously have constrained collaborative ecosystem management. In particular, user groups with valuable local ecological knowledge have not been included in collaborative arenas. Our case points out the inherent double-nature of all social networks as they facilitate some collective actions, yet constrain others. The paper argues for incorporating social network structure in theories and applications of adaptive governance and co-management.