Ostrom’s social-ecological systems (SES) framework infrequently has been applied to civil society research. But its focus on collective action may help explain why some national parks are more successful at attracting philanthropic resources to supplement stagnant public funding. We examine two types of charitable supporting organizations: "Friends of" Groups (FOGs), which typically emphasize fundraising, and Cooperating Associations (CAs), which typically emphasize visitor support. We identify their partnership patterns across more than 300 national park units. Our findings suggest that FOGs and CAs fill different niches. CAs are drawn to more popular parks or memorials, and FOGs are found in parks with smaller budgets or offering fewer activities. Actor characteristics play a secondary role in explaining nonprofit incidence. The holistic approach of the SES perspective demonstrates the importance of connecting resource systems to institutional settings and actor attributes.