The institutions of global governance have changed dramatically in recent years. New organizational forms—including informal institutions, transgovernmental networks, and private transnational regulatory organizations (PTROs)—have expanded rapidly, while the growth of formal intergovernmental organizations has slowed. Organizational ecology provides an insightful framework for understanding these changing patterns of growth. Organizational ecology is primarily a structural theory, emphasizing the influence of institutional environments, especially their organizational density and resource availability, on organizational behavior and viability. To demonstrate the explanatory value of organizational ecology, we analyze the proliferation of PTROs compared with the relative stasis of intergovernmental organizations (IGOs). Continued growth of IGOs is constrained by crowding in their dense institutional environment, but PTROs benefit from organizational flexibility and low entry costs, which allow them to enter “niches” with limited resource competition. We probe the plausibility of our analysis by examining contemporary climate governance.