In policy domains characterised by complexity, international organizations (IOs) with overlapping mandates and governance functions regularly interact in ways that have important implications for global governance. Yet the dynamics of IO interactions remain understudied. This article breaks new ground by building on the theoretical insights of organizational ecology to examine IO competition, cooperation, and adaptation in the domain of energy. Drawing on original empirical data, I consider three related hypotheses: (1) competition between IOs in the same population is likely to centre on material resources; (2) IOs are more likely to cooperate when they have a shared governance goal; and (3) individual IOs can adapt by changing their goals and boundaries. In considering these hypotheses, this article highlights the limits of the organizational ecology approach and the need to broaden it to account for the possibility that IOs do cooperate, and that individual IOs, such as the International Energy Agency, have the capacity to adapt to changes in their environment.