The high complexity of ecological systems provides a key challenge for understanding ecosystem structure and functioning (including their biodiversity), as well as for understanding how ecosystems respond to environmental change. Given current massive changes to the global environment and the expected even stronger future changes (notably in climate) improved conceptual knowledge and predictive power are crucial for society in order to predict impacts on ecosystem services and for developing effective adaptation and mitigation strategies to minimize negative effects (e.g., loss of populations and species, expansion of pathogens and pests, changes in carbon sequestration or other ecosystem functions). CIRCE is in a unique position to achieve progress on this important topic. Its researchers provide leading, synergistic expertise on ecological complexity, from population biology over network biology, food web and community ecology to macro- and ecosystem ecology. With their proven dedication to interdisciplinary collaboration, they constitute an exceptional constellation with strong potential for achieving major synthetic understanding of ecological complexity. CIRCE will therefore also be in an exceptional position to provide improved, much needed integration of complexity into predictive models of biodiversity and ecological impacts of global environmental change. Given the topic’s importance, CIRCE therefore has high potential to become internationally leading in ecological research, on the 2012-2016 time scale as well as in the longer term.