We developed a new method, known as reference coupling, for detecting technical dependencies between projects. The method establishes dependencies through user-specified cross-references between projects. We used our method to identify ecosystems in GitHub-hosted projects, and identified several characteristics of the identified ecosystems. We find that most ecosystems are centered around one project and are interconnected with other ecosystems. The predominant type of ecosystems are those that develop tools to support software development. Studying the properties of the ecosystems we identified, we also found that the project owners’ social behaviour aligns well with the technical dependencies within the ecosystem, but project contributors’ social behaviour does not align with these dependencies.