Background: Accurate representation of developer expertise has always been an important research problem. While a number of studies proposed novel methods of representing expertise within individual projects, these methods are difficult to apply at an ecosystem level. However, with the focus of software development shifting from monolithic to modular, a method of representing developers’ expertise in the context of the entire OSS development becomes necessary when, for example, a project tries to find new maintainers and look for developers with relevant skills . Aim: We aim to address this knowledge gap by proposing and constructing the Skill Space where each API, developer, and project is represented and postulate how the topology of this space should reflect what developers know (and projects need). Method: we use the World of Code infrastructure to extract the complete set of APIs in the files changed by open source developers and, based on that data, employ Doc2Vec embeddings for vector representations of APIs, developers, and projects. We then evaluate if these embeddings reflect the postulated topology of the Skill Space by predicting what new APIs/projects developers use/join, and whether or not their pull requests get accepted. We also check how the developers’ representation in the Skill Space align with their self-reported API expertise. Result: Our results suggest that the proposed embeddings in the Skill Space appear to satisfy the postulated topology and we hope that such representations may aid in the construction of signals that increase trust (and efficiency) of open source ecosystems at large and may aid investigations of other phenomena related to developer proficiency and learning.