Firms are increasingly relying on third-party developers to innovate by building open digital ecosystems. We draw on a network biology approach to explore the structural pattern of how individual modules in a digital ecosystem interact with one another to produce an ever-evolving ecosystem landscape. We perform an empirical study of WordPress.org, the world's largest blog service platform. Using text mining and network analyses, we extract API (Application Programming Interfaces) used in plug-ins developed by third party developers, characterizing each plug-in as a combination of APIs. We further construct a topological overlap plot of the WordPress ecosystem comprising of APIs that are used with each plug-in allowing us to examine the underlying structure of generativity in the WordPress ecosystem through a co-expression network of APIs. Our results show that APIs form a complex and dynamic ecosystem that follow a discernible pattern distinctive from known distributions, even though there is no central designer who coordinates the design of all plug-ins and overall growth of the entire WordPress ecosystem. Our analysis shows that while APIs created by the focal firm play a central role in shaping the architecture of generativity of its ecosystem, some of the external APIs developed by other firms also play an important role in shaping the ecosystem, showing the distributed nature of the architecture of the generativity.