The goal of this chapter is to document the evolution of a portfolio of open source communities. These are communities formed around a set of related projects with common governance, which often produce artifacts shared among all projects. It helps to think of a portfolio of project communities as an ecology, in which the projects are mutually dependent, and there is both cross-project collaboration and competition for resources among the communities. As a case study, we explore the ecology of communities within the Apache project, one of the largest and most visible open source projects. We infer the community structure from developer mailing lists, and study how the communities evolve and interact over time. The analysis lends support to the often-stated hypothesis that open source communities grow by a process of preferential attachment. However, we show that the influx of external developers is not the only factor affecting community growth. The structure and dynamics of a community is also impacted by inter-project information flow, and the migration of developers between projects (including the formation of spin-offs).