Metamodels, defining the determinant concepts of a domain, constitute the core components in Model-Driven Engineering. Together with their depending artifacts, e.g., models and transformations, they form modeling ecosystems. To be operable, it is essential for a modeling ecosystem to be in a valid state with respect to the various interdependencies between the metamodel and its depending artifacts as well as among the depending artifacts. Consequently, in case of metamodel evolution, caused by, e.g., changing requirements, the depending artifacts have to be co-evolved accordingly to keep the system in a valid state. With respect to modeling ecosystems, special effort has to be laid to a consistent co-evolution across the different kinds of artifacts and their relationships. Although several approaches for the co-evolution of depending artifacts have been proposed, there was no special focus on an ecosystem-wide perspective of co-evolution, yet. Therefore, this paper focuses on co-evolution in modeling ecosystems by discussing the various components of a modeling ecosystem and their relationships, depicting the respective co-evolution process, proposing an evaluation framework for co-evolution, and applying this framework to current approaches. Based on this evaluation we derive lessons learned and present future research directions.