Abstract Host-symbiont communities present a unique interaction structure, in which host-symbiont interdependent fitness and the possible formation of novel host-symbiont associations can give rise to a broad range of dynamics. Understanding mechanisms that can facilitate or prevent invasion in these communities, there-fore, requires to embrace a community perspective that accounts for host-host, symbiont-symbiont and host-symbiont interactions. Empirical work has shown that the formation of novel host-symbiont associations between native and invasive species can lead to multiple outcomes affecting invasion success, such as increased fitness of introduced hosts or symbionts, or decreased fitness of natives. Here, we present a unifying theoretical framework to contextualize these individual scenarios and investigate their significance at the community level, along a continuum ranging from mutualistic to parasitic host-symbiont interactions. We discuss interesting scenarios emerging from our analysis and multiple pathways through which host-symbiont interactions can facilitate or prevent host invasion, symbiont invasion, and the invasion of both hosts and their co-introduced symbionts. Our framework provides a new, mechanistic, cohesive, and intuition-enhancing tool for theoretically exploring the ways in which the subtleties of the relationships between hosts and symbionts affect invasion dynamics.

Toward a unified theory of microbially mediated invasion