Toward a unified theory of microbially mediated invasion

Microbially mediated invasion is an emerging science that aims to account for the role of microbial agents in facilitating or preventing biological invasion. Progress has been made in identifying possible scenarios occurring when microbes can transmit either from native to invasive hosts (\`symbiont spillback') or from invasive to native hosts (\`symbiont spillover'). For example, the presence of pre-existing mycorrhizal networks in the soil may facilitate plant invasion, and invaders are more likely to be successful if native species are infected with parasites co-introduced with their invasive hosts. However, a unifying theoretical framework to contextualize these individual scenarios and explore the consequences of microbial transmission between hosts along a continuum of beneficial to harmful host-microbial interactions is lacking. Here, we present and analyse such a framework. We discuss interesting scenarios emerging from our analysis and multiple pathways through which microbes can facilitate (or prevent) host invasion, microbial invasion, and the invasion of both hosts and their co-introduced microbes. Our framework provides a new, cohesive and intuition-enhancing perspective on microbially driven dynamics and the way in which the subtleties of the relationships between hosts and microbes affect invasion outcomes. ### Competing Interest Statement The authors have declared no competing interest.