Background/Question/Methods
Although global change clearly influences species invasion, the exact mechanisms by which global change either intensifies or limits invasive spread remain elusive. Global change can affect invasion directly by altering abiotic conditions, as well as indirectly by altering the abundance and composition of interacting species. Here we examine the relative impacts of direct and indirect effects of saltwater intrusion on the expansion of a model invasive plant species, Phragmites australis, in freshwater marshes of coastal Louisiana. Using experimental mesocosms, we test two hypotheses: (1) saltwater intrusion facilitates invasion of P. australis in freshwater marshes and (2) this process relies primarily on an indirect mechanism, alterations in composition of microbial symbionts. To tease apart the relative impacts of salinity and microbial communities on invasion, we experimentally manipulate these factors in three community contexts: P. australis monoculture, a native community, and a mixed P. australis and native community. We predict that the indirect effects of salinity will be stronger than the direct effects of salinity on invasion across all community types.
Results/Conclusions
We find that overall salinity strongly increases invasion, but the direction and magnitude of direct and indirect effects are context dependent. Indirect effects of salinity (via alterations in soil microbial composition) increase invasive performance in monoculture and decrease native performance in native communities. However, in mixed communities, direct effects of salinity are stronger than indirect effects. When P. australis and natives are grown together, salinity increases invasion regardless of changes in microbial composition. Results suggest that indirect effects of saltwater intrusion will promote P. australis expansion and hinder native productivity in freshwater wetlands. However, the relative strength of direct and indirect effects of salinity on invasion will be contingent on community context. More broadly, these results underscore the importance of considering microbially-mediated indirect effects of global change in investigating the long-term outcomes of plant species interactions.