Plant invasions can cause changes in arbuscular mycorrhizal fungal (AMF) abundance and community composition. These changes can influence the success of exotic plants by altering their competitive interactions with natives (driver analogy). However, it is also possible that changes in fungal communities simply reflect a response by AMF to shifts in host quality, abundance and diversity with no functional consequence (passenger analogy). Both scenarios assume that invasive and native plants differ in how they interact with AMF, but the extent and direction of such differences are unknown. We used meta-analysis to quantify if invasive and native plants respond differently to AMF, and if AMF abundance change in directional ways with invasions. We then used field surveys and experiments to assess the ability of various grassland invaders to alter AMF communities in Intermountain West grasslands. Finally, to determine if the observed changes in fungal communities make the invaders more competitive toward native plants (driver analogy), or likely have no functional consequences for invasion outcomes (passenger analogy), we grew native and invasive plants in soils collected from native and invaded areas where fungicides had been added or not.
Results/Conclusions
The meta-analysis revealed that plant invasions are not associated with directional shifts in AM responsiveness and AMF abundance. Rather, plant responses to AMF—as well as changes in AMF abundance—were best predicted by plant functional group identity of the invader relative to the native community being invaded. Forbs tended to benefit from AMF when grown alone and in competition and to increased AMF abundance, whereas grasses responded negatively to AMF and suppressed their abundance. As such, AMF have the greatest potential to affect invasion outcomes when the invasive and native plants belong to different functional groups. Our empirical results mirrored this, because the annual cheatgrass (Bromus tectorum) reduced AMF abundance whereas the perennial forbs spotted knapweed (Centaurea stoebe) and leafy spurge (Euphorbia esula) increased AMF abundance and richness relative to grass-dominated native communities. Whether or not these observed changes in AMF communities influence interactions between invasive and native plants (driver vs. passenger analogy), and if outcomes depend on the functional group identity of the two plants is tested in our ongoing greenhouse experiment and will be discussed.