For our next Invasive Species Week blog post, Rebecca Bunn and Ylva Lekberg have contributed a piece about their paper, which is included in our special Invasive Species Virtual Issue. Read their full paper here, free to access for a limited time.
For an exotic plant to become invasive it must outcompete native plants. One of the most popular hypotheses in invasion biology is that invasive plants can do this because they escape enemies present in their home range. An alternative hypothesis is that exotic plants form stronger mutualisms, or somehow disrupt these beneficial associations and weaken the competitive ability of native plants.
In our paper, we analysed previously published data to find out if invasive and native plants associate differently with one group of soil-dwelling mutualists, arbuscular mycorrhizal fungi (AMF), and whether these associations affect competitive interactions.
AMF colonise plant roots and can increase plant nutrient uptake, pathogen protection and drought tolerance in exchange for carbon. Previous research has shown that spotted knapweed, a grassland invader in the Intermountain West of the United States, becomes more competitive against many native plants in the presence of AMF. On the other hand, garlic mustard, which invades the understory in deciduous forests, does not associate with AMF and produces antifungal compounds that suppress fungal colonisation of native plants. It is clear that these high-profile invaders interact very differently with AMF, but whether these associations are typical of invasive plants and fundamentally different from native plants is unknown.
To find out if native and invasive plants respond differently to AMF in ways that affect competitive interactions, we consolidated data on plant-AMF associations involving 70 native and 55 invasive plant species. Responses of AMF to native and invasive plants were also quantified to see if these fungi benefited more from one group of plants than the other.
Native grasslands in Missoula MT in 1974 (photo Sue Brown) and in 2000 after invasion by spotted knapweed (photo Dean Pearson)
We found that native and invasive plants do not interact differently with AMF. Instead, plant functional group identity appears to be important. For instance, forbs were more colonised and benefited more from AMF than grasses, regardless of plant provenance. This implies that forb invasions into grass-dominated native communities may very well be aided by AMF, whereas native forbs may be more competitive and better resist invasions by grasses. Grass invasions are more likely to decrease AMF abundance, but forb invasions are likely to increase it, which could further affect competitive interactions between native and invasive plants.
Our findings indicate that AMF are most likely to influence the outcome of plant invasions when invasive and native plants belong to different functional groups. Also, shifts in fungal abundances may be predictable based on whether the invader was a forb or a grass. We are now asking what the consequences of these shifts are for restoring native plants into previously invaded areas. We are also addressing biogeographical patterns of AMF communities to determine if exotic plants encounter new AMF species in their invasive range.
Rebecca Bunn (Western Washington University) and Ylva Lekberg (MPG Ranch and University of Montana)
Read the full paper. All papers included in the special virtual issue are free to access for a limited time.
Journal of Ecology Blog 