When species invade a new habitat, the invasion is typically confounded with a change in community diversity. Loss of species is common, but in some cases increased overall species richness has been observed. To understand the mechanisms underlying the impacts of invasion, we must distinguish between the effects of invaders and the effects of altered diversity. We examined how plant invasion and plant community diversity affect arbuscular mycorrhizal (AM) fungi, which can be dramatically altered by the presence of exotic plant species. The focal species were five congeneric pairs of grasses, with one native and one exotic species from each genus (Bothriochola, Festuca, Panicum, Eragrostis and Agropyron). We used a standard diversity design with combinations of one, two, or four species, but replicated across four plant origin treatments: (1) native species only, (2) exotic species only, (3) mixtures of native & exotic congeners, and (4) mixtures of native and exotic non-congeners. Grasses were grown in field-collected soils and maintained in a greenhouse for six months. We characterized the AM community in terms of root colonization, size of hyphal networks in soil, and community composition.

Results/Conclusions

Different aspects of the mycorrhizal community were dependent on either invasion or diversity. Inside roots, AM colonization was determined only by the origin of the plants: colonization was lower for exotic compared to native congeners, despite the fact that the exotic grasses were consistently larger both above- and belowground in every treatment. We also found differential effects of individual grass species on AM colonization of roots, with generally more hyphae and arbuscules in native Panicum roots compared to other species. In contrast, in the soil outside of roots, the abundance of AM hyphae increased with increasing plant diversity regardless of plant origin. The impact of invasive species on the overall structure of the AM fungal community can thus be affected by a combination of both the invasion and any resulting changes to plant community diversity. We are currently characterizing the shift in AM community composition in both soils and roots using a DNA-based approach. Differentiating between the independent effects of exotic species and altered plant community diversity that can occur as a result of the invasion process increases our understanding of the controllers of community structure and diversity.