Soil fauna may buffer the negative effects of drought on alien plant invasion

Although many studies have tested the direct effects of drought on alien plant invasion, less is known about how drought affects alien plant invasion indirectly via other groups of organisms such as soil fauna. To test for such indirect effects, we grew single plant of nine naturalized alien target species in pot-mesocosms with a native community of five native grassland species under four combinations of two drought (well-watered vs drought) and two soil-fauna-inoculation (with vs without) treatments. We found that drought decreased the absolute and the relative biomass production of the alien plants, and thus reduced their competitive advantage in the native community. Drought decreased the abundance of soil fauna, particularly the soil mites, but did not affect abundance and richness of soil herbivores. Soil-fauna inoculation increased the biomass of the native plant community and thereby decreased the relative biomass production of the alien species. The increased invasion resistance due to soil fauna, however, tended (p = 0.09) to be stronger for plants growing under well-watered conditions than under drought. Our multispecies experiment thus shows that soil fauna might help native resident communities to resist alien plant invasions, but that this effect might be diminished by drought.