Can soil animals help native plants resist alien invaders during drought?

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Many studies have tested drought’s direct effects on alien plant invasion, but less is known about indirect effects through interactions with organisms such as soil mesofauna.

To test for these indirect effects, researchers grew single plants of nine naturalized alien target species in pot mesocosms alongside a community of five native grassland species. They used four combinations of two watering treatments (well-watered or drought) and two soil-mesofauna inoculation treatments (with or without inoculation).

Drought decreased both the absolute and relative biomass production of the alien plants, reducing their competitive strength in the native community. It also decreased the abundance of soil mesofauna, particularly soil mites, but did not affect the abundance or richness of soil herbivores.

Soil-fauna inoculation did not affect alien plant biomass, but increased the biomass of the native plant community and thereby decreased the relative biomass production of the alien plants. This increase in invasion resistance tended to be stronger under well-watered conditions than under drought, although the difference was uncertain (p = 0.09).

In synthesis, the multispecies experiment suggests that soil fauna might help native communities resist alien plant invasions, but that this effect might be weakened under drought. In other words, soil mesofauna may buffer the negative effects of drought on alien plant invasions.