Close Encounters

Invasive plants can homogenize soil pathogens

The decreased number of pathogen species appears to benefit the invaders.

When invasive plants take root in a new environment, they not only affect above-ground communities, but those below-ground as well. While it has been proposed that their overall effect is to drive down the species richness in the community, this hypothesis has not been tested for soil microorganisms.

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In a new article published in Annals of Botany, lead author Meiling Wang and colleagues set out to determine the effect on soil fungal communities of the invasive species Alternanthera philoxeroides versus its native congeneric, A. sessilis, in China. They did this via a plant-soil feedback experiment followed by high-throughput sequencing of soil fungi. The fungi assessed included pathogenic, saprotrophic, and arbuscular mycorrhizal fungi. The experiment was designed to account for density-dependent effects from both the native and invasive plants.

The researchers found that at high densities, the native plant was associated with more highly varied assemblages of soil pathogens than its invasive cousin. Low densities of the native species in the presence of the invasive had lowered pathogen species richness. Arbuscular mycorrhizal fungi, on the other hand, became more varied when associated with high densities of the invasive plant A. sessilis. Overall, the changes appeared to benefit the invasive species, perhaps because of a decrease in pathogens the plants have no natural resistance to, though the authors note this advantage may not persist over the long term.

“Although the underlying reason remains unclear, it is possible that the within-species genetic diversity and/or root metabolite variation, both of which can influence soil microbial composition, of the native plant increased at a high density and in turn promoted more specific and/or rare pathogens than at lower densities, corresponding to a high proportion of unique [operational taxonomic units] found in this study,” write the authors.

This finding is important because, as the authors note, “manipulative studies across plant densities are essential for disentangling the mechanisms, dynamics and consequences of changes in soil microbial composition due to plant invasions.”