Soil fungi quietly decide which invasive plants win

Beneath the soil surface, an invisible marketplace of fungi may be quietly deciding which invasive plants dominate the landscapes they colonize. A new study published in the journal Plant and Soil reveals that arbuscular mycorrhizal fungi, or AMF, do not simply boost every invader equally. Instead, their effects shift dramatically depending on how abundant each invading species is within a community, a finding that could reshape how ecologists understand and manage multi-species plant invasions.

The research, led by Shanshan Qi of Jiangsu University together with colleagues from institutions across China and Australia, focused on two notorious invasive plants: Bidens pilosa, commonly known as hairy beggarticks, and Sphagneticola trilobata, the creeping wedelia. Both species, native to the Americas, have spread aggressively across tropical and subtropical regions of Asia, where they frequently appear together in disturbed habitats. When two invasive species establish in the same location, ecologists call this co-invasion, and it can accelerate biodiversity loss far more severely than a single invader acting alone.

The team began with a field survey, examining naturally co-occurring populations of the two invaders to measure how heavily their roots were colonized by mycorrhizal fungi. These fungi form symbiotic partnerships with most land plants, threading into root tissues and extending networks of hyphae into the soil. In exchange for plant-derived carbon, the fungi deliver hard-to-obtain nutrients, particularly phosphorus, to their hosts. The survey produced a striking result: mycorrhizal colonization rates were significantly higher in communities where the two invaders grew together than in sites dominated by a single species. Something about coexistence appears to encourage the fungal partnership.

To untangle the mechanisms behind this pattern, the researchers moved into the greenhouse, where they could control the proportions of each species and the presence of AMF with precision. Their experimental design allowed them to measure not just growth, but the intensity of competition between and within the two invaders under different conditions. This is where the story became genuinely surprising, because the fungi’s influence proved anything but uniform.

The greenhouse results showed that the growth promotion induced by AMF decreased as the proportional abundance of an invader increased. In other words, when a species was rare within the mixture, the fungi gave it a substantial lift; when it dominated the pot, that benefit faded. This proportion-dependent response suggests that the value of the symbiosis is not a fixed trait of either plant species but a dynamic property of the community’s composition. A rare invader, perhaps freed from intense competition for fungal services, reaps outsized rewards from the partnership.

Even more intriguing was the effect on competition between the two species. AMF effectively alleviated interspecific competition, softening the struggle between Bidens pilosa and Sphagneticola trilobata when they grew side by side. However, this buffering effect weakened as competition intensified. When the two invaders were locked in a fierce contest for space and resources, the fungi could no longer smooth over the conflict. The symbiosis, it seems, acts as a moderator only up to a threshold of competitive pressure.

The picture inverted completely under monoculture conditions. When either species grew alone with members of its own kind, AMF exacerbated intraspecific competition, making life harder for plants competing against their own kind. This asymmetry, where fungi ease conflict between species while sharpening conflict within species, has profound implications for how co-invading populations structure themselves. It hints that the fungi may push communities toward mixtures rather than pure stands, subtly reshuffling the competitive balance in favor of coexistence between the invaders.

These findings connect to a broader and increasingly urgent body of research on invasional meltdown, the hypothesis that positive interactions among non-native species can accelerate their collective impact. Previous work has documented how soil microorganisms can mediate such meltdowns, with invasive plants sometimes cultivating fungal communities that favor themselves or other invaders over native species. The new study adds a crucial layer of nuance: the direction and strength of these microbial effects depend on proportional abundance, meaning the same fungal community could facilitate an invader at low density and constrain it at high density. Ecologists have long recognized that density dependence shapes plant populations, but demonstrating that mycorrhizal fungi modulate both interspecific and intraspecific competition in a proportion-dependent way is a significant advance.

The technical implications extend to invasion management. Control strategies that target a single invader may inadvertently create conditions in which the remaining invader, now at lower proportional abundance, receives a stronger mycorrhizal boost and rebounds aggressively. Conversely, understanding that AMF intensify intraspecific competition in monocultures suggests that dense, single-species invasions may harbor their own internal brakes, ones that could potentially be leveraged in restoration efforts. The authors note that their work reveals a complex, density-dependent role for AMF in modulating competition between co-occurring invaders, and that the fungi can shift the competitive balance depending on plant proportional abundance.

As biological invasions continue to impose mounting economic and ecological costs worldwide, studies like this one underscore that the battle against invasive species is not fought only above ground. The hidden fungal networks threading through the soil are active participants, capable of amplifying or dampening the fortunes of co-invading plants in ways that shift with every change in community composition. For land managers and restoration ecologists, the message is clear: to predict which invader will prevail, and when, they must look below the surface and account for the proportional arithmetic of the invasion itself.

Subject of Research: Density-dependent effects of arbuscular mycorrhizal fungi on competition between co-invading plant species

Article Title: Proportion-dependent asymmetric facilitation of co-invaders by arbuscular mycorrhizal fungi

Article References: Qi, S., Chen, R., Lei, P., Shu, Z., Zhai, D., Ren, G., Wang, H., Yan, X., Dai, Z., Du, D., & Bonser, S. P. (2026). Proportion-dependent asymmetric facilitation of co-invaders by arbuscular mycorrhizal fungi. Plant and Soil. https://doi.org/10.1007/s11104-026-09045-x

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09045-x

Keywords: arbuscular mycorrhizal fungi, co-invasion, Bidens pilosa, Sphagneticola trilobata, plant competition, invasional meltdown, density dependence, soil microbiome, invasive plants, symbiosis, plant ecology, biodiversity loss