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Productivity benefits from diversity can arise when compatible pathogen hosts are buffered by unrelated neighbors, diluting pathogen impacts. However, the generality of pathogen dilution has been controversial and rarely tested within biodiversity manipulations. Here, we test whether soil pathogen dilution generates diversity- productivity relationships using a field biodiversity-manipulation experiment, greenhouse assays, and feedback modeling. We find that the accumulation of specialist pathogens in monocultures decreases host plant yields and that pathogen dilution predicts plant productivity gains derived from diversity. Pathogen specialization predicts the strength of the negative feedback between plant species in greenhouse assays. These feedbacks significantly predict the overyielding measured in the field the following year. This relationship strengthens when accounting for the expected dilution of pathogens in mixtures. Using a feedback model, we corroborate that pathogen dilution drives overyielding. Combined empirical and theoretical evidence indicate that specialist pathogen dilution generates overyielding and suggests that the risk of losing productivity benefits from diversity may be highest where environmental change decouples plant-microbe interactions. Soil pathogen dilution can improve plant production. Here, the authors combine an empirical and theoretical approaches finding that specialist pathogens suppress yield in monocultures while pathogen dilution drives yield benefits in diverse plant communities.

Ecological restoration efforts can increase the diversity and function of degraded areas. However, current restoration practices cannot typically reestablish the full diversity and species composition of remnant plant communities. We present evidence that restoration quality can be improved by reintroducing key organisms from the native plant microbiome. In particular, root symbionts called arbuscular mycorrhizal fungi are crucial in shaping grassland communities, but are sensitive to anthropogenic disturbance, which may pose a problem for grassland restoration. In the present article, we highlight the conceptual motivation and empirical evidence evaluating native mycorrhizal fungi, as opposed to commercial fungi. Reintroduction of the native microbiome and native mycorrhizal fungi improves plant diversity, accelerates succession, and increases the establishment of plants that are often missing from restored communities. The example of mycorrhizal fungi serves to illustrate the value of a more holistic view of plant communities and restoration that embraces the intricacies and dynamics of native microbial communities.

Symbioses exert strong influence on host phenotypes; however, benefits from symbionts can increase or degrade over time. Understanding the context-dependence of reinforcing or degrading dynamics is pivotal to predicting stability of symbiotic benefits. Host phylogenetic relationships and host life history traits are two candidate axes that have been proposed to structure symbioses. However, the relative influence of host evolutionary history and life history on symbiont composition, and whether changes in symbiont composition translate into stronger mutualistic benefits is unknown. We tested the influence of plant phylogenetic relationships and plant life history on the composition of arbuscular mycorrhizal (AM) fungi, perhaps the most ancestral and influential of plant symbionts, and then tested whether AM fungal differentiation resulted in improved mutualism as expected from coadaptation. We constructed mycobiomes composed of seven AM fungal isolates derived from tallgrass prairie and grew them for two growing seasons with 38 grassland plant species. We found that host phylogenetic structure was a significant predictor of the composition of AM fungal communities and the genetic composition of AM fungal species, patterns consistent with phylosymbiosis. However, the phylogenetic structure of AM fungi failed to translate to improved benefits to their host. While AM fungi generally improved plant growth and mycorrhizal feedback was generally positive, the strength of feedback was not predicted by plant phylogenetic distance. The composition of the AM fungal community and genetic composition within AM fungal species were also significantly influenced by plant life history and feedbacks between early and late successional species were generally positive. Interestingly, positive mycorrhizal feedback was predicted by changes in genetic composition of the two most abundant AM fungal species, not by changes in species composition. Positive mycorrhizal feedbacks across life history can mediate plant species turnover during succession and suggests that consideration of mycorrhizal dynamics could improve ecosystem restoration.

Arbuscular mycorrhizal (AM) fungi are root symbionts that can facilitate plant growth and influence plant communities by altering plant interactions with herbivores. Therefore, AM fungi could be critical for the conservation of certain rare plants and herbivores. For example, North American milkweed species are crucial hosts for monarch butterflies (Danaus plexippus). Understanding how mycorrhizal composition affects milkweeds will have direct impacts on the conservation and restoration of both increasingly threatened guilds. We present data from three studies on the effect of AM fungal composition on milkweed growth, latex production, and establishment. First, we grew seven milkweed species with and without a mixture of native mycorrhizal fungi. We assessed how important fungal composition is to milkweed growth and latex production by growing four milkweed species with seven fungal compositions, as single‐species inoculations with four native fungi, a mixture of native fungi, a single commercial fungus of presumably non‐native origin, and noninoculated controls. Finally, we assessed the field establishment of two milkweed species with and without native mycorrhizal inoculation. Milkweed species grew 98% larger and produced 82% more latex after inoculation with native mycorrhizae. Milkweeds were strongly affected by fungal composition; milkweeds were inhibited by commercial fungi (average of −14% growth) and showed variable but positive responses to native fungal species (average of +3% to +38% biomass). Finally, we found that restoration establishment was dependent on inoculation with native fungi and milkweed species. Overall, our findings indicate that some milkweed species (i.e., Asclepias syriaca and A. incarnata) are not responsive to mycorrhizal fungal presence or sensitive to mycorrhizal composition while others are, including endangered species (A. meadii) and species of high conservation value (A. tuberosa). We conclude that the reintroduction of native AM fungi could improve the establishment of desirable milkweed species and should be considered within strategies for plantings for monarch conservation.

Soil aggregate stability is an important ecosystem property that is altered by anthropogenic disturbance. Yet, the generalization of these alterations and the identification of the main contributors are limited by the absence of cross-site comparisons and the application of inconsistent methodologies across regions. We assessed aggregate stability in paired remnant and post-disturbance grasslands across California, shortgrass and tallgrass prairies, and in manipulative experiments of plant composition and soil microbial inoculation. Grasslands recovering from anthropogenic disturbance consistently had lower aggregate stability than remnants. Across all grasslands, non-native plant diversity was significantly associated with reduced soil aggregate stability. A negative effect of non-native plants on aggregate stability was also observed in a mesocosm experiment comparing native and non-native plants from California grasslands. Moreover, an inoculation study demonstrated that the degradation of the microbial community also contributes to the decline in soil aggregate stability in disturbed grasslands. Anthropogenic disturbance consistently reduced water-stable aggregates. The stability of aggregates was reduced by non-native plants and the degradation of the native soil microbial community. This latter effect might contribute to the sustained decline in aggregate stability following anthropogenic disturbance. Further exploration is advocated to understand the generality of these potential mechanisms.

Plant colonization of islands may be limited by the availability of symbionts, particularly arbuscular mycorrhizal (AM) fungi, which have limited dispersal ability compared to ectomycorrhizal and ericoid (EEM) as well as orchid mycorrhizal (ORC) fungi. We tested for such differential island colonization within contemporary angiosperm floras worldwide. We found evidence that AM plants experience a stronger mycorrhizal filter than other mycorrhizal or non-mycorrhizal (NM) plant species, with decreased proportions of native AM plant species on islands relative to mainlands. This effect intensified with island isolation, particularly for non-endemic plant species. The proportion of endemic AM plant species increased with island isolation, consistent with diversification filling niches left open by the mycorrhizal filter. We further found evidence of humans overcoming the initial mycorrhizal filter. Naturalized floras showed higher proportions of AM plant species than native floras, a pattern that increased with increasing isolation and land-use intensity. This work provides evidence that mycorrhizal fungal symbionts shape plant colonization of islands and subsequent diversification.Delavaux et al. uses global plant databases to examine how the types of mycorrhizal fungi influence the colonization of plants on islands. Their results show that arbuscular mycorrhizal (AM) symbiosis limits plant species’ establishment on islands more than other mycorrhizal associations, and that this initial filter of AM plant species impacts diversification and plant invasion risks.

The establishments of new organisms that arrive naturally or with anthropogenic assistance depend primarily on local conditions, including biotic interactions. We hypothesized that plants that rely on fungal symbionts are less likely to successfully colonize remote environments such as oceanic islands, and this can shape subsequent island ecology. We analyzed the mycorrhizal status of Santa Cruz Island, Galapagos flora compared with the mainland Ecuador flora of origin. We experimentally determined plant responsiveness and plant–soil feedback of the island flora and assessed mycorrhizal density and soil aggregate stability of island sites. We found that a greater proportion of the native island flora species belongs to families that typically do not associate with mycorrhizal fungi than expected based upon the mainland flora of origin and the naturalized flora of the island. Native plants benefited significantly less from soil fungi and had weaker negative soil feedbacks than introduced species. This is consistent with the observation that field sites dominated by native plant species had lower arbuscular mycorrhizal (AM) fungal density and lower soil aggregate stability than invaded field sites at the island. We found support for a mycorrhizal filter to the initial colonization of the Galapagos.

The International Culture Collection of (Vesicular-) Arbuscular Mycorrhizal Fungi—INVAM—the largest living culture collection of arbuscular mycorrhizal fungi (AMF) celebrated its 35th year in 2020. The authors record here the mission and goals of INVAM, its contribution as a living culture collection, some historical aspects of INVAM, and describe the advances in mycorrhizology and AMF systematics after INVAM moved to West Virginia University. This commentary emphasizes the importance of a living culture collection to preserve germplasm and to educate and assist researchers in mycorrhizal science.

Soil microbial communities contribute to ecosystem function and structure plant communities, but are altered by anthropogenic disturbance. Successful restoration may require microbial community restoration. Inoculation of plants with arbuscular mycorrhizal fungi (AMF) may improve ecological restoration, but AMF species that are locally adapted to native plant communities are often unavailable and commercially propagated AMF are not necessarily locally adapted to the desired plant community target. The disconnect between readily available commercial fungi and later‐successional plants may inhibit successful establishment of the restoration. We tested this concept using four mid‐ to late successional prairie plant species planted with one of three inoculum sources: a locally adapted AMF mix cultured from native prairie, a non‐locally adapted commercial AMF product, or a sterilized background soil control. The inoculated plants (termed nurse plants) were planted in the middle of field plots. In each plot, uninoculated plants (test plants) were planted at 0.5, 1, and 2 m from the nurse plants in order to test whether growth and survival of test plants could be affected by inoculum source. Generally, plants grew larger when inoculated with native AMF compared to commercial inoculum or the control. Later successional species responded most positively to native AMF. Benefits of inoculation also spread to neighbors, as uninoculated late successional test plant, Sporobolus heterolepis, grew larger when its' neighbors were inoculated with native AMF than with commercial AMF or the control. Due to an unanticipated herbivory event, we also assessed the degree to which rate of herbivory or plant tolerance to herbivory is affected by inoculum source. The mid‐ successional nurse plant, Ratibida pinnata, received the majority of the herbivore damage, and when it was inoculated with commercial AMF, it experienced significantly more herbivory than plants inoculated with native AMF or the control. R. pinnata inoculated with native AMF grew significantly larger one month following herbivory, though there was no significant difference in growth in the second year of sampling. This study suggests that native, locally adapted AMF can improve restoration of prairie plant species and these benefits can extend to neighbors up to two meters from the inoculation point.

Bever et al have worked intensely on the ecology of the plant-arbuscular mycorrhizal fungal interactions within a one-hectare field in North Carolina. They describe the process of discovering this diversity, detail mechanisms that might maintain fungal diversity, and then discuss their understanding of what this diversity means for ecology as a whole.