The results suggest that plant–soil feedbacks may help these invaders sustain dense stands, but the greenhouse design could not determine whether microbes, nutrients or other soil properties produced the effects. The experiment used field soil from three nearby locations near Springhill Community in Gallatin County, Montana. One treatment came from land historically confirmed to be uninvaded by either grass; the others came from sites occupied by B. tectorum but not V. dubia , or by V. dubia but not B. tectorum .

Each pot contained pasteurized potting soil topped with a 1-centimeter layer of one of these field soils. Seeds were planted at densities ranging from one to 16 plants, with monocultures and mixed-species arrangements. After the first 10-week growth period, plants were harvested, the pots were left dry for two weeks, and the same planting arrangements were restored for a second 10-week period.

No additional soil or sterilization was applied between phases, allowing the second planting to experience both the field legacy and the conditioning produced during the first phase. In the first phase, which tested growth in soils carrying their pre-existing field legacies, B. tectorum had the greatest average per-capita biomass in V. dubia soil, at 0.112 grams, followed by B. tectorum soil at 0.094 grams and neutral soil at 0.063 grams. V. dubia also grew best in soil from the two invaded sites: mean per-capita biomass was 0.020 gram in V. dubia soil and 0.018 gram in B. tectorum soil, with no significant difference between them, versus 0.014 gram in neutral soil.

Thus, before experimental conditioning, plants of both species performed better in soils associated with invasive grass occupancy than in the uninvaded reference soil. Density nevertheless shaped performance substantially. Each additional B. tectorum plant reduced its own per-capita biomass by an estimated 15% in neutral soil and by 11% in B. tectorum soil, while the corresponding decline was not significant in V. dubia soil.

The influence of V. dubia on B. tectorum varied by soil: biomass fell by 3% for each added V. dubia plant in neutral soil, by 9% in V. dubia soil, and showed no significant change in B. tectorum soil. Across soils, an additional V. dubia plant reduced mean per-capita V. dubia biomass by 4%, whereas an additional B. tectorum reduced it by 15%. These phase-one responses indicate that both aboveground competition and soil origin influenced the grasses, with the strength of some effects depending on the soil treatment.

The second phase produced more distinct patterns associated with experimental soil conditioning. For B. tectorum , soil origin alone did not significantly affect per-capita biomass, but its interactions with plant density did. Intraspecific competition was pronounced in neutral soil, where each added B. tectorum plant reduced biomass by 22%, and weaker in V. dubia soil, where the reduction was 14%.

In soil previously conditioned by B. tectorum , however, increasing its own density produced less than a 1% change in per-capita biomass. That near stabilization was interpreted as evidence of positive plant–soil feedback: the grass may have altered its soil in a way that reduced the density-dependent limitation experienced by its own kind. The conditioning effect for V. dubia was more complex and did not support the study’s prediction that B. tectorum soil would benefit it.

Mean per-capita biomass was highest in V. dubia soil at 0.014 gram, lowest in B. tectorum soil at 0.004 gram, and intermediate in neutral soil at 0.008 gram. The difference between V. dubia and B. tectorum soils was significant, whereas the neutral-soil value did not differ significantly from either. Intraspecific density dependence was absent in neutral and V. dubia soils, but appeared in B. tectorum soil, where each additional V. dubia reduced per-capita biomass by 16%.

By contrast, B. tectorum strongly reduced V. dubia biomass: the estimated decrease was 42% per added B. tectorum in neutral soil, 32% in V. dubia soil and 18% in B. tectorum soil. The response of V. dubia therefore depended on both soil origin and the competing plants present. Nonlinear competition models offered a second, partly contrasting view.

Their bootstrapped coefficients generally indicated stronger intraspecific than interspecific competition for both grasses, with overlapping confidence intervals preventing firm comparisons among soil treatments. B. tectorum was consistently less affected by competition than V. dubia . The estimated effect of V. dubia on B. tectorum remained below the estimated effect of B. tectorum on V. dubia in every treatment and phase.

These models use a different parameterization from the mixed-effects analyses and assume the same resource-use efficiency for both species, so their coefficients are not directly comparable. The apparent tension between the two approaches illustrates that the evidence for feedback depends on the model and the soil treatment rather than pointing to one uniform mechanism. Why the two grasses can form dense monocultures is therefore partly a question about what happens beneath their feet, not only about competition for light above ground.

B. tectorum has shallow fibrous roots, can alter litter cover and nitrogen cycling, and has been associated with neutral or negative effects on arbuscular mycorrhizal fungi in invaded areas. V. dubia produces leaves with high silica content, which may slow litter decomposition, and its effects on mycorrhizal fungi vary among study locations. A soil legacy could also involve shifts in nutrient partitioning or interactions with pathogens and other organisms.

The experiment did not include a sterilized-soil control and did not directly measure microbial communities, so its treatment differences represent combined soil effects and cannot be assigned to microbes or abiotic properties individually. The results carry implications for secondary invasion—the establishment or expansion of one invasive species after another is removed—without demonstrating that outcome in the field. Both grasses germinate in autumn and are commonly targeted with pre- or early post-emergent herbicides, but V. dubia also has a spring cohort that shorter-residual treatments may miss.

Because B. tectorum exerted the stronger estimated effect on V. dubia under the experiment’s watering regime, removing B. tectorum could release V. dubia from competition in some circumstances. That is a plausible management scenario, not a tested field prediction. The authors also caution that the relationship may change with temperature, moisture and the density of established conspecifics, while noting that a dense V. dubia population could reduce the per-plant effect of invading B. tectorum .

Overall, the study provides greenhouse evidence that soil legacies can modify density dependence in two invasive annual grasses associated with western rangelands. The strongest positive-feedback signal occurred when B. tectorum grew in soil conditioned by B. tectorum , whereas V. dubia showed a more context-dependent response and performed worst in B. tectorum -conditioned soil. Neither species displayed a simple, universal advantage through soil conditioning.

Field experiments across moisture and temperature gradients, together with direct measurements of arbuscular mycorrhizal fungi, nutrient cycling and other soil biota, will be needed to determine whether the observed feedback helps sustain monocultures, promotes species replacement or alters the consequences of management in western rangelands.

Subject of Research: Greenhouse testing of plant–soil feedbacks and competition between Bromus tectorum and Ventenata dubia

Article Title: Plant–Soil Feedbacks May Underlie Competitive Dynamics Between Co‐Occurring Invasive Annual Grasses Bromus tectorum and Ventenata dubia

Article References: Sencenbaugh, L., Mangold, J. M., Maxwell, B. D., Ulrich, D. E. M., & Rew, L. J. (2026). Plant–Soil Feedbacks May Underlie Competitive Dynamics Between Co‐Occurring Invasive Annual Grasses Bromus tectorum and Ventenata dubia. Ecology and Evolution, 16(10), Article e74516. https://doi.org/10.1002/ece3.74516

Image Credits: AI Generated

DOI: 10.1002/ece3.74516

Keywords: plant–soil feedback, soil legacy, invasive grasses, Bromus tectorum, Ventenata dubia, plant competition, greenhouse experiment, rangeland invasion, density dependence, mycorrhizal fungi, Plant, Soil

The study, conducted in a controlled greenhouse environment, utilized four plant species from the family Amaranthaceae: two native to China, Alternanthera sessilis and Achyranthes bidentata, and two alien species introduced to the region, Celosia argentea and Amaranthus retroflexus. The experimental design crossed two levels of soil nutrient heterogeneity, homogeneous versus heterogeneous, with the presence or absence of a native generalist herbivore, the moth Spodoptera litura. Plants were grown either alone or in competitive pairs of one native and one alien species. In the heterogeneous soil treatment, nutrients were concentrated in small, discrete patches, simulating the fine-scale spatial variation commonly found in natural soils.

Contrary to the common expectation that plants benefit from nutrient heterogeneity by precisely targeting rich patches, the results indicated an overall negative effect of soil nutrient heterogeneity on the total biomass of both the native Alternanthera sessilis and the alien Celosia argentea. This suggests that the energetic costs of searching for and exploiting small, discrete nutrient patches may outweigh the benefits gained from increased nutrient uptake. The fine-scale nature of the heterogeneity created in the experiment, with patches only one centimeter in diameter, likely imposed a high metabolic burden on the plants, leading to reduced whole-plant growth compared to plants grown in homogeneous soils where nutrients were evenly distributed.

However, post-hoc comparisons uncovered species-specific patterns in how herbivory modified these responses. For the native plant Alternanthera sessilis, the lowest total biomass was observed in heterogeneous soils under herbivory. This plant exhibited clear root foraging behavior in the absence of herbivores, allocating more roots to nutrient-enriched patches. When herbivores were present, this foraging capacity appeared to be constrained, resulting in a dual penalty of reduced nutrient uptake and aboveground damage. In contrast, the alien plant Celosia argentea showed significant biomass differences between homogeneous and heterogeneous soils only in the absence of herbivores. Under herbivore pressure, the suppressive effect of soil heterogeneity on C. argentea was diminished, suggesting a different belowground strategy or greater tolerance to herbivore damage.

The belowground biomass analyses provided further insight into these contrasting responses. In the absence of herbivores, Alternanthera sessilis produced significantly greater root biomass in nutrient-enriched patches compared to low-nutrient patches, confirming its foraging behavior. However, under herbivory, this patch-specific root allocation was no longer significant, indicating that the plant could not maintain its foraging precision when under attack. Celosia argentea, on the other hand, did not show significant differences in root biomass between patch types under either herbivory condition. This lack of strong root foraging response may have limited the costs associated with exploiting fine-scale heterogeneity, allowing the alien plant to maintain growth even when herbivores were present.

These findings highlight the role of co-evolutionary history in shaping plant-herbivore-soil interactions. Native plants like Alternanthera sessilis may be more susceptible to the combined stresses of herbivory and the energetic demands of foraging in heterogeneous soils due to long-term co-evolution with native herbivores. Alien plants, such as Celosia argentea, may benefit from enemy release or greater physiological tolerance, allowing them to cope with herbivore damage while still navigating the challenges of patchy soils. The study suggests that local native generalist herbivores can alter the expression of soil nutrient heterogeneity effects in a species-specific manner, potentially influencing the competitive balance between native and alien plants.

Competition between native and alien plants also played a significant role in the experimental outcomes. When grown together, the aboveground biomass of both native plants was reduced compared to when they were grown alone, while the alien plants were unaffected by competition with natives. This asymmetry in competitive effects is consistent with previous studies showing that alien plants often possess traits that allow them to outcompete native species, such as rapid growth rates or allelopathic capabilities. However, the study found no evidence that competition modified the effects of soil nutrient heterogeneity or herbivory, suggesting that these factors operate independently of interspecific competition in this context.

The magnitude of the effects varied considerably across species and treatments. Competition-induced reductions in aboveground growth were substantial for the native plants, with Alternanthera sessilis experiencing a 49.6 percent reduction and Achyranthes bidentata a 62.2 percent reduction. In contrast, heterogeneity-induced reductions were smaller, around 31 to 33 percent for the affected species. Herbivory had a more variable impact, reducing growth in Alternanthera sessilis by 14.3 percent but actually increasing growth in Achyranthes bidentata by 25.1 percent. These results underscore the complexity of plant responses to multiple environmental stressors and the importance of considering species identity when predicting ecological outcomes.

The study concludes that soil nutrient heterogeneity and native generalist herbivores jointly shape the performance of native and alien plants in a species-specific manner. The negative effects of fine-scale nutrient heterogeneity on plant biomass can be modulated by herbivory, with native and alien plants responding differently due to variations in their foraging strategies and tolerance to herbivore damage. These findings have important implications for understanding plant invasions and ecosystem dynamics, as they suggest that the impact of soil heterogeneity on alien plant success may be contingent on the presence of native herbivores. Further research is needed to explore how these interactions scale up to natural ecosystems and how they may be influenced by other environmental factors.

As climate change and land-use alterations continue to modify soil nutrient patterns and herbivore communities, understanding these intricate interactions will be crucial for predicting and managing plant community composition. The study provides valuable insights into the mechanisms underlying plant responses to patchy soils and herbivory, highlighting the need for integrated approaches that consider belowground and aboveground processes simultaneously. By revealing the species-specific nature of these responses, the research opens new avenues for investigating the role of soil-herbivore-plant interactions in shaping biodiversity and ecosystem function.

Subject of Research: Effects of herbivory on plant responses to soil nutrient heterogeneity

Article Title: Contrasting effects of herbivory on responses of a native and an alien plant to soil nutrient heterogeneity

Article References: Ou-Yang, K., Na, X.-Y., Wu, Y.-N., Huang, L., Xue, W., & Yu, F.-H. (2026). Contrasting effects of herbivory on responses of a native and an alien plant to soil nutrient heterogeneity. NeoBiota, 106, 91-105. https://doi.org/10.3897/neobiota.106.180549

Image Credits: AI Generated

DOI: 10.3897/neobiota.106.180549

Keywords: soil nutrient heterogeneity, herbivory, alien plants, native plants, root foraging, plant competition, Spodoptera litura, ecosystem dynamics, NeoBiota, plant performance, Contrasting, effects

Senecio inaequidens, a perennial subshrub in the daisy family, is one of Europe’s most successful plant invaders. Native to the highlands of Southern Africa, it hitchhiked into Europe in the late nineteenth century as a contaminant of wool shipments. Since then it has spread aggressively along roadsides, railways, quarries, vineyards, and dry grasslands, reaching heights of 40 to 100 centimeters. Its arsenal includes pyrrolizidine alkaloids, allelopathic chemicals that deter herbivores both above and below ground and can reshape soil microbial communities. Only tetraploid populations, carrying forty chromosomes, made it to Europe, while both diploid and tetraploid forms persist in its native range. This history made it an ideal test subject for disentangling how shared evolutionary history versus soil conditions shape competitive outcomes.

The research team, working under controlled growth chamber conditions, designed a fully factorial experiment. Individual ragwort plants occupied the center of each pot, surrounded by one of three competitive scenarios: a community of five species from the invader’s native South African range, a community of five species from the invaded range in Northern Italy, or no competitors at all. Half of the pots contained soil that had been autoclaved at 120 degrees Celsius for 45 minutes to suppress microbial life, while the other half retained the natural bacterial communities collected from the invaded quarry site at Collepedrino, in the Italian province of Bergamo. The South African suite included Aristida congesta, Hibiscus trionum, Salvia disermas, and two Wahlenbergia species, while the Italian suite comprised Bromopsis erecta, Hypericum perforatum, Onobrychis viciifolia, Poterium sanguisorba, and Trifolium repens.

The rationale behind this design draws on a century-old idea. Darwin’s naturalization hypothesis proposes that introduced species should struggle to establish in communities dominated by closely related natives, because related species tend to share traits, resource requirements, and enemies shaped by common evolutionary history. Phylogenetic distance therefore serves as a proxy for niche overlap and for the strength of competition an invader will face. At the same time, a parallel line of research suggests that soil microbes can tip the scales. Introduced plants may escape coevolved specialist pathogens from home, accumulate novel mutualists or antagonists abroad, or actively cultivate bacterial communities that favor their own growth. Which of these mechanisms dominates in practice has remained remarkably unclear.

After 84 days of growth at 29 degrees Celsius under a 14-hour day cycle mimicking ruderal Mediterranean habitats, the answer was unambiguous. Competitor community identity had the strongest effect on ragwort’s performance, with plants growing significantly shorter when surrounded by species from their native South African range compared with plants growing alone. Autoclaving the soil, by contrast, had no significant effect on plant height, and neither community identity nor soil treatment significantly influenced lateral spread. Statistical analysis using two-way ANOVA revealed a significant community effect on height, while soil effects fell well short of significance. In short, the evolutionary familiarity of the competitors, not the living soil, was the dominant force suppressing the invader.

Phylogenetic analysis sharpened the picture. The researchers calculated two complementary measures of relatedness: a community-weighted phylogenetic distance that integrated all competitor species according to their abundance, and the distance between ragwort and the single most abundant competitor in each pot. The community-wide metric showed no relationship with ragwort’s growth, but the distance to the dominant species mattered. The closer the dominant competitor was to ragwort on the angiosperm tree of life, the smaller the invader grew. The two Wahlenbergia species from South Africa were the most phylogenetically similar competitors in the experiment, and the South African communities produced the smallest ragwort individuals overall. This pattern supports Darwin’s naturalization hypothesis in its strongest form, but with a crucial refinement: it is the dominant, closely related species that deliver biotic resistance, not the community as a whole.

The soil bacterial results added an unexpected twist. Sequencing the V5–V6 hypervariable regions of the bacterial 16S rRNA gene yielded nearly 690,000 valid sequences and 8,471 amplicon sequence variants across the samples. Pseudomonadota dominated the communities at 46.8 percent of sequences, followed by Actinomycetota and Bacteroidota. Both competitor identity and soil treatment significantly shaped bacterial community structure, together explaining nearly half of the variation. Most intriguingly, taller and wider ragwort plants were consistently associated with lower bacterial diversity in their pots, a statistically significant negative relationship for both species richness and the Shannon diversity index. Soils under South African communities, where ragwort performed worst, harbored the most diverse bacterial assemblages.

What could explain the link between vigorous invader growth and depauperate bacterial communities? The authors caution that their experiment cannot establish causality, but several mechanisms are plausible. Ragwort’s pyrrolizidine alkaloids may suppress susceptible microbial taxa, echoing earlier work showing that the species’ traits, particularly those linked to competitive ability and root allocation, correlate with rhizosphere bacterial diversity. Alternatively, the pattern may reflect the actions of co-introduced or generalist soil pathogens associated with native-range competitors, which could have compounded the aboveground suppression of ragwort in South African treatments. The authors also note an important methodological caveat: autoclaving can alter soil chemistry, nutrient availability, and physical structure, potentially confounding purely biotic interpretations, although residual DNA from cells killed some ninety days before sampling is likely negligible.

The findings also reconcile an apparent contradiction in the literature. An observational study in Northern Italy had found ragwort performing better in phylogenetically clustered communities, consistent with the pre-adaptation hypothesis, which holds that invaders succeed where they share environmental adaptations with natives. The new experimental results, showing the opposite pattern at fine spatial scales, fit an emerging synthesis in which both hypotheses operate simultaneously but at different scales. At broad geographic scales, environmental filtering favors aliens resembling native species; at the fine scale of individual interactions, where competition for limiting resources is intense, phylogenetic distance becomes an advantage and close relatedness becomes a liability. Context, scale, and resource levels determine which force prevails.

For land managers battling ragwort across Europe’s disturbed landscapes, the study’s implications are refreshingly concrete. Restoration efforts might do well to sow multi-species suites of native species that are phylogenetically close to the invader and competitively strong, deploying them at high abundance to maximize biotic resistance. The suggestion that diverse soil microbiota also constrain the invader opens a second avenue: testing soil amendments that boost microbial diversity and resilience in degraded sites. More broadly, the work demonstrates that the ecological naivety of recipient communities, their lack of evolutionary experience with a newcomer, can outweigh belowground feedbacks in determining invasion outcomes. As global trade continues to shuffle species across continents, understanding which natives are best equipped to resist them, and why, may prove one of the most valuable tools in restoration ecology’s growing arsenal.

Subject of Research: Eco-evolutionary experience and soil bacterial communities as drivers of the invasive success of Senecio inaequidens

Article Title: Eco-evolutionary experience outweighs soil effects in shaping Senecio inaequidens invasion success

Article References: Quaglini, L. A., Yannelli, F. A., Gandolfi, I., Franzetti, A., Caronni, S., Montagnani, C., Carbutt, C., Jeschke, J. M., Citterio, S., & Gentili, R. (2026). Eco-evolutionary experience outweighs soil effects in shaping Senecio inaequidens invasion success. NeoBiota, 106, 141-160. https://doi.org/10.3897/neobiota.106.182956

Image Credits: AI Generated

DOI: 10.3897/neobiota.106.182956

Keywords: invasive species, Senecio inaequidens, eco-evolutionary experience, Darwin's naturalization hypothesis, soil bacteria, plant competition, phylogenetic distance, biotic resistance, restoration ecology, allelopathy, soil microbiome, NeoBiota

Halophila stipulacea is no ordinary newcomer. Originating in the Red Sea and Indian Ocean, this small, fast-growing seagrass is a Lessepsian migrant, meaning it crossed into the Mediterranean Sea through the Suez Canal after its opening in 1869. It was first observed in the Mediterranean in 1894 and later reached the Caribbean Sea, likely transported by boats, with the first Caribbean sighting reported in Grenada in 2002. Today it ranks among only three known invasive seagrass species worldwide, and its tolerance of wide ranges in light, depth, temperature, salinity, and water movement has long been credited with its spread. What has remained unclear is whether its associated microbes contribute to that remarkable ecological flexibility.

To probe that question, a research team led by scientists at the University of Puerto Rico Río Piedras, working with collaborators in Spain, the United States, Cyprus, and Puerto Rico, built on a previously published controlled competition experiment conducted in Cyprus. Intact cores of naturally co-occurring seagrass containing both species were collected by SCUBA from donor beds and transplanted into 24-liter flow-through mesocosms at a marine aquaculture facility in Larnaka. The experiment ran for six weeks under natural conditions and included four treatments with ten mesocosm replicates each: native and invasive grown together at natural densities, both species together at reduced densities, invasive monoculture, and native monoculture. Monocultures were created by selectively removing one species, while reduced-density mixed treatments were established by removing roughly half the shoots of both species while maintaining their co-occurrence.

The team focused on the phyllosphere, the microbial community living on the surface of seagrass leaves, because shoot dynamics had responded strongly to competition in the earlier experiment. At the start and end of the six-week period, researchers scraped epibionts from blades of both species, washed and centrifuged the samples, and extracted DNA using a commercial soil kit. They then sequenced the V4 region of the 16S rRNA gene, the workhorse marker for characterizing bacterial communities, following the Earth Microbiome Project protocol. Sequencing was performed on an Illumina MiSeq platform, and the resulting data were processed with the QIIME 2 microbiome platform using the DADA2 pipeline to denoise sequences and assign them to amplicon sequence variants, or ASVs, which represent distinct bacterial taxa. Taxonomy was assigned with a naive Bayes classifier trained on the SILVA database.

Alongside the microbial work, the researchers measured an array of biochemical and morphological indicators of plant health. They extracted photosynthetic pigments including chlorophyll a, chlorophyll b, and carotenoids from leaf tissue, quantified hydrogen peroxide as a marker of oxidative stress using the potassium iodide method, and measured malondialdehyde, a product of lipid peroxidation that signals cellular damage. Growth was assessed by counting changes in shoot numbers over the experiment, and the team also recorded aboveground and belowground biomass, rhizome internode length, and sediment nitrate and nitrite concentrations. This combination of microbial, biochemical, and morphological data allowed the researchers to look for associations between bacterial communities and the physiological state of each seagrass species under different competitive conditions.

The results revealed a striking asymmetry. In the competition experiment, the invasive H. stipulacea increased by an average of 3.7 shoots, roughly a 59 percent gain, when grown in mixed assemblages with the native seagrass, while the native C. nodosa lost about one shoot, a 29 percent decline, under the same conditions. When density was reduced in mixed treatments, the invader grew by 4.3 shoots, an increase of roughly 150 percent, while the native showed a modest gain of 0.2 shoots. Aboveground biomass followed the same pattern: natives produced significantly more biomass when grown alone than when mixed, whereas the invader maintained or increased its biomass in mixed treatments. Notably, the native seagrass had internode lengths roughly three times longer than the invader, a morphological difference that had not been documented in the earlier work.

Biochemical measurements added another layer of intrigue. Hydrogen peroxide concentrations were more than sevenfold higher in the native C. nodosa, averaging 4.18 micromoles per gram compared with just 0.62 in the invader, indicating substantially greater oxidative stress in the native species. Malondialdehyde levels were also higher in the native seagrass, and the difference varied by treatment. Photosynthetic pigment levels told an opposite story for the two species: the native seagrass had higher pigment concentrations when grown in mixed communities than when alone, while the invader showed the reverse pattern, producing significant treatment-by-species interactions for chlorophyll a, chlorophyll b, and carotenoids. Sediment nitrate and nitrite concentrations, by contrast, did not differ significantly between treatments or species.

On the microbial side, sequencing of 41 libraries yielded 301,907 raw reads, of which 169,068 high-quality reads survived quality control, chimera removal, and filtering. After rarefaction to a standardized depth of 1,500 reads per sample, the final dataset comprised 826 bacterial ASVs with an average of 220 ASVs per sample. Alpha diversity, measured as Shannon diversity and Pielou evenness, did not differ significantly between species or treatments. However, beta diversity analysis using Bray-Curtis dissimilarities revealed significant overall differences in community composition among groups, with group identity explaining 63.7 percent of the variation in a PERMANOVA test. Complementary ANOSIM analysis supported this separation, and tests for homogeneity of multivariate dispersion confirmed that the differences were not driven by unequal within-group variability. Results from an alternative normalization approach, Scaling with Ranked Subsampling, were consistent with the rarefied analyses, strengthening confidence in the findings.

The taxonomic composition of the two seagrass microbiomes diverged sharply. Proteobacteria dominated across all samples, accounting for 72.1 percent of sequence reads, followed by Bacteroidota at 7.7 percent and Planctomycetota at 3.7 percent. The native C. nodosa in monoculture harbored the richest microbiome, with elevated proportions of genera such as Oceanicella, uncultured Microtrichaceae, Hyphomonas, Methylotenera, and Lentimonas. When the native grew alongside the invader, its microbial diversity declined and several taxa associated with nutrient cycling and stress tolerance, including the sulfate-reducing genus Desulfobulbus and the hydrocarbon-degrading genus Bermanella, became strongly associated with monoculture conditions. Conversely, the invader’s microbiome was most abundant in mixed environments: the genus Pseudoalteromonas reached 32.7 percent relative abundance in full mixed density compared with just 6.5 percent in monoculture, and genera such as Vibrio, Photobacterium, Zeaxanthinibacter, and Tenacibaculum were more prevalent when the invader grew with the native. Modeling with MaAsLin 2, which integrates microbial data with experimental covariates while correcting for multiple comparisons, confirmed these associations, and a sensitivity analysis with reduced predictor sets produced qualitatively consistent results.

The authors propose several hypotheses to explain these patterns. One possibility is microbe-mediated allelopathy: the invader may harbor bacterial associates that produce compounds harmful to the native species, a mechanism well documented in terrestrial plants but not yet reported between seagrasses. Supporting this idea, recent work has shown that H. stipulacea produces a distinct volatilome compared with native seagrasses, including allelopathic compounds such as apocarotenoids and isothiocyanates. Another hypothesis is that the invader actively selects beneficial microbes when competing, or alternatively passively acquires its microbiome from environmental sources. The study has limitations: it examined only phyllosphere bacteria, leaving rhizosphere and sediment communities unexplored, and the compositing strategy used to reduce technical variation may have underestimated within-treatment variability. Sequencing depth was also constrained by quality filtering. Nevertheless, the correlative patterns are compelling, and the authors emphasize that future work must determine whether the microbial shifts are a cause or a consequence of competitive success. As climate change is expected to further promote the expansion of H. stipulacea in the Mediterranean, understanding whether these microbes are drivers or passengers of invasion could prove critical for predicting and managing the future of seagrass ecosystems on two continents.

Subject of Research: Microbial communities associated with enhanced growth of the invasive seagrass Halophila stipulacea when mixed with native Mediterranean seagrass Cymodocea nodosa

Article Title: Better together? Microbial communities associated with enhanced growth of the invasive seagrass Halophila stipulacea when mixed with a native Mediterranean seagrass Cymodocea nodosa

Article References: Hoza-Frederick, E., Martínez-Campos, S., Barber, P. H., Vasquez, M. I., Fotopoulos, V., Antoniou, C., Drakou, K., Godoy-Vitorino, F., & Chiquillo, K. L. (2026). Better together? Microbial communities associated with enhanced growth of the invasive seagrass Halophila stipulacea when mixed with a native Mediterranean seagrass Cymodocea nodosa. Frontiers in Marine Science, 13, Article 1871379. https://doi.org/10.3389/fmars.2026.1871379

Image Credits: AI Generated

DOI: 10.3389/fmars.2026.1871379

Keywords: seagrass, invasive species, microbiome, Mediterranean Sea, Halophila stipulacea, Cymodocea nodosa, 16S rRNA, phyllosphere, marine ecology, plant competition, oxidative stress, Frontiers in Marine Science

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

The study, led by Jian-Kun Sun, Wei-Wei Feng, Ming-Chao Liu, Ji-Xin Chen, Fa-Zhao Qi, and Yu-Long Feng of Southwest Forestry University and Shenyang Agricultural University in China, focused on a dramatic head-to-head contest between two plants. On one side was Solanum rostratum, an aggressive invader native to North America that has spread widely across China and other regions, armed with spines and a voracious appetite for resources. On the other was Astragalus laxmannii, a native species that shares the same habitats. The researchers grew the two species together in mixed field plots, forcing them to compete directly under different nitrogen regimes, and then measured everything from total biomass to the microscopic economics of their leaves and roots.

The central question was deceptively simple: when soils receive more nitrogen, and when that nitrogen arrives in different chemical forms, do invasive and native species respond in the same way? The answer, according to the data, is a resounding no. Across nearly all treatments, Solanum rostratum outperformed its native neighbor in total biomass, and the gap between the two species widened as nitrogen availability increased. Crucially, the mechanism behind that widening gap was not just faster growth but a coordinated shift in how the invader builds and deploys its tissues.

To understand that mechanism, it helps to think of plants as investors operating under a strict carbon budget. Ecologists describe this through the framework of the plant economics spectrum, a continuum that runs from acquisitive strategies—thin, cheap tissues built for rapid resource capture—to conservative strategies, which favor dense, durable structures that conserve resources over the long term. The leaves and roots of a plant tend to align along this spectrum, and the new study shows that the two species sit at opposite ends of it.

Solanum rostratum displayed the classic acquisitive profile. It had a higher specific leaf area, meaning it constructed more light-intercepting surface per gram of leaf tissue, and a higher net photosynthetic rate, converting sunlight into sugars faster. It also achieved a higher photosynthetic nitrogen-use efficiency, extracting more carbon gain from every unit of nitrogen invested in its photosynthetic machinery. Below ground, the invader paired these leaf traits with a higher specific root length—more foraging root length per unit of root mass—along with thinner roots and lower root tissue density. In other words, it built an inexpensive, high-throughput system for capturing both light and soil nutrients. Astragalus laxmannii, by contrast, showed the conservative syndrome: thicker, denser roots, a higher root-to-shoot ratio, and slower, more frugal resource economics.

The most striking finding emerged when nitrogen was added. For the invader, nitrogen addition pushed its acquisitive traits even further: specific leaf area, photosynthetic rate, photosynthetic nitrogen-use efficiency, and specific root length all increased. For the native species, those same responses were smaller or in some cases statistically indistinguishable from zero. Because the invader could flexibly scale up its resource-capture apparatus while the native could not, the growth difference between the two species grew larger under nitrogen enrichment. Nitrogen pollution, in effect, acted as an amplifier for a pre-existing competitive asymmetry rather than creating one from scratch.

The chemical form of the nitrogen mattered as well. Soils supply inorganic nitrogen primarily in two forms: nitrate and ammonium. Plants expend different amounts of energy assimilating each, and species differ in their preferences and transport capacities. The experiment revealed that adding nitrate was generally more effective at boosting the invader’s performance in some cases, while ammonium additions could more effectively increase biomass for the native Astragalus laxmannii. This detail carries real-world weight, because atmospheric nitrogen deposition changes not just the total amount of nitrogen entering ecosystems but also the relative proportions of nitrate and ammonium that plants encounter. Landscapes receiving nitrate-dominated deposition may therefore be tipping the competitive balance further in favor of invaders with nitrate-hungry, acquisitive physiologies.

Statistical analysis of the trait data confirmed that these leaf and root characteristics were not incidental correlates of the growth difference—they significantly contributed to it. The acquisitive traits of the invader and the conservative traits of the native together explained a substantial portion of why Solanum rostratum accumulated more biomass under competition. This coordinated leaf-root perspective is important because much earlier invasion research focused on leaves alone, treating below-ground economics as a black box. By measuring both organ systems simultaneously, the study demonstrates that invasion success rests on a whole-plant strategy, with roots and leaves working in concert to convert abundant nitrogen into rapid growth.

The findings arrive at a moment of mounting global concern. Nitrogen deposition has intensified dramatically over industrialized and agricultural regions, with East Asia among the most affected areas, and recent global assessments have documented rising impacts of plant invasions on terrestrial ecosystems worldwide. If nitrogen enrichment systematically favors acquisitive invaders over conservative natives, then the two environmental pressures—nutrient pollution and biological invasion—may be reinforcing each other, producing ecosystems that are simultaneously more eutrophic and more dominated by exotic species. That feedback could accelerate the displacement of native flora and erode the biodiversity that underpins ecosystem services.

There are also practical implications for land managers. Controlling invasive plants in nitrogen-enriched landscapes may require more than mechanical removal or herbicide treatment; it may demand interventions that address the underlying soil fertility, such as restoring low-nitrogen conditions, managing fertilizer runoff, or favoring native species with sufficient plasticity to respond to elevated nutrients. The study’s authors suggest that increasing soil nitrogen availability, especially in the form of nitrate, may facilitate invasions of exotic plants characterized by coordinated resource-acquisitive strategies—a conclusion that turns a global pollution problem into a predictive tool for invasion risk. As nitrogen continues to accumulate in soils around the world, the plants best positioned to exploit it may increasingly be the ones we least want.

Subject of Research: How soil nitrogen availability and form influence competitive trait differences between an invasive plant and a native species

Article Title: Increasing soil availability of nitrogen, especially nitrate, amplifies growth advantage associated with acquisitive leaf–root traits in Solanum rostratum compared with co-occurring native species

Article References: Sun, J.-K., Feng, W.-W., Liu, M.-C., Chen, J.-X., Qi, F.-Z., & Feng, Y.-L. (2026). Increasing soil availability of nitrogen, especially nitrate, amplifies growth advantage associated with acquisitive leaf–root traits in Solanum rostratum compared with co-occurring native species. Plant and Soil. https://doi.org/10.1007/s11104-026-09091-5

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09091-5

Keywords: biological invasion, Solanum rostratum, nitrogen deposition, nitrate, ammonium, plant traits, leaf economics, root economics, photosynthetic nitrogen-use efficiency, specific leaf area, specific root length, plant competition

The study, published in Nature Plants, brings together results from 19 CO₂-enrichment experiments conducted in glasshouses, growth chambers and field environments. The experiments included plant communities ranging from grass-dominated ecosystems to multispecies assemblages. By comparing plants grown alone with plants grown in mixtures, the researchers examined a question that has often been overlooked in climate-change experiments: does elevated CO₂ help all species equally, or does competition determine which plants are able to use the additional carbon?

The contrast was striking. When plants grew without neighbours, most species responded positively to elevated CO₂, or eCO₂. This response is biologically plausible because CO₂ is the raw material plants use during photosynthesis. Inside leaves, carbon dioxide is fixed into sugars through the Calvin cycle, providing the carbon skeletons needed to build new tissues. Higher atmospheric CO₂ can also reduce the amount of water plants lose while taking up carbon, because leaf pores known as stomata may not need to remain as open. Under controlled conditions, these effects can translate into faster growth and greater biomass.

Yet those benefits often disappeared when plants had to share their environment. More than half of the species examined showed neutral or negative responses to eCO₂ when growing in mixtures. In other words, a plant that grew better under elevated CO₂ by itself was not necessarily able to capitalize on the same conditions when surrounded by other species. The finding challenges the tendency to use single-species experiments as a guide to how natural plant communities will respond to a carbon-rich atmosphere.

Competition can block the conversion of extra carbon into additional growth in several ways. Plants may compete for sunlight, water, nitrogen and phosphorus, all of which are required to turn photosynthetic products into leaves, stems, roots and seeds. A plant may absorb more CO₂ and produce more sugars, but if nutrients are scarce, it may be unable to construct the proteins and tissues needed to use that carbon. Similarly, a taller or faster-growing neighbour may capture the available light before a smaller species can benefit, creating a physical barrier between elevated CO₂ and actual plant growth.

The community-level result was more subtle than a simple collapse in productivity. Although many individual species experienced neutral or negative responses, dominant species often drove modest increases in total mixture biomass under eCO₂. This means a plant community can appear to benefit from elevated CO₂ even while many of its members do not. A rise in total biomass therefore does not necessarily indicate that growth has been shared evenly, or that the community has become more productive in a way that benefits all species.

That distinction could have major implications for biodiversity. If elevated CO₂ consistently favours species that are already dominant, they may capture an even larger share of light, nutrients and space. Subdominant species could then face stronger suppression, reducing their growth and potentially altering the composition of plant communities over time. The result may be a greener-looking ecosystem with fewer species, rather than a uniformly healthier or more productive one.

The researchers’ trait analysis offers a clue about why some plants succeed while others fall behind. Acquisitive strategies, associated broadly with rapid resource capture and fast growth, were better predictors of performance under competition than a species’ intrinsic sensitivity to CO₂ alone. This suggests that the key question is not simply whether a plant can respond physiologically to elevated carbon dioxide. It is whether the plant can acquire enough light, water and nutrients quickly enough to turn that response into a competitive advantage.

The findings also expose a limitation in how climate models and ecological forecasts often treat CO₂ fertilization. If models assume that species respond independently, they may overestimate the benefits that rising CO₂ will deliver to plant communities. Real ecosystems are networks of interactions, and the outcome for one species depends partly on the traits and abundance of its neighbours. The same atmospheric change can therefore stimulate growth in an isolated plant, suppress a subordinate competitor and produce only a small increase in total community biomass.

As atmospheric CO₂ continues to rise, understanding this uneven distribution of benefits will become increasingly important. The new synthesis does not suggest that elevated CO₂ has no effect on vegetation. Instead, it shows that its effects are filtered through competition. Dominant plants may gain enough to lift overall biomass, while many less competitive species receive little benefit or even lose ground. The future of plant communities may therefore be shaped not by a universal CO₂ boost, but by an intensified struggle over which species can claim it.

Subject of Research: The effects of elevated carbon dioxide on plant growth and competition in multispecies communities.

Article Title: Interspecific competition negates CO₂ benefits for most plant species.

Article References: Raubenheimer, S.L., Simpson, K.J., Ripley, B.S. et al. “Interspecific competition negates CO₂ benefits for most plant species.” Nature Plants (2026). https://doi.org/10.1038/s41477-026-02360-2

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41477-026-02360-2

Keywords: Elevated CO₂, climate change, plant competition, biodiversity, plant communities, ecosystem productivity, dominant species, subdominant species, CO₂ fertilization, plant traits