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Globally, numerous ecosystems have been co‐invaded by multiple exotic plant species that can have competitive or facilitative interactions with each other and with native plants. Invaded ecosystems often exhibit spatial heterogeneity in soil moisture and nutrient levels, with some habitats having more nutrient‐rich and moist soils than others. The stress‐gradient hypothesis predicts that plants are likely to engage in facilitative interactions when growing in stressful environments, such as nutrient‐deficient or water‐deficient soils. In contrast, when resources are abundant, competitive interactions between plants should prevail. The invasional meltdown hypothesis proposes that facilitative interactions between invasive species can enhance their establishment and amplify their ecological impact. Considering both hypotheses can offer insights into the complex interactions among invasive and native plants across environmental gradients. However, experimental tests of the effects of soil moisture and nutrient co‐limitation on interactions between invasive and native plants at both interspecific and intraspecific levels in light of these hypotheses are lacking. We performed a greenhouse pot experiment in which we cultivated individual focal plants from five congeneric pairs of invasive and native species. Each focal plant was subjected to one of three levels of plant–plant interactions: (1) intraspecific, in which the focal plant was grown with another individual of the same species; (2) interspecific, involving a native and an invasive plant; and (3) interspecific, involving two native or invasive individuals. These plant–plant interaction treatments were fully crossed with two levels of water availability (drought vs. well‐watered) and two levels of nutrient supply (low vs. high). Consistent with the stress‐gradient and invasional meltdown hypotheses, our findings show that under low‐nutrient conditions, the biomass production of invasive focal plants was facilitated by invasive interspecific neighbors. However, under high‐nutrient conditions, the biomass production of invasive focal plants was suppressed by invasive interspecific neighbors. When competing with native interspecific neighbors, high‐nutrient conditions similarly enhanced the biomass production of both invasive and native focal plants. Invasive and native focal plants were neither competitively suppressed nor facilitated by conspecific neighbors. Taken together, these results suggest that co‐occurring invasive exotic plant species may facilitate each other in low‐nutrient habitats but compete in high‐nutrient habitats. [ABSTRACT FROM AUTHOR]

India has numerous invasive alien plant species (IAPSs), which seriously impact biodiversity, ecosystem services, and economic development. The availability of reliable occurrence records of IAPSs is of great importance for their successful management, prediction of distribution across time and space, and other research and development efforts. Global databases of occurrence data, like the Global Biodiversity Information Facility (GBIF), are often not exhaustive, especially for India, and poorly represent the actual distribution of IAPSs in the country. Our objective in creating this data set was threefold: (1) to compile occurrence data for the invasive and naturalized alien plant species of India, (2) to collect spatial and temporal information associated with occurrence records, and (3) to share the data in an accessible format so every record is traceable to its sources and allow users to submit data to increase the resolution of the data set. To achieve these objectives, we extracted data from 3137 literature records and 357 herbarium sheets. For each occurrence record, we curated information for 20 variables, which were arranged in a table with the Darwin Core (DwC) terms as column names. All data were subjected to technical validation before being included in the database. A total of 12,347 occurrence records were obtained for 362 species (195 invasive aliens and 167 naturalized aliens). The number of collected occurrence records was much higher for the invasive aliens (73.7%) than for the naturalized alien species (26.3%). Our data set will supplement the GBIF data by 60.39%, and occurrence records will be added for 64 invasive and naturalized alien plant species. The data set, as a part of the larger database of the Indian Alien Flora Information (ILORA) database, is made available without any restrictions on use as long as this data paper is properly cited. We have also made provisions for users to submit occurrence‐related data following a data standard. The users are encouraged to cite the original reference when using a specific data record. The data set is expected to assist a wide range of stakeholders involved in India's scientific research, policy formulation, and decision‐making related to IAPSs. [ABSTRACT FROM AUTHOR]

Ephemeral disturbances are common in many systems. Often, these brief events are assumed to be a nuisance with little long-term ecological impact. We quantified the impact of the ephemeral forest disturbance caused by gypsy moth canopy defoliation on exotic plant invasion in eight hardwood forests in the Delaware Water Gap National Recreation Area, USA. Six years following the gypsy moth disturbance, we were able to predict 59% of the increase in Alliaria petiolata abundance and 42% of the increase in Microstegium vimineum abundance with models incorporating an interaction between disturbance severity and propagule pressure. In addition, we found that the disturbance timing had substantial impacts on the invasion dynamics of Alliaria petiolata. Our results suggest that ephemeral disturbances can have important and long-lasting impacts on plant communities, and highlight the need to account for variations in disturbance characteristics and the role of propagule pressure in determining the disturbance-invasion relationship. [ABSTRACT FROM AUTHOR]

Although exotic plants comprise a substantial portion of floristic biodiversity, their contributions to community and ecosystem processes are not well understood. We manipulated plant species richness in old-field communities to compare the impacts of native vs. exotic species on plant biomass, seed production, and arthropod community structure. Plants within diverse communities, regardless of whether they were native or exotic, had higher biomass and seed production than in monocultures and displayed positive complementarity. Increasing native or exotic plant richness also enhanced the richness of arthropods on plants, but exotics attracted fewer arthropod species for a given arthropod abundance than did natives. Additionally, when exotic and native plants grew together, exotics suppressed seed production of native species. Thus, exotic plants appear to contribute positively to some biodiversity functions, but may impact native communities over longer time frames by reducing native seed production and recruiting fewer arthropod species. [ABSTRACT FROM AUTHOR]

Exotic plant invasions threaten ecosystems globally, but we still know little about the specific consequences for animals. Invasive plants can alter the quality of breeding habitat for songbirds, thereby impacting important demographic traits such as dispersal, philopatry, and age structure. These demographic effects may in turn alter song-learning conditions to affect song structure and diversity. We studied Chipping Sparrows (Spizella passerina) breeding in six savannas that were either dominated by native vegetation or invaded by spotted knapweed (Centaurea stoebe), an exotic forb known to diminish food resources and reproductive success. Here, we report that the prevalence of older birds was relatively low in knapweed-invaded habitat, where recruitment of yearlings compensated for diminished site fidelity to sustain territory abundance. In both habitat types, yearling males tended to adopt songs similar to their neighbors and match the songs of older birds rather than introducing new song types, a pattern seen in many songbird species. As a consequence, in invaded habitat where age structure was skewed away from older birds serving as potential song models, yearlings converged on fewer song types. Similarity of songs among individuals was significantly higher and the overall number of song types averaged nearly 20% lower in invaded relative to native habitat. Degradation of habitat quality generally impacts site fidelity and age ratios in migratory songbirds and hence may commonly alter song-learning conditions. Associated shifts in song attributes known to influence reproductive success could in turn enforce demographic declines driven by habitat degradation. Local song structure may serve as an important indicator of habitat quality and population status for songbirds. [ABSTRACT FROM AUTHOR]

Knowledge from basic plant ecology suggests that impact of one plant species on another is driven by either competition for the same limiting resources, or by unique plant traits. These processes might be context specific, explaining a differential impact of exotic plant invaders in the native vs. introduced range. With the help of a conceptual framework, we aimed at identifying the relationship between invader biomass and impact in the invasive Centaurea stoebe by conducting pairwise competition experiments with 15 European (old) and 15 North American (new) neighboring species. Old neighbors grew larger and could use available soil moisture more efficiently for growth than new neighbors. Interestingly, biomass of C. stoebe explained a substantial amount of the variation in biomass of the coevolved neighbors, but not of the new "naïve" neighbors. Thus, impact in the home range appears to be driven by competition for the same limiting resources, but by other factors in the introduced range, possibly by exploitation of resources that are not used by the new neighbors or by interference competition. This distinction has important consequences for the management of invasive species, as in our study ecosystem recovery is less likely after simple biomass reduction. [ABSTRACT FROM AUTHOR]

The widely held belief that riparian communities are highly invasible to exotic plants is based primarily on comparisons of the extent of invasion in riparian and upland communities. However, because differences in the extent of invasion may simply result from variation in propagule supply among recipient environments, true comparisons of invasibility require that both invasion success and propagule pressure are quantified. In this study, we quantified propagule pressure in order to compare the invasibility of riparian and upland forests and assess the accuracy of using a community's level of invasion as a surrogate for its invasibility. We found the extent of invasion to be a poor proxy for invasibility. The higher level of invasion in the studied riparian forests resulted from greater propagule availability rather than higher invasibility. Furthermore, failure to account for propagule pressure may confound our understanding of general invasion theories. Ecological theory suggests that species-rich communities should be less invasible. However, we found significant relationships between species diversity and invasion extent, but no diversity-invasibility relationship was detected for any species. Our results demonstrate that using a community's level of invasion as a surrogate for its invasibility can confound our understanding of invasibility and its determinants. [ABSTRACT FROM AUTHOR]

The impacts of exotic plants on soil nutrient cycling are often hypothesized to reinforce their dominance, but this mechanism is rarely tested, especially in relation to other ecological factors. In this manuscript we evaluate the influence of biogeochemically mediated plant-soil feedbacks on native shrub recovery in an invaded island ecosystem. The introduction of exotic grasses and grazing to Santa Cruz Island, California, USA, converted native shrublands (dominated by Artemisia californica and Eriogonurn arborescens) into exotic-dominated grasslands (dominated by Avena barhata) over a century ago, altering nutrient-cycling regimes. To test the hypothesis that exotic grass impacts on soils alter reestablishment of native plants, we implemented a field-based soil transplant experiment in three years that varied widely in rainfall. Our results showed that growth of A vena and Artemisia seedlings was greater on soils influenced by their heterospecific competitor. Theory suggests that the resulting plant-soil feedback should facilitate the recovery of Artemisia in grasslands, although four years of monitoring showed no such recovery, despite ample seed rain. By contrast, we found that species effects on soils lead to weak to negligible feedbacks for Eriogonum arborescens, yet this shrub readily colonized the grasslands. Thus, plant-soil feedbacks quantified under natural climate and competitive conditions did not match native- plant recovery patterns. We also found that feedbacks changed with climate and competition regimes, and that these latter factors generally had stronger effects on seedling growth than species effects on soils. We conclude that even when plant-soil feedbacks influence the balance between native and exotic species, their influence may be small relative to other ecological processes. [ABSTRACT FROM AUTHOR]

Introduced species inevitably experience novel selection pressures in their new environments as a result of changes in mutualist and antagonist relationships. While most previous work has examined how escape from specialist enemies has influenced herbivore or pathogen resistance of exotic species, post-introduction shifts in exotic dependence on mutualists have not been considered. In a common environment, we compared dependence on AM fungi of North American and European populations of Hypericum perforatum (St. John's Wort), a forb native to Europe. Introduced North American populations responded less to inoculation with AM fungi than did European populations. Root architecture was strongly correlated with mycorrhizal response, and introduced populations had finer root architecture than native populations. Finally, introduced populations exhibited decreased root and increased reproductive allocation relative to European populations, consistent with a transition to a weedier life history; however, biomass allocation patterns were uncorrelated with mycorrhizal response. These findings are the first demonstration of a genetically based reduction of mycorrhizal dependence and shift in root architecture in an introduced species. [ABSTRACT FROM AUTHOR]

The extensive research on plant communities of natural-habitat islands has primarily focused on the "islands." The island analogy, however, potentially limits understanding of processes influencing composition on habitat islands because the nature of their matrix is overlooked. We determine how plant community structure of the surrounding matrix influences vegetation on volcanic outcrops in the modified landscape of Banks Peninsula, New Zealand. Our primary purpose is to address whether the matrix is more important for recently established exotic species than it is for well-established native species and whether such invasion by exotics has led to homogenization of the outcrop flora. To test this, we examined our data at three spatial scales: that of the entire outcrop flora, between individual outcrops and their immediate surrounding matrix, and between individual outcrop faces and the individual relevés of the immediate surrounding matrix. We found that 81% of the native flora and 90% of the exotic flora also occur in the matrix: This high level of species shared between the outcrop and matrix persists at the scale of individual outcrop faces (68% of the total flora of individual faces is shared with the matrix). We predicted that floras from different outcrops would vary in their distinctiveness from their immediate matrix. We found Bray-Curtis distance coefficient values to range from 0.26 to 0.64; these were even higher at the outcrop-face scale. Variability in outcrop distinctiveness relates primarily to the outcrop face properties of area, vegetation height, and soil depth, and matrix properties of vegetation structure and vegetation heterogeneity. The effect of the vegetation structure of the matrix is more pronounced on the exotic than on the native outcrop flora. The component of composition and structure of the matrix that was independent of outcrop properties and local environment accounts for similar levels of explainable variation in total and native composition (29-31%), but considerably more (40%) in composition of exotic species. Our results support our prediction that, as the surrounding matrix becomes more modified, invasion by exotics makes outcrop vegetation less distinct from its matrix. [ABSTRACT FROM AUTHOR]

Invasive species richness often is negatively correlated with native species richness at the small spatial scale of sampling plots, but positively correlated in larger areas. The pattern at small scales has been interpreted as evidence that native plants can competitively exclude invasive species. Large-scale patterns have been understood to result from environmental heterogeneity, among other causes. We investigated species richness patterns among submerged and floating-leaved aquatic plants (87 native species and eight invasives) in 103 temperate lakes in Connecticut (northeastern USA) and found neither a consistently negative relationship at small (3-m2) scales, nor a positive relationship at large scales. Native species richness at sampling locations was uncorrelated with invasive species richness in 37 of the 60 lakes where invasive plants occurred; richness was negatively correlated in 16 lakes and positively correlated in seven. No correlation between native and invasive species richness was found at larger spatial scales (whole lakes and counties). Increases in richness with area were uncorrelated with abiotic heterogeneity. Logistic regression showed that the probability of occurrence of five invasive species increased in sampling locations (3 m2, n = 2980 samples) where native plants occurred, indicating that native plant species richness provided no resistance against invasion. However, the probability of three invasive species' occurrence declined as native plant density increased, indicating that density, if not species richness, provided some resistance with these species. Density had no effect on occurrence of three other invasive species. Based on these results, native species may resist invasion at small spatial scales only in communities where density is high (i.e., in communities where competition among individuals contributes to community structure). Most hydrophyte communities, however, appear to be maintained in a nonequilibrial condition by stress and/or disturbance. Therefore, most aquatic plant communities in temperate lakes are likely to be vulnerable to invasion. [ABSTRACT FROM AUTHOR]

The Evolution of Increased Competitive Ability (EICA) hypothesis predicts that introduced plants should lose enemy resistance and in turn evolve increased size or fecundity. We tested the first prediction of this hypothesis by growing introduced North American and native European genotypes of St. John's Won (Hypericum perforatum) in common gardens in the state of Washington, USA, and in Girona, Spain. In both gardens we measured levels of hypericin and pseudohypericin (and in Washington, hypericide)- compounds known to be toxic to generalist pathogens and herbivores. In a third common garden, in Spain, we experimentally manipulated native pathogen pressure (by treating plants with fungicides) and quantified how pathogen resistance varied between North American and European genotypes. North American St. John's Wort had lower levels of hypericin than European conspecifics in common gardens in Washington and Spain. North American plants also produced less hypericide (in Washington) and pseudohypericin (in Spain) than did European plants. In Spain, individuals were attacked by three generalist pathogens: Colletotrichum sp. (Coelomycetes), Alternaria sp. (Hyphomycetes), and Fusarium oxysporum (Hyphomycetes), A higher percentage of individuals from North American populations were infected by pathogens and died from pathogen attack compared to European genotypes. Infection also appeared to reduce plant size and fecundity; these negative effects were similar in magnitude for North American and European genotypes. Taken together, results indicate that introduced St. John's Wort has lost enemy resistance. However, contrary to EICA, current and previous results indicate that these changes have not been associated with an increase in plant size or fecundity. [ABSTRACT FROM AUTHOR]

This dataset provides the Global Naturalized Alien Flora (GloNAF) database, version 1.2. GloNAF represents a data compendium on the occurrence and identity of naturalized alien vascular plant taxa across geographic regions (e.g. countries, states, provinces, districts, islands) around the globe. The dataset includes 13,939 taxa and covers 1,029 regions (including 381 islands). The dataset is based on 210 data sources. For each taxon‐by‐region combination, we provide information on whether the taxon is considered to be naturalized in the specific region (i.e. has established self‐sustaining populations in the wild). Non‐native taxa are marked as "alien", when it is not clear whether they are naturalized. To facilitate alignment with other plant databases, we provide for each taxon the name as given in the original data source and the standardized taxon and family names used by The Plant List Version 1.1 (http://www.theplantlist.org/). We provide an ESRI shapefile including polygons for each region and information on whether it is an island or a mainland region, the country and the Taxonomic Databases Working Group (TDWG) regions it is part of (TDWG levels 1–4). We also provide several variables that can be used to filter the data according to quality and completeness of alien taxon lists, which vary among the combinations of regions and data sources. A previous version of the GloNAF dataset (version 1.1) has already been used in several studies on, for example, historical spatial flows of taxa between continents and geographical patterns and determinants of naturalization across different taxonomic groups. We intend the updated and expanded GloNAF version presented here to be a global resource useful for studying plant invasions and changes in biodiversity from regional to global scales. We release these data into the public domain under a Creative Commons Zero license waiver (https://creativecommons.org/share-your-work/public-domain/cc0/). When you use the data in your publication, we request that you cite this data paper. If GloNAF is a major part of the data analyzed in your study, you should consider inviting the GloNAF core team (see Metadata S1: Originators in the Overall project description) as collaborators. If you plan to use the GloNAF dataset, we encourage you to contact the GloNAF core team to check whether there have been recent updates of the dataset, and whether similar analyses are already ongoing. [ABSTRACT FROM AUTHOR]