1. Introduction

Biological invasion is an issue of great concern on a global scale, with the number of invasive alien species increasing yearly (Board of Trustees of the Royal Botanic Gardens, 2017; IPBES, 2019). Invasive alien plants (IAS) threaten local ecosystems. Specifically, IAS have been shown to reduce the growth rate and species number of native species (Vila et al., 2011; Goossens et al., 2024; Sheppard and Lüpke, 2024; Trevenen et al., 2024). Consequently, one priority for research on invasion ecology is the elucidation of the mechanisms that drive invasion success (Lu et al., 2020; Xu et al., 2023; Park et al., 2024; Swierszcz et al., 2024).

Within a given plant community, multiple IAS can coexist, resulting in a co-invasion referred to as an invasion meltdown (Banerjee et al., 2023; Beshai et al., 2023; Li et al., 2024; Lone et al., 2024). Importantly, co-invasions may alter the structure and function of ecosystems, particularly with regard to the diversity of plant species and the invasibility of plant communities. Previous studies have shown that the effects of co-invasion can be additive, antagonistic, or synergistic, perhaps due to interspecific facilitation or interference between invasive species (Wang et al., 2020, 2022; Lone et al., 2024; Ahmad et al., 2025). Invasion by increasing numbers of species is predicted to affect total coverage and invasion intensity. Consequently, it is imperative to determine the effects of co-invasion on plant taxonomic diversity, community invasibility, as well as total coverage and invasion intensity. Similarly, research is needed to analyze how the number of invasive species affects plant taxonomic diversity on community invasibility.

Although for many species co-invasion is rare and limited to specific areas (Lone et al., 2024; Ahmad et al., 2025), in China species from the family Asteraceae frequently engage in co-invasion over extensive areas of the same habitat. Asteraceae invasive species are predominantly sourced from the Americas, i.e., they share a common evolutionary history (Yan et al., 2014; Wang et al., 2016). Studies have shown that IAS with a shared origin and greater trait similarity are more prone to co-invade a specific habitat than those with distinct origins and lesser trait similarity (Sheppard et al., 2018; Guo et al., 2024). Asteraceae co-invasions of two or three species have been found to generate novel challenges to environmental health and ecological security, especially the loss of the diversity of native plant species (Wang et al., 2020, 2023; Yu et al., 2023; Li et al., 2024).

Three Asteraceae species have been documented to co-invade plant communities in China, namely Erigeron canadensis, E. annuus, and Solidago canadensis. The period of maximum growth for these species typically occurs between April and August. Furthermore, the height of these three species is comparable, reaching appropriately 2–3 m. These three species are also found in similar habitats, including wastelands, agro-ecosystems, and areas near main roads in China. Research has shown that E. canadensis, E. annuus, and S. canadensis are capable of establishing expansive, monodominant communities with minimal latency following the successful invasion of their habitats in China (Li et al., 2024). Furthermore, these co-invasion by these Asteraceae species frequently occur in the same habitats (Figs. S1–S4).

The present study tests two hypotheses. First, we hypothesize that co-invasion by two or three Asteraceae species increases community invasibility and decreases plant taxonomic diversity of plant communities. Second, we hypothesize that the effects of co-invasion on plant diversity and community invasibility are correlated with the number of species invading the community. To test these hypotheses, we evaluated differences in plant taxonomic diversity, community invasibility, and total coverage, and invasion intensity of three Asteraceae species in plant communities invaded by one, two, or three species. We also estimated the influence of the number of invasive species on total coverage, plant taxonomic diversity, and community invasibility, as well as the influences of the total coverage of invasive plant species and plant taxonomic diversity on community invasibility in the invaded plots.

2. Materials and methods 2.1. Study design

To determine the effect of invasive Asteraceae species on plant communities, we surveyed several plots on the outskirts of Zhengjiang (119.47–119.69°E, 32.07–32.24°N). The geographical locations of sampling points in this study are shown in Fig. S5. The survey was conducted from May to August, 2022 to 2024. Habitats under investigation were mostly heathland that lacked tall trees. The dominant plant species were herbaceous. Plant plots with the same invasion scenario were separated by at least 100 m.

We compared plant plots (size: 2 m × 2 m) invaded by one, two, or three invasive species of the Asteraceae family to native community control plots. The following plots were surveyed: (1) control plots (no invasion by Asteraceae) (n = 221); (2) invasion by Erigeron canadensis (n = 275); (3) invasion by Solidago canadensis (n = 237); (4) invasion by E. annuus (n = 51); (5) co-invasion by E. canadensis and S. canadensis (n = 28); (6) co-invasion by E. canadensis and E. annuus (n = 28); (7) co-invasion by S. canadensis and E. annuus (n = 42); (8) co-invasion by S. canadensis, E. annuus, and E. canadensis (n = 31).

For each plot, species numbers, the relative coverage per plant species, and the number of individuals per plant species were determined. The number of individuals of clonal plant species was assessed by counting the number of ramets, because there is no noteworthy difference in the space colonized between one individual of non-clonal plant species and one ramet of clonal plant species if both have an equal or similar height (Wang et al., 2019a, 2019b, 2020; Zhang et al., 2024).

2.2. Determination of corresponding community variables

We assessed the effect of invasive Asteraceae species on native plant communities by measuring the relative coverage of plant species, plant taxonomic diversity (i.e., Shannon's diversity index, Simpson's dominance index, Pielou's evenness index, and Margalef's richness index), the degree of influence index of IAS on plant taxonomic diversity, the community invasibility index, the invasion intensity index of IAS, and the Pianka's niche overlap index. Calculation of indices is shown in Table S1.

2.3. Statistical analysis

Deviations from normality and homogeneity of the measured variables were evaluated through the application of the Shapiro–Wilk's test and Bartlett's test, respectively. The assumptions of the ANOVA model were met.

Differences in the values of plant taxonomic diversity, the degree of influence of IAS, the total coverage and invasion intensity of IAS, and community invasibility across different invasion scenarios were determined using a one-way analysis of variance (ANOVA) with the Tukey's test for multiple comparisons.

Correlations between the total coverage of IAS, community invasibility, plant taxonomic diversity, and the degree of influence of IAS on plant taxonomic diversity in the invaded plots was assessed by using the correlation analysis based on the Pearson's coefficient (r) (Hüsser, 2017; Runkler, 2025).

Path analysis based on the path coefficient (i.e., the standardized regression coefficient) was used to examine the influences of species number of IAS on total coverage, plant taxonomic diversity, and community invasibility, as well as the influences of the total coverage of IAS and plant taxonomic diversity on community invasibility in the invaded plots.

Statistical analyses were conducted by using IBM SPSS Statistics 26.0 (IBM, Armonk, NY, USA).

3. Results 3.1. Differences in plant taxonomic diversity indices

Plant diversity was lower in experimental plots invaded by one or two invasive Asteraceae species than in control plots (p < 0.05; Fig. 1a–e). Plant diversity was higher in experimental plots co-invaded by three invasive Asteraceae species than in experimental plots invaded by either one or two of the same species.

The degree of influence index for one and two invasive Asteraceae species was positive for all plant taxonomic diversity indices (Fig. 2a–e). When we examined single species invasions, the degree of influence index indicated that the invasive Asteraceae species with the highest impact on plant diversity was Erigeron annuus (p < 0.05). When we examined co-invasion by multiple species, the degree of influence index indicated that the number of invasive species with the lowest impact on plant diversity was three (p < 0.05).

3.2. Differences in the total coverage, invasion intensity index, and Pianka's niche overlap index of these three IAS as well as the community invasibility index

Total coverage of invasive Asteraceae species and community invasibility were highest following invasion by E. canadensis (p < 0.05; Fig. 3a and b). When plots were co-invaded by Asteraceae species, total coverage of invasive species and community invasibility were generally higher following co-invasion by three Asteraceae species than by either one or two species (p < 0.05).

Invasion intensity for Erigeron canadensis was lower when plot were invaded with two additional Asteraceae species than when invaded with E. annuus (p < 0.05; Fig. 4a). Invasion intensity for Solidago canadensis was lower when plots were invaded with E. canadensis than when invaded singly, with E. annuus, and with all three Asteraceae species (p < 0.05; Fig. 4b). E. annuus invasion intensity was higher when plots were invaded singly than when invaded with E. canadensis, with S. canadensis, or with all three Asteraceae species (p < 0.05; Fig. 4c). Pianka's niche overlap are shown in Fig. 5.

3.3. Correlations between the assessed variables

The total coverage of Asteraceae invasive species was positively correlated with the community invasibility (p < 0.0001; Table 1). Total coverage and community invasibility of Asteraceae invasive species were positively associated with the degree of influence index of Asteraceae invasive species on all plant taxonomic diversity (p < 0.0001). Accordingly, total coverage and community invasibility of Asteraceae invasive species were negatively correlated with all indices of plant taxonomic diversity (p < 0.0001).

3.4. The effects of the invasive species number and total coverage

Species number of co-invading Asteraceae species positively influenced total coverage, Shannon's diversity index, Simpson's dominance index, Pielou's evenness index, and community invasibility (p < 0.05; Fig. 6). Community invasibility of Asteraceae invasive species was influenced by total coverage, Pielou's evenness index, and Margalef's richness (p < 0.05). Community invasibility of Asteraceae invasive species was negatively influenced by Shannon's diversity index (p < 0.05).

4. Discussion

Previous studies have shown that invasive alien species reduce plant diversity, particularly in smaller plots (Carboni et al., 2021; Cheng et al., 2021; Wang et al., 2021; Beshai et al., 2023). In this study, we found that invasion by one and two alien species also decreases plant diversity. Furthermore, the degree of influence of one and two invasive species was positively correlated with several diversity indices (Fig. 2a–e). However, we found that co-invasion by three Asteraceae increases plant diversity in communities.

Invasive plants may reduce taxonomic diversity by decreasing community stability. This would be consistent with previous studies that found diversity and community stability are positively correlated (Li and Charnov, 2001; López-Villalta, 2008; Kang et al., 2020; Li et al., 2022). Low community stability, in turn, can increase community invasibility (Cheng et al., 2021; Wang et al., 2021; Beshai et al., 2023; Zhang et al., 2024). We found that community invasibility was negatively associated with plant taxonomic diversity (Table 1). In addition, community invasibility was positively associated with the degree of influence of invasive alien species on plant taxonomic diversity. These findings indicate that high plant taxonomic diversity creates more stable plant communities that are more resistant to plant invasion.

Communities with multiple species have greater population asynchrony. Over time, these populations become more stable because of compensatory fluctuations. In addition, overyielding also stabilizes productivity by increasing levels of average biomass production relative to temporal variability. Furthermore, communities with multiple species grow better together by increasing complementary effects when species interactions increase niche partitioning or facilitation. Species interactions may also decrease selection effects. High plant diversity can also mitigate the decrease in abundance of some species by increasing the abundance of another species. In communities with greater diversity, the abundance of competing species covaries negatively. Finally, temporal variance in species abundances scales with mean species abundance (Hector et al., 2010; Byun et al., 2013; Gherardi and Sala, 2015; Wang et al., 2019a).

In our experimental plots, the total coverage of alien species was high (Fig. 3a). Specifically, in plots with one and two alien species, the three alien species collectively occupied more than 50% of the area. In addition, community invasibility in experimental plots was relatively high (Fig. 3b), and was positively associated with total coverage of invasive plant species. Taken together, these findings indicate that the observed decline in plant taxonomic diversity may also be attributed to an increase of coverage of invasive alien species, which creates an asymmetry between invasive and non-invasive plant coverage, and an increase in invasibility (Table 1). This is consistent with our finding that the total coverage of invasive alien species positively regulates community invasibility (Fig. 6). One explanation for how greater alien species coverage affects invasibility is that it increases the competitive advantage of invasive alien species and decreases the competitive advantage of native species. Increased invasibility in communities ultimately results in the extinction of some native plant species.

Of the three invasive Asteraceae species examined, the most detrimental impact to plant communities was caused by Erigeron annuus. Plant communities invaded solely by E. annuus had the lowest levels of plant taxonomic diversity (Fig. 1a–e). In addition, the degree of influence of invasive alien species was highest for E. annuus (Fig. 2a–e). These findings are likely explained by the high total coverage of E. annuus, which increased invasibility as well (Fig. 3a and b). In addition, the invasion intensity of E. annuus was markedly higher than that of E. canadensis or S. canadensis (Fig. 4a–c). We recommend that early warning and prevention of plant invasion in Zhengjian prioritize E. annuus, although full monitoring of the distribution and spread of all invasive Asteraceae species should continue.

Co-invasion by three Asteraceae invasive species had a negligible impact on plant taxonomic diversity (with the exception of evenness) (Fig. 1a–e). Plant diversity was higher when communities were invaded by three invasive species than when they were invaded by one or two species. In addition, the degree of influence of the co-invasion of these three invasive alien species was generally lower than that of one or two such species (Fig. 2a–e). We also found that the number of invasive species introduced into plots positively regulated plant diversity, plant dominance, and plant evenness (Fig. 6). This finding is inconsistent with our hypothesis that co-invasion by two or three Asteraceae species increases community invasibility and decreases plant taxonomic diversity of plant communities. The increased plant taxonomic diversity triggered by co-invasion of three alien species may be the factor that allows these three invasive species to act as a positive driver rather than a passenger when they invaded togethe

Our finding that co-invasion by three Asteraceae species increases plant diversity indicates that co-invasion by three species may be antagonistic. This finding is inconsistent with the second hypothesis that the effects of co-invasion on plant diversity and community invasibility are correlated with the number of species invading the community. Instead, this finding indicates that when multiple IAS invade simultaneously, they may interact with each other, resulting in a phenomenon known as invasion interference. Invasion interference between multiple IAS can result in a reduction in their collective impacts on native plant communities. Invasion interference has been previously demonstrated in several studies (Lenda et al., 2019; Wang et al., 2020, 2023; Ferenc et al., 2021), and has been thought to be attributed to their competition for resources, which arises from interspecific and intraspecific competition increasing progressively as they invade together. This may also be more likely when co.

Although species with the same origin and greater similarity are more readily able to invade the same habitat and thus form a co-invasion (Sheppard et al., 2018; Guo et al., 2024), their similar species identities and ecological niches result in similar nutrient requirements, which in turn leads to a more intense competition for resources, as evidenced by the Gause's hypothesis and the niche differentiation hypothesis (Gause and Witt, 1935; Lambdon et al., 2008; D'Andrea and Ostling, 2017; Quiroga et al., 2021).

To date, little research has investigated the differences in the invasion intensity of the same IAS under different invasion scenarios. Interestingly, our results varied. Specifically, the invasion intensity of E. canadensis reached a high level under co-invasion by two IAS (Fig. 4a), whereas the invasion intensity of E. annuus reached a high level under single invasion facilitated by one IAS (Fig. 4c). The invasion intensity of S. canadensis reached a high level under single invasion facilitated by one IAS and co-invasion facilitated by three IAS (Fig. 4b). Taken together, these findings indicate the two key factors that significantly affect invasion intensity are species identity and number of invasive species. These two factors should be considered when developing measures to prevent and control the biological invasions by multiple species.

Several factors may complicate our findings. This study did not fully consider the effects of infiltration and migration between different invasion scenarios, differences in the growth stage, invasion stage, invasion history, or spatial distribution scale of IAS under different invasion scenarios. In addition, a variety of invasion scenarios may not be completely independent. Therefore, more comprehensive analysis is needed in the future to clarify the effects of co-invasion mediated by multiple IAS on native communities.

5. Conclusions

This study examined how the number of invasive Asteraceae species affects taxonomic diversity and community invasibility. We found that the invasive Asteraceae species that most affects plant taxonomic diversity is Erigeron annuus. In addition, total coverage of Asteraceae invasive species was positively associated with community invasibility. The total coverage of Asteraceae invasive species and community invasibility were negatively associated with plant taxonomic diversity, but positively associated with the degree of influence of IAS on plant taxonomic diversity. The total coverage of Asteraceae invasive species, plant evenness, and plant richness positively regulated community invasibility. The number of Asteraceae invasive species in an invasion was positively correlated with total coverage, plant diversity, plant dominance, plant evenness, and the community invasibility.

Acknowledgements

We are very grateful to the anonymous reviewers for the insightful and constructive comments that greatly improved the clarity of this manuscript. The study was funded by the following sources: State Key Research Development Program of China (2024YFF1307500), Open Science Research Fund of Key Laboratory of Ocean Space Resource Management Technology, Marine Academy of Zhejiang Province, China (KF-2024-112), Special Research Project of School of Emergency Management, Jiangsu University (KY-C-01), Jiangsu Collaborative Innovation Center of Technology and Material of Water Treatment (no grant number), and Research project on the application of invasive plants in soil ecological restoration in Jiangsu (20240110).

CRediT authorship contribution statement

Yingsheng Liu: Writing – review & editing, Methodology, Investigation, Data curation. Yizhuo Du: Writing – review & editing, Methodology, Investigation, Data curation. Chuang Li: Writing – review & editing, Methodology, Investigation, Data curation. Yue Li: Writing – review & editing, Methodology, Investigation, Data curation. Congyan Wang: Writing – original draft, Supervision, Project administration, Funding acquisition, Formal analysis, Conceptualization. Daolin Du: Writing – review & editing, Supervision, Project administration, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this manuscript.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.pld.2025.05.013.