With the exponential rate of trade and globalization, alien plants invasion has attracted the attention of governments, scientists, environmentalists and biodiversity conservationists with regard to ecosystem imbalance and diversity loss, as well as economic loss (Bajwa et al. 2016; Pyšek et al. 2020; Schaffner et al. 2020; Vila et al. 2011). Previous studies have revealed that numerous biotic factors (e.g., parasitism, competition, microorganisms and species richness) influence the invasion success of alien plants (Li et al. 2012; Omer et al. 2022; Reinhart and Callaway, 2006; Zhang et al. 2020). However, as few studies integrate these factors to study this issue, our understanding of all the factors that interactively affect invasion is still limited.
It is inevitable that alien plants will compete for resources with native plants when they reach their non-native range. Elton proposed that alien plants had greater competitive ability than native plants, and numerous studies supported it (Elton, 1958; Ragan and Erik, 2000). For instance, a previous meta-analysis revealed that alien species possess greater suppressive and tolerance abilities when they coexisted with natives (Golivets and Wallin, 2018; Kuebbing and Nunez, 2016). Therefore, competition has been regarded as a crucial factor in determining plant resistance and is recognized as a fundamental mechanism underlying the successful invasion of alien species (Callaway and Aschehoug, 2000; Inderjit and Van Der Putten, 2010).
Apart from direct competition, the biotic resistance hypothesis postulates that communities with higher native plant richness maintain greater community stability and resistance to prevent alien invasion (Elton, 1958; Levine et al. 2004; Wang et al. 2020). Perhaps communities with higher native richness have narrow niches or accumulate more native-range pathogens to prevent the naturalization of alien plants (Zhang et al. 2020). Anthropogenic activities have created empty niches for alien plants through trade and construction, providing opportunities for entry. Moreover, the soil legacy effect of native richness has been overlooked. We predict that the soil legacy effect of richness may also inhibit alien plant invasion (Kuťáková et al. 2023).
Community resistance can also be regulated by the soil biota, but this topic is still unclear. The enemy release hypothesis predicts that alien species are not suppressed by pathogens due to the limited coevolutionary history of these organisms. Mitchell and Power (2003) found that invasive plants were actually released from fungal and viral pathogens in their non-native range. However, aliens would also be inhibited compared to native plants due to the limited number of mutualists, as confirmed by the Pinaceae invasion of Isla Victoria (Nuñez et al. 2009). Therefore, after the invasion of aliens, it remains unclear whether the soil microbial effects mentioned earlier is positive or negative.
Beyond soil microorganisms, the presence of aboveground parasitic plants may also play a crucial role in this process. Parasitic plants are widely occurred that extract nutrients, carbon and water from the host’s roots or stems, which can have direct negative impacts on the growth, reproduction and survival of host plants (Bardgett et al. 2008; Brunel et al. 2020). Numerous studies have revealed that parasitism can indirectly increase biotic resistance to invasive plants through host plants (Dunn et al. 2012; Prider et al. 2009; Těšitel et al. 2020). Thus, parasitism also influences the community resistance of alien plants.
More importantly, the above four factors, both above- and belowground, might interactively affect communities. For instance, soil microbes might influence diversity-invasibility (Zhang et al. 2020), plant‒plant interactions (Yan et al. 2022), and host-parasite interactions (Cai et al. 2023). Additionally, the competition between alien and native plants might also be influenced by factors such as parasitism (Mellado and Zamora, 2020) and soil legacy effects attributed to native plant richness (Li et al. 2023). Overall, further exploration is needed to understand how these four factors collectively shape community resistance.
Chromolaena odorata, a perennial herb, is a notorious invasive species in tropical and subtropical areas and has severely disrupted ecosystem biodiversity and integrity in China (Muniappan et al. 2009; Zheng et al. 2018). Cuscuta chinensis is a common holoparasite plant that obtains nutrients, water and other resources via the host stem and connected tissue from numerous wild or cultivated species (Donnapee et al. 2014; Jayasinghe et al. 2004). Some scientists have wanted to utilize C. chinensis to control its invasion and restrain its growth (Wan et al. 2010). We also observed that C. odorta was parasitized by C. chinensis in nature.
Here, we conducted a full factorial common garden experiment with C. odorta and two native plants (Artemisia leucophylla and Desmodium sequax) to address the following question: How do these four factors shape community resistance individually and interactively? We aim to explore whether the complex interactions within the community can successfully resist the invasion of alien plants. Simultaneously, this study contributes to a theoretical foundation for understanding the invasion of C. odorta.