Introduction
Freshwater ecosystems play vital roles in nutrient and water cycling, biodiversity maintenance, and providing various ecological services to human society (Dudgeon et al. 2006, Carpenter et al. 2011, Rinke et al. 2019). Hundreds of exotic plants have invaded freshwater ecosystems around the world over the past centuries (Strayer 2010, Hussner 2012, Wang et al. 2016). These invaders reduce biodiversity, change community composition, habitat, and ecosystem function, interrupt ecosystem services, and degrade human health (Gallardo et al. 2016). Furthermore, the invasion of exotic aquatic plants is expected to become increasingly serious due to the rise in international commerce and global change (Seebens et al. 2017). Therefore, there is an urgent need to determine the factors that influence the invasion of exotic aquatic plants and their relative importance since it is necessary for the management of invaders and biodiversity conservation.
Numerous studies have examined the roles of various biotic and abiotic factors in the plant invasion process. Some studies have found that plant communities with higher species richness and abundance exhibit greater competitive resistance to the invasion of exotic aquatic plants (Levine et al. 2004, Capers et al. 2007, Michelan et al.2013, Petruzzella et al. 2018, Zhang et al. 2021a). The consumptive resistance of native generalist herbivores and introduced specialist enemies also reduces the establishment and performance of exotic aquatic plants (Parker and Hay 2005, Coetzee et al. 2011, Ribas et al. 2017). However, climate warming (Rahel and Olden 2008, Gillard et al. 2017), water eutrophication (Henry-Silva et al. 2008; Salgado et al. 2019), and disturbances (Trebitz and Taylor 2007; Quinn et al. 2011) are considered to facilitate aquatic plant invasions. Water depth is a crucial factor that determines the distribution, reproduction, and growth of exotic aquatic plants because it is closely related to light, temperature, dissolved oxygen, nutrient availability, and water movement (Gerhardt and Collinge 2003; Santos et al. 2011; Bornette and Puijalon 2011; Fleming et al. 2021). Additionally, in communities with multiple invasive plants, an invader is influenced by co-occurring invaders (Kuebbing and Nuñez 2015). The invasive plants may facilitate each other by suppressing shared resident competitors, increasing resource availability, providing protection from herbivores, enhancing pollinator visitation rates as magnet species, and promoting their dispersal and establishment as nurse species (Simberloff 2006, Tecco et al., 2007, Molina-Montenegro et al. 2008, Cushman et al., 2011, Flory and Bauer, 2014), or interfere with each other by competing for resources and pollinators (Belote and Weltzin 2006, Yang et al. 2011, Zhang et al. 2021b).
Evaluating the relative importance of various factors in contributing to exotic plant invasion is important (Eschtruth and Battles 2009, Bansal and Sheley 2016, Szymura et al. 2018). In freshwater ecosystems, native plant coverage has been found to account for most of the variation in exotic plant richness (Yu et al. 2018). Whereas few studies have quantitatively assessed the relative importance of various factors influencing the biomass of exotic aquatic plants. Different measures of exotic plants convey different information; richness indicates the number of species that have successfully invaded, while biomass quantifies the dominance and impact of exotic plants (Catford et al. 2012). Furthermore, because the living environment of four life-forms of aquatic plants (emergent, submerged, floating-leaved, and free-floating) varies greatly (Gallego et al. 2015, Hussner et al. 2021, Hu et al. 2021), the effect and relative importance of the same factors on different life-form exotic plants may differ. Additionally, freshwater ecosystems often experienced multiple invasions (Zhang et al. 2021b), a general understanding of the relative importance of various factors in the invasion process necessitates taking into account the overall performance of all invasive plants, as well as the performance of single invaders.
In this study, we surveyed the communities invaded by exotic plants in freshwater ecosystems in China. Various biotic and abiotic factors that may affect invasions were determined, including the richness, biomass, and coverage of native communities, water nutrient status, climate, habitat features, herbivory level, and anthropogenic disturbance. We used biomass to measure the overall invasion extent of all exotic plants in communities and the invasion extent of different life-form exotic plants, and employed regression analysis, structural equation modeling, and hierarchical partitioning to (1) examine which factors determine the invasion of exotic plants in freshwater ecosystems and how they work; (2) quantify the relative importance of these factors; and (3) determine the differences in the effects of the same factors on the overall invasion extent of all exotic plants and the invasion extent of different life-form exotic plants.