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.