Discussion
In our study, competitive exclusion was the most important factor affecting plant invasion in freshwater ecosystems. Previous studies considered climate and habitat-related factors were the most important factors driving plant invasion at broad scales, the biotic interactions became more important at fine scales (Pearson and Dawson 2003, Milbau et al. 2009, Hortal et al. 2010, Bellard et al. 2016, Catford et al. 2017). However, our study showed that habitat characteristics, such as water depth, habitat size, and anthropogenic disturbance, had relatively weak impacts on the biomass of exotic aquatic plants, and just affected the latter indirectly by influencing native plants. Our surveys covered a vast geographical area and targeted communities that had been invaded by exotic species, which suggested that even at a large scale, once the exotic plants adapt to the local climate and environment and successfully establish their population, interspecific competition plays a more important role in their population expansion than climate and physical environment. The competition came from both native plants and co-occurring exotic plants. In fact, A. philoxeroides andE. crassipes experienced stronger competitive exclusion from co-occurring exotic plants than from native plants. Invasive plants are characterized by fast growth and strong competitiveness, therefore, the competition among invasive plants, especially plants with the same niche, is more intense (Van Kleunen et al. 2010, Zhang and van Kleunen 2019). The submerged plant C. caroliniana was not affected by the co-occurring exotic plants, because there is only one exotic submerged plant, Elodea nuttallii , in its distribution area, and their growth periods are different: E. nuttallii in winter and early spring, while C. caroliniana in summer and autumn (Yu 2017).
Unlike interspecific competition, we found that natural enemies did not inhibit the population expansion of exotic aquatic plants in the field. This may be because native generalist herbivores can only effectively control invasive plants when plant density is below some threshold, but they can’t bring sufficient herbivore pressure to limit exotic plants after plant density crosses the threshold (Maron and Vila 2001). Although some specialist herbivores are introduced as biological control agents for invasive aquatic plants (such as Neochetina eichhorniae and Agasicles hygrophila ) and reach a high population density (Jayanth 1988, Julien et al.1995), these specialist herbivores only establish populations and persist in sites where the abundance of the host plant is enormous. And they do not reduce plant performance once the plant develops a large population (Yu and Fan 2018). In addition, invasive aquatic plants often have strong compensatory abilities (Soti and Volin 2010, Lu and Ding 2012). Therefore, specialist herbivores may lower the population expansion speed of invaders but cannot eliminate or reduce the developed population. What’s more, in multi-invader communities, it is rare that specialist herbivores of each invasive plant appear and all plants are effectively controlled (Zhang et al. 2021b). Even if enemies can successfully control one invasive plant, the co-occurring invasive plants can still dominate the communities (Center et al. 2005, Hill et al. 2020). Taken together, top-down controls are not the primary drivers affecting the population dynamics of invasive plants in the field natural community (Davis 2009, Bohl Stricker and Stiling 2014, Silveira et al. 2018). Conversely, fragmentation by herbivory can facilitate the spread of some aquatic exotic plants (Ribas et al. 2017).
Our results showed that water eutrophication accelerated the invasion of exotic aquatic plants by directly favoring them and indirectly weakening the resistance of native plants. The growth and reproduction of exotic aquatic plants are enhanced in high nutrient availability environments (Hussner et al. 2009, Henry-Silva et al. 2008, Xie et al. 2010, Wersal and Madsen 2011). Excessive nutrients in water are harmful to submerged plants because a high density of periphyton and phytoplankton and low light availability caused by eutrophication can inhibit the growth of submerged plants, subsequently reducing the biodiversity, coverage area, and biomass of aquatic plants (Hautier et al. 2009, Arthaud et al.2017, O’Hare et al. 2018). However, when compared to the site-level mean biomass of exotic plants and the quadrat-level biomass of C. caroliniana , the relative effects of water nutrient status on the quadrat-level biomass of E. crassipes and A. philoxeroideswere weak. This may be due to these two plants’ inherent high growth rate, which enables them to colonize a large area and reach peak biomass at a fine scale in a short time, regardless of the nutrient concentrations (Edwards and Musil 1975, Pan et al. 2007). In addition, our results showed that the effect of water nutrient status on invasion was lower than that of community properties. Previous research has also found that the characteristics of the resident plant community are more critical than resource fluctuations in determining the invasion of exotic plants, which may be because the biotic resistance of native plants buffers the effects of nutrient enrichment on invasions (Walker et al. 2005, Teixeira et al. 2017). This finding indicates that as long as resident plants occupy enough space and resources, they can prevent plant invasions even when resources are in excess in the community.
We found that climate had completely different impacts on the overall performance of all exotic plants and the population performance of individual species. The optimal climates for various species are different, as a result, different exotic plants respond differently to climate change (Hoveka et al. 2016, Merow et al. 2017). In multi-invader communities, there must be one or some invaders that perform best under a given climate, and maximize the ecosystem’s productivity, so the overall performance of invaders does not vary in different climate regions. Whereas climate warming can facilitate the establishment and dispersal of exotic aquatic plants by altering streamflow and thermal regimes, reducing ice cover in waterbodies, and increasing water development activities (Rahel and Olden 2008). In addition, climate change has the potential to influence plant invasions by influencing native communities and co-occurring exotic plants (Li et al. 2017, Zhang et al. 2021b). Given the complexity of the effects of climate change on biological invasion, the study on predicting biological invasion under climate change should involve numerous invasive species, multiple measures of invaders, and the interactions between native and invasive plants, as well as among invaders.