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.