Discussion
Despite decades of study,
quantifying the relative importance of deterministic versus stochastic
processes in natural community assembly remains a key challenge to
ecologists (Dini-Andreote et al. 2015; Tucker et al. 2016), especially
in dynamic species-rich landscapes (Ruhí et al. 2017). In this study, we
investigated the temporal dynamics of the benthic fish communities in
two habitat types (i.e. natural vs modified) within a large
river-floodplain system over a water level rising-and-drawdown cycle for
three consecutive years. We found distinct assembly processes operating
between habitat types, contributing to the variation in observed
community dynamics, such as temporal β-diversity community stability,
and species synchrony. Moreover, our results suggested that the nutrient
gradients strongly mediate the community assembly processes (Donohue et
al. 2009), exhibiting significant effects on temporal community
dynamics.
Distinct normalized ecological stochasticity in natural and
modified habitats
Null models have become a valuable tool to infer assembly processes from
spatial variations in community structure (Chase et al. 2011; Mori et
al. 2015). However, few studies have applied null models to explore the
temporal dimension of community turnover. Here, we used the NST based on
a null modelling approach (Ning et al. 2019) to quantify the ecological
stochasticity using monthly samples of benthic fish community. NST
reflects the contribution of stochastic assembly relative to
deterministic assembly, providing a better quantitative measure of
stochasticity than other randomization-based measurements such as
standardized effect size (Ning et al. 2019). While the mean NST was much
less than 0.50 in the modified sites, it was greater than 0.50 at the
natural sites. These results suggested that deterministic processes,
either environmental filtering or biotic interactions or both
(Dini-Andreote et al. 2015; Ning et al. 2019) were the main mechanisms
underlying the temporal variation in community structure in the modified
habitat. In contrast, in the natural sites especially in open waters,
stochastic processes, i.e. demographic (e.g. probabilistic dispersal and
random birth - death events) and environmental stochasticity (e.g.
changes in water level, pH and water clearance) (Chase & Myers
2011;Shoemaker et al. 2020) could be the prevailing processes driving
the temporal structural changes in fish communities.
The lower stochasticity (thus, higher deterministic forcing) in modified
habitats could be attributed to many interconnected abiotic and biotic
factors. First, modification reduces hydrological connectivity (Li et
al. 2020), which depresses temporal variation and creates relatively
stable environmental conditions. In turn, the stable conditions might
trigger environmental filtering and biotic interaction, such as
competition. Second, habitat modification cleared submerged macrophytes
through excavation of sediment (field observation by authors), leading
to further environmental homogenization in terms of structural
complexity. Third, modification could impose dispersal limitation
through habitat isolation. Dispersal (e.g., random chance for
colonization) can be considered as a more stochastic process that
induces deviations from expectations based solely on niche theory (Chase
and Myers 2011). Studies have shown that ecological connectivity and
dispersal play a central role in structuring communities (Cadotte et al.
2006; Vellend et al. 2014). Our findings demonstrated the importance of
considering the ensemble of local communities as an integrated
metacommunity, and approach which improves our understanding of the
processes maintaining biodiversity in complex dynamic landscapes.
Stochasticity is positively related with temporal
β-diversity, synchrony and positive covariation
Previous studies have demonstrated that human disruption, such as
habitat modification, could cause biotic homogenization (Iacarella et
al. 2018), leading to reduced spatial β-diversity in fish communities in
floodplain environments (Quintero et al. 2010; Li et al. 2020). In this
study, we found significantly lower temporal β-diversity of benthic fish
communities in the modified plantations than in the natural Carexsedges and open waters. Moreover, a significantly positive relationship
between ecological stochasticity and total temporal β-diversity was
confirmed.
Although the difference in species variance rate between habitat types
was not significant, species synchrony in communities was generally
higher in natural habitats than in the modified sites. In addition, both
species synchrony and variance rate were significantly positively
related to NST. The higher synchrony in natural habitats can be
partially explained by life history theory (Mims & Olden 2012). Native
fish have developed life history strategies adapted to the natural
inter- and intra- variations in hydrology (McManamay & Frimpong 2015).
Therefore, the life cycles of native fish, such as reproduction, larval
and juvenile survival, migration and movement of young and mature fish,
are synchronized with long-term hydrological regime (King et al. 2003).
In the plantation, to ensure the survival of young trees, the natural
hydrological regime was greatly modified, especially during low water
periods (Li et al. 2020), resulting an interruption to how native biota
respond to environmental cues. There is general agreement that dispersal
can synchronise spatially distinct subpopulations (Goldwyn & Hastings
2008; Vogwill et al. 2009). Therefore, the lower species synchrony in
modified habitats could also be attributed to the dispersal limitation.
The community stability, measured by the time series of a univariate
index (the sum of fish abundance), was significantly higher in the
modified habitat than in natural habitats. It had significantly negative
relationships with NST, species synchrony and variance rate. As we have
shown that ecological stochasticity could lead to high synchrony and
positive covarying (i.e. abundance changes in the same direction) among
fish species in river-floodplain system, these negative relationships
suggested that temporal asynchrony (i.e. compensatory dynamics, as in
Gonzalez and Loreau 2009) helps to stabilize abundance fluctuations in
the modified habitats. Thus, the persistence of the generalists such asCarassius auratus was higher at the modified plantation sites,
where environmental conditions are relative stable. As the use of a
single univariate index can potential give misleading impression of the
overall picture of community stability (Death & Winterbourn 1994), time
lag analysis using community distance (Collins et al. 2000 & 2008;
Hallett et al. 2016) can provide insights into community stability
(Jones et al. 2017). However, the duration of our dataset is not long
enough to have a meaningful analysis using this approach.
Nutrient gradients mediate community temporal
dynamics
Nutrient enrichment affects temporal dynamics of biotic communities
(Cook et al. 2018; Avolio et al. 2014), and many studies documented that
elevated nutrient levels cause biotic homogenization, reducing the
temporal β-diversity of biotic communities, such as plants (Zhou et al.
2020) and macroinvertebrates (Huttunen et al. 2020). We found
significant relationships between nutrient level (TN or TP) and
community temporal dynamics. Species synchrony and variance rate had
negative relationship with TN whereas the temporal β-diversity decreased
significantly with TP. Note that TP and TN were highly correlated
(Pearson’s r = 0.73), and we used the variable with the closer
relationship in all models. Johnson and Angeler (2014) also found the
negative relationship between TP and temporal β-diversity in a
meta-analyses of European fish community diversity. The low phylogenetic
diversity and loss of functional diversity in high nutrient conditions
might be mediated by periodically hypoxic conditions in floodplain
settings (King et al. 2012) or the responses of food sources such as
benthic diatoms (Allen 2004; Johnson and Angeler 2014).