Introduction
Globally, freshwater ecosystems have undergone massive changes and the
increasing loss of freshwater biodiversity is widespread (Palmer & Ruhí
2019; Reid et al. 2019). Floodplain ecosystems of large river are some
of the most heavily exploited and valued ecosystems and also among the
most modified and degraded (Bayley 1995; Vörösmarty et al. 2010;
Castello & Macedo 2016). Human modifications such as river regulation
and land reclamation have greatly changed the inundation regimes such as
extent, duration, timing and frequency (Poff et al. 1997, 2006; Olden &
Naiman 2010; Lu et al. 2018); and have diminished the benefits of
hydrological connectivity for vegetation (Guan et al. 2016), fishes
(Fontenot et al. 2001; Li et al. 2020), waterbirds (Xia et al. 2017) and
many other aquatic organisms (Kupferberg et al. 2012). Thus, floodplain
system restoration programs, such as environmental water allocation
(Stewart & Harper 2002), reconnecting floodplain with its rivers
(Pander et al. 2015) and returning croplands to wetland (Xu et al.
2018), remain a focus of regional biodiversity conservation.
Understanding the underlying abiotic and biotic assembly mechanisms
controlling temporal and spatial community structure and patterns is not
only a central issue in ecology (Cottenie 2005; Leibold et al. 2004;
Magurran et al., 2018 & 2019; McGill 2003; Ning et al. 2019), but also
has practical implications for biodiversity conservation (Economo 2011;
McGill et al. 2015; Yurkonis et al. 2005). Many mechanisms have been
hypothesized as explaining spatial and temporal variations in community
structure, i.e. the number of taxa (richness) and their relative
abundance (composition). The current advances in metacommunity theory
generally suggest that the two broad types of processes, i.e.
deterministic and stochastic, work simultaneously in shaping ecological
communities (Chase & Myers 2011; Gaston & Chown 2005; Gravel et al.
2006; Daniel et al. 2019). The deterministic assembly generally refers
to ecological niche-based mechanisms, including environmental filtering
(e.g., climate, pH, nutrients, and salinity) and biological interactions
(e.g., competition and facilitation) (Levi et al. 2019; Silvertown
2004). Considering that all species are ecologically equivalent in the
probabilistic sense (i.e. individuals have equal chances of reproduction
or death, Hubbell 2001; Rosindell et al. 2012), the stochastic
processes, however, concerns ecological processes that generate
community patterns indistinguishable from neutral simulations through
random birth– death events, probabilistic dispersal, and ecological
drift (random walk, either to extinction or monodominance) (Hubbell
2001; Gravel et al. 2006; Ning et al. 2019; Zhou et al. 2014). A range
of methods have been used to quantify the relative importance of
deterministic and stochastic processes in community assembly (Anderson
et al. 2011; Baselga 2010), of which the null model approach is the most
widely used (Mori et al. 2015; Mori et al. 2013). Through randomizing
the empirical community matrix (Gotelli 2000), null models can detect if
and by how much the observed variation in community structure (i.e.
β-diversity) deviates from the null expectation given a regional species
pool (i.e. gamma diversity) (Chase et al. 2011). Large deviations are
suggestive of deterministic processes driving community assembly while
close to zero deviations are interpreted as neutral community (Gotelli
2000; Tucker et al. 2016).
Research on the temporal dimensions of community ecology is becoming
increasingly urgent in the face of accelerating loss of biodiversity in
the Anthropocene (Dirzo et al. 2014; Garcia-Moreno et al. 2014; McGill
et al. 2015). The study of temporal dynamics can help to enhance our
predictions on the responses of communities to natural environmental
fluctuations and anthropogenic disruptions (Collins et al. 2000; Kéfi et
al. 2019; Rosset et al. 2017) by revealing how species respond to
community-level constraints (Musters et al. 2019) and how species
interactions change over time (Gonzalez & Loreau 2009; Hallett et al.
2014). This knowledge has strong conservation implications. For example,
identifying keystone communities that disproportionally contribute to
the maintenance of regional diversity is critical to conservation
planning (Ruhí et al. 2017). Many statistical metrics and analytical
tools, such as community stability (Tilman 1999), species turnover and
community change rate (Collins et al. 2000; Collins et al. 2008),
synchrony (Hallett et al. 2016) and temporal beta-diversity index
(Legendre 2019) have been developed to quantify and compare the temporal
dynamics.
Over the last two decades, studies on spatial variation in community
structure (i.e. spatial beta-diversity) have played a central role in
understanding how communities are organized along environmental
gradients, and how stochastic and deterministic processes influence the
functioning of ecosystems (Jia et al. 2020; Heino et al. 2015; Mori et
al. 2018). In comparison, far fewer studies have investigated the
temporal dynamics of communities (see Korhonen et al. 2010 and Jones et
al. 2017 for examples), limiting our understanding on how communities
respond to the widespread and rapid changes in the Anthropocene (Dirzo
et al. 2014; Ruhí et al. 2017), which may lead to contradictory
predictions (see Gonzalez et al. 2016 and Vellend et al. 2017). This
limitation is particularly significant for the highly variable
ecosystems, such as floodplains, where communities exhibit high intra-
and inter- annual variability in community assembly processes (Heino et
al. 2015).
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 collected monthly samples of benthic fish
communities in 12 sites, of which six are located at highly modifiedPopulus nigra plantations and six are from natural Carexsedges and open waters, over three consecutive years in a large
floodplain with high intra-annual water level fluctuations. Our main
purpose is to quantify the relative importance of deterministic and
stochastic processes on the temporal dynamics of fish community, and to
investigate if these processes differ between the modified and natural
habitats. In natural habitat, inter- and intra- annual variabilities in
exogenous physico-chemical factors rising from natural climatic and
environmental events, such as fluctuations of water level, pH, salinity
and nutrients, are relatively high in comparison with the modified
habitat (Bunn & Arthington 2002). Life histories of native biota,
especially the exploitative taxa, are finely tuned to capitalize on the
specialized temporal niches associated with this variability (Chase et
al. 2011). Consequently, temporal diversity can be promoted through an
array of mechanisms, such as migration to exploit resources and escape
competition (Shaw 2016), synchronous seasonal reproduction, and seasonal
fluctuations in abundance (King et al. 2003). Moreover, habitat
modification is likely to create dispersal barriers (Li et al. 2020),
which may reduce chance colonization by opportunistic species. Thus, our
working hypothesis is that the ecological stochastic processes will
prevail in the natural sites where dispersal is not constrained and
environmental variability is high; and the deterministic processes will
dominate the human modified sites, where dispersal is limited by
isolation and environment is relatively stable.