4. DISCUSSION
Hydrologic connectivity disruption triggered by dams can affect flow
regimes, habitat and physicochemical parameters of river systems
(Tiemann et al., 2004; Yan et al., 2013; Hitchman et al., 2018).
Previous studies have shown that low-head dams may modify local habitats
in streams, including deeper water, slower flow, smaller substrate in
impoundments upstream, and faster flow and larger substrate in the
plunging areas downstream (Yan et al., 2013; Fencl et al., 2015; Li et
al., 2021). Furthermore, the PERMDISP analyses suggested that the local
habitat heterogeneity did not significantly differ between the
impoundments and free-flowing segments, and the habitat differentiation
in impoundments was slightly higher than that of the free-flowing
segments. Habitat conditions are highly heterogeneous along the
upstream-downstream gradients of streams, and the larger spatial scales
should encompass greater environmental heterogeneity (Vannote et al.,
1980; López-Delgado et al., 2019). When all spatial units share the same
range of habitat characteristics at relatively large scales, the common
changes of habitat conditions in impoundments
(i.e.,
water-depth increasing, flow slowing and substrate-size decreasing) may
reduce the spatial heterogeneity
and further cause habitat differentiation among different impoundments
(Bu et al., 2017). In contrast,
dams would result in greater environmental differences and induce
habitat differentiation across impoundments at smaller scales (Smith &
Mather, 2013; Liu et al., 2019), which is due to that the degree to
which local habitat is modified often varies with
dam
size and operation (Poff & Hart,
2002; Li et al., 2021). In this study, all the 53 low-head dams surveyed
were located within the first-order headwater streams in Wannan
Mountains areas, therefore, the slight habitat differentiation in
impoundments was observed at finer spatial scales.
As low-head dams are relatively small with weaker storage capability,
the extent and degree to which low-head dams altered the local fish
assemblages could be relatively weak (Yan et al., 2013; Fencl et al.,
2015; Brewitt & Colwyn, 2020). Although a fraction of native species
lost and native-invasive species gained in impoundments, a large number
of the remaining species (especially for abundant species) often tend to
decline or increase in abundance
(Tiemann et al., 2004; Liu et al.,
2019). In terms of species composition, twenty-nine species occurred in
both the impoundments and free-flowing segments, and just a few species
gained (C. idella and M. swinhonis ) and lost (L.
guilinensis and P. assimilis ) compared with that of free-flowing
segments (Table 1). However, changes in species abundance can also
affect the degree to which communities differ in composition despite
pairwise communities with the same species identities (Cassey et al.,
2008; Barwell et al., 2015). We also found the significant changes in
community composition between the impoundments and free-flowing
segments. Compared with that of free-flowing segments, some
native species such as Z.
platypus , Ctenogobius sp. , and V. stenosoma decreased in
abundances, and other native-invasive species such as M.
anguillicaudatus , O. potamophila , and C. auratusincreased (Table 2). Earlier works have shown that the transformation of
lotic to lentic habitats in impoundments created by low-head dams can
not only cause native fish species population declining and even local
extinguishing but also favor the occurrence and spread of native
invaders into upland streams (Chu et al., 2015; Smith et al., 2017; Liu
et al., 2019).
In this study, our results demonstrated the overall tendency of fish
taxonomic differentiation for abundance-based approaches in impoundments
relative to free-flowing segments (Figure 4a), which is consistent with
the slight habitat differentiation observed in impoundments.
Surprisingly, taxonomic homogenization detected is often observed
at relative large scales
(inter-ecoregion or basins), which is the results of both the
establishment of alien/cosmopolitan species and the loss of native
species (Toussaint et al., 2016; Daga et al., 2020; Kirk et al., 2020).
However, our results are not consistent with those studies at broader
spatial scales. First signs of changes in fish diversity patterns
affected by low-head dams may be mainly reflected in the abundance
changes (Tiemann et al., 2004; Fencl et al., 2015; Liu et al., 2019). If
the number of shared species between communities does not change, but
the abundances do, the Bray-Curtis index based on abundance approaches
will show changes that depend on the magnitude of shifts in abundance
(Cassey et al., 2008; Baselga, 2013). Apart from the decline in
abundance of shared dominant species (Z. platypus andCtenogobius sp.), formerly dominant species became rarer
(V. stenosoma ) and some unique species abundance markedly
increased (M.
anguillicaudatus and C. auratus ) in impoundments, which may
result in taxonomic differentiation of current fish assemblages based on
abundance approaches. Additionally, previous studies
also revealed that fish assemblages
experienced a transition phase from taxonomic differentiation to
homogenization over time (Petesse
&
Petrere, 2012; Pool & Olden, 2012). Given that
the alien/cosmopolitan species
introduction and colonization may take considerable time to negatively
affect native species (Ding et al., 2017), the observed taxonomic
differentiation of current fish assemblages is probably an early warning
sign of further homogenization.
We found that changes in functional similarity of fish assemblages
tended to demonstrate homogenization (Figure 4b), which showed opposite
trends to changes in taxonomic similarity. Such patterns are often the
result of the decrease in abundance of shared native species with unique
functional roles and the
establishment and colonization of alien/cosmopolitan species with
similar functional roles (Olden & Rooney, 2006; Pool & Olden, 2012).
Previous research in this region (Li et al., 2021) has shown that some
native species (i.e., typical of lotic species) could bring unique
functional traits, and
the
native-invasive and native species potentially have functional
redundancy. In other words, the native species (Z. platypus andV. stenosoma ) with unique functional traits decreased sharply in
abundance and some native-invasive species (M. anguillicaudatusand C. auratus ) increased, both of which contributed to
functional homogenization despite diverging taxonomies based on
abundance approaches. More importantly, even if a significantly positive
correlation was observed between changes in mean taxonomic and
functional similarities (Figure 5b), approximately a third of pairwise
assemblages presented contrasting outcomes (Table 3). The pattern of
fish faunal homogenization and differentiation depended in large part on
spatiotemporal changes in species abundance, and ecological functions of
those species were gained and lost among communities (Cassey et al.,
2008; Villéger et al., 2014; Socolar et al., 2016). Taxonomic
differentiation but functional homogenization can occur when shared
native species with unique functional traits decline in range or
abundance, whereas Taxonomic homogenization but functional
differentiation can occur when unshared species having similar traits
are lost (Villéger et al., 2014;
Campbell & Mandrak, 2020). This discrepancy between changes in
taxonomic and functional similarities could be explained by functional
redundancy among different species, which was supported by previous
studies (Villéger et al., 2014; Su et al., 2015).