1. INTRODUCTION
Anthropogenic
disturbances
accelerate the rates of species invasion and extinction, resulting in a
dramatic reconfigurationn of the current biodiversity (Toussaint et al.,
2014;
Villéger
et al., 2014; Zhang et al., 2019). These disturbances alter not only the
number of species in each locality (α-diversity) (Sax & Gaines, 2003)
but also the species composition among localities
(β-diversity)
(Villéger et al., 2011; Olden et al., 2018; Erős et al., 2020). More
importantly, the combination of species introductions and extinctions
can increase (i.e., homogenization) or decrease (i.e., differentiation)
the similarity among communities (McKinney & Lockwood, 1999; Olden &
Poff, 2004). Changes will continue to occur as new species become
established and others are lost, causing the formation of “novel
communities” (Hobbs et al., 2006), which will have notable ecological
and evolutionary implications (Olden et al., 2004).
Taxonomic homogenization and differentiation are typically quantified,
treating all species identically despite the fact that they may have
different ecological functions (Rahel, 2000; Taylor, 2010; Olden et al.,
2018). Functional diversity, as an important component of biodiversity,
represents
the variation of functional traits of all species within a community
that influences composition and ecosystem function (Tilman, 2001). Given
the
functional complementarity and/or redundancy among species
traits, functional diversity may not necessarily coincide with the
patterns of taxonomic diversity under anthropogenic disturbances (Pool
&
Olden, 2012; Villéger et al., 2014;
Campbell & Mandrak, 2020). For
instance,
Villéger et al. (2014) found that functional homogenization of
freshwater fishes in Europe exceeded taxonomic homogenization six times,
and approximately 40% of the paired communities showed taxonomic
differentiation, but functional homogenization. Furthermore, a pair of
assemblages that exhibit taxonomic differentiation
(i.e.,
an increase in dissimilarity) can actually be
functionally homogenized when
nonnative introduced species fill similar functional roles, such as
ecological redundancy of traits into new geographical locations (Olden
& Rooney, 2006; Pool & Olden, 2012). In contrast,
taxonomic
homogenization may be accompanied by functional differentiation due to
the loss of unique species with similar functional roles
(i.e.,
functionally redundant native species) in each community (Campbell &
Mandrak, 2020). Therefore, the analysis of changes in functional
dissimilarity is complementary to the assessment of changes in taxonomic
dissimilarity
(Villéger
et al., 2014). Assessing the combination of taxonomic and functional
components will permit a better understanding of the potential
consequences of biodiversity change on ecosystem functioning under
pressure.
Freshwater fish faunas are among the most intensively threatened by
anthropogenic disturbances such as nonnative fish introductions, dam
construction, habitat alteration and habitat fragmentation (Dias et al.,
2017; Reid et al., 2019; Couto et al., 2021). The influence of human
activities on fish diversity has become a primary environmental problem,
especially in dam construction (Brewitt & Colwyn, 2020; Couto et al.,
2021). Dams obstruct the dispersal and migration of organisms and modify
habitat conditions, both of which are directly linked to the patterns of
species speciation and invasion for the most part (Johnson et al.,
2008;
Petesse & Petrere, 2012; Smith et al., 2017). In addition, dams
can break down the geographic constraints from physical barriers by
allowing the dispersal of fish into systems outside their natural range
(Vitule et al., 2012; Clavero et al., 2013) and may contribute to biotic
homogenization through habitat changes and homogenization (Rahel, 2007;
Vitule et al., 2012; Daga et al., 2015). Low-head dams have a hydraulic
height < 15m and are typically overflow or spillway structures
(Poff & Hart, 2002; Brewitt & Colwyn, 2020), which are a ubiquitous
human disturbance in headwater streams (Jumani et al., 2020). Meanwhile,
similar to large dams, low-head dams fragment river ecosystems,
constrain the upstream movement of fish, create impoundments in place of
running water, and alter local habitat (i.e., water deepening, flow
slowing, and substrate size decrease) (Yan et al., 2013; Fencl et al.,
2015; Hitchman et al., 2018). Until now, whether low-head dams can
promote biotic homogenization/differentiation with the same ecological
effect as larger dams have received less attention (Bu et al., 2017; Liu
et al., 2019).
Previous studies on fish
homogenization/differentiation driven by low-head dams have primarily
focused on changes in taxonomic similarity of assemblages, but two
important knowledge gaps remain. First, whether the taxonomic and
functional components of fish assemblage similarities differ in their
responses to low-head dams remain poorly understood. Only accounting for
the taxonomic component may mask whether a community is functionally
saturated, when among communities have the same number of species but
may occupy different portions of functional space (Villéger et al.,
2014). Meanwhile, many circumstances such as geographic barriers and
harsh environmental conditions may obstruct whether a community become
functionally saturated (Głowacki & Penczak, 2013; Mateo et al., 2017).
Second, limited empirical evidence has showed that the varying role of
population abundance in structuring homogenization and differentiation
of fish assemblages (Cassey et al., 2008; Legendre, 2014); however, few
investigations have assessed the role of species abundance in defining
the trends towards homogenization/differentiation due to the limitation
on sampling and accessibility for abundance data (Cassey et al., 2008).
Although the incidence-based approaches may play a crucial role when
communities differ mostly in species compositions and are geographically
far apart on global or regional scales (Legendre, 2014), the
abundance-based approaches may exhibit higher sensitivity in structuring
homogeneous or heterogeneous patterns when species composition
differ primarily in species
abundance in small spatial extents (Cassey et al., 2008; Liu et al.,
2019; Dai et al., 2020). Indeed, the population size (i.e., abundance)
of the same species may differ substantially across locations, and the
number of individuals is not equally distributed across different
species in the same locations. Particularly at small scales, the
incidence-based approaches may underestimate the functions of dominant
species in biotic homogenization if species abundance data are
overlooked (Liu et al., 2019; Dai et al., 2020).
In the present study, based on the data collected from 53 impoundments
created by low-head dams (i.e., the treatment sites) and 53 free-flowing
segments blew low-head dams (i.e., the reference sites) within the
first-order streams (stream order, Strahler, 1957) in the Wannan
Mountains, China, we examined the effects of low-head dams on patterns
of fish faunal homogenization and differentiation in headwater streams
based on abundance data. We aimed to (1) quantify the spatial changes in
taxonomic and functional similarity of fish assemblages driven by
low-head dams and (2) examine whether the direct of changes in taxonomic
and functional similarity of fish assemblages differ in their responses
to low-head dams.