Beta diversity patterns of fish and conservation implications in the Luoxiao Mountains, China

Abstract The Luoxiao Mountains play an important role in maintaining and supplementing the fish diversity of the Yangtze River Basin, which is also a biodiversity hotspot in China. However, fish biodiversity has declined rapidly in this area as the result of human activities and the consequent environmental changes. Beta diversity was a key concept for understanding the ecosystem function and biodiversity conservation. Beta diversity patterns are evaluated and important information provided for protection and management of fish biodiversity in the Luoxiao Mountains. The results showed that the spatial turnover component was the main contributor to beta diversity of Hemiramphidae, Amblycipitidae, Catostomidae, Clariidae, Balitoridae and Percichthyidae in the Luoxiao Mountains, which indicated that a number of protected areas would be necessary to conserve fish biodiversity and that these families would need conservation measures. Most protected areas are currently limited to some regions; therefore, in order to protect fish diversity, conservation efforts must target an increase in the number of protected areas which should be spread across each of the regions.


Introduction
Biodiversity patterns and their formation mechanisms have been one of the hot issues, and it is also an important foundation for conservation (Kennedy and Norman 2005;Sutherland et al. 2009). Biodiversity is important for the future sustainability of freshwater natural resources (Hiddink et al. 2008). While it is axiomatic that biodiversity is essential for sustainable productive fisheries there is surprisingly little supporting evidence (Dulvy et al. 2000;Hilborn et al. 2003). Freshwater fishes are among the most diverse assemblages on Earth, which provide important economic value (e.g., nutrition) and valuable ecosystem services (e.g., natural water filtration; Naylor et al. 2000;Cressey 2009;De Silva 2012). However, due to dam construction, overfishing (commercial fish fishing), pollution, deforestation, and other human activities, fish numbers have declined rapidly in global terms (Fu et al. 2003;Arthington et al. 2016;Liu et al. 2017) and they are thus one of the most threatened assemblages.
Beta diversity is an important tool for conservation planning (Anderson et al. 2006); knowledge on beta diversity patterns can aid the decision on the number of protected areas needed and their sizes (Margules and Pressey 2000;Wiersma and Urban 2005). Beta diversity can be decomposed into species turnover (species replacement) and nestedness (richness difference; Baselga 2010; Carvalho et al. 2012). The species turnover component (species replacement) is the replacement of some species by others leading to a low number of shared species among two communities where turnover is high (Baselga 2010). In addition, the nestedness component (richness difference components) represents the differences between two communities only in terms of species richness, with the poorer community as a subset of the richer one (Baselga 2010). According to the percentage of spatial turnover and nestedness components in total beta diversity, different conservation strategies can be selected. If species turnover is the main component of beta diversity, a larger number of protected areas would be necessary to conserve regional biodiversity (Baselga 2010;Carvalho et al. 2012). If the nestedness is the main component of beta diversity, one large protected area comprising a high species richness could be sufficient (Baselga 2010;Carvalho et al. 2012).
The Luoxiao Mountains range is located in the southeast of China's mainland and has a long history and complex environmental factors (Liao et al. 2014;Wei et al. 2015). The northern part of the mountains is connected with the Yangtze River, and the southern part is connected with the Nanling Mountain (Gong et al. 2016). It is the most important ecotone and fragile zone in the third step of eastern China, and is an important channel for the migration and diffusion of terrestrial organisms in the Northern Hemisphere (Liao et al. 2014;Gong et al. 2016). In addition, the Luoxiao Mountains is also a biodiversity hotspot in China (Liao et al. 2014;Gong et al. 2016). At the same time, as being the watershed of the Poyang Lake Basin and the Dongting Lake Basin in the middle reaches of the Yangtze River, the Luoxiao Mountains are a refuge to many endemic and endangered fishes (Liao et al. 2014;Gong et al. 2016). Therefore, fish resources of the Luoxiao Mountains play an important role in maintaining and supplementing the aquatic biodiversity of the Yangtze River Basin. However, due to dam construction, overfishing, pollution, deforestation, and other human activities, fish diversity declined rapidly in this region. Here, we aim to evaluate beta diversity patterns and to provide useful information for the protection and management of fish biodiversity in the Luoxiao Mountains.

Study area
The Luoxiao Mountains (25°32'-29°28'N, 113°09'-114°26'E) are a large system of mountain ranges, located in the southeast of China's mainland with an overall north-south trend, stretching across Hubei, Hunan, and Jiangxi provinces. It consists of Mufu Mountain, Jiuling Mountain, Wugong Mountain, Zhuguang Mountain, and others. The total length of the Luoxiao Mountains is 400 km and altitude ranges are 82-2120 m. Lingfeng Peak (2122 m) is one of the highest mountains in the southeastern Eurasia. Its average precipitation range is 1341-1943 mm and forest coverage in the watershed reaches 90% (Table 1). The tributaries of the Ganjiang River from the eastern stream of the Luoxiao Mountains flow into Poyang Lake. The tributaries of Xiangjiang River from the western stream of it flow into the Dongting Lake. The Fushui River alone flows into the Yangtze River.

Sampling methods
Sampling sites were selected by considering habitats, variations, and anthropogenic activities in the Luoxiao Mountains. Fish samples were collected from April 2014 to 2017 in eleven streams of the Luoxiao Mountains. We selected eleven streams (42 sampling sites) (Figure 1), including the (1) Fu River (sampling code is FR; three sampling sites), Xiuhe River (sampling code is XH; six sampling sites), Jinjiang River (sampling code is JJ; three sampling sites), Yuanshui River (sampling code is YS; three sampling sites), Heshui River (sampling code is HS; three sampling sites), Shushui River (sampling code is SS; two sampling sites), Suichuan River (sampling code is SC; five sampling sites), Shangyou River (sampling code is SY; three sampling sites), Miluo River (sampling code is ML; four sampling sites), Liuyang River (sampling code is LY; four sampling sites), Mishui River (sampling code is MS; six sampling sites). We collected the fish catch from professional fishermen who captured fish using fully standardized five gillnet clusters, each consisting of six gillnets of 50-80 m in length 4-10 m in height (mesh size = 1.0-10.0 cm) in the Luoxiao Mountains rivers. In addition, we assumed similar capture efficiencies from gillnet samples at each site. At the same time, we surveyed and collected fish in the township markets along the river which enhanced the species checklists at each section. All fish specimens were identified according to Chen (1998), Chu et al. (1999), and Yue (2000), and the scientific name was corrected according to Fishbase (http://www.fishbase.org/search.php). The division of endangered categories of fish was decided according to Jiang et al. (2016) and IUCN (2017).

Data analysis
Beta diversity is represented by the difference in species composition between different communities, which was determined by species turnover (species replacement) and nestedness (richness difference; Baselga 2010; Carvalho et al. 2012). In order to quantify the effects of two processes, Baselga (2010) systematically proposed the beta diversity decomposition method (BAS frameworks) based on the Sørensen index (β sor ), which was decomposed into species spatial turnover components (β sim ) and nestedness components (β sne ). Podani and Schmera (2011) and Carvalho et al. (2012) proposed the beta diversity decomposition method (POD frameworks) based on the Jaccard index (β jac ), which was decomposed into species replacement components (β -3 ) and richness difference components (β rich ). Here, we analyzed the fish biodiversity based on both the BAS and POD frameworks.

BAS frameworks (Sørensen index):
POD frameworks (Jaccard index): where a was the number of shared species among two streams, and b and c were the number of species only present in the first and second stream, respectively. Sørensen and Jaccard indices ranged from 0 to 1, representing respectively no species and all species in common among the two streams (Appendix 1). A principal component analysis (PCA) was performed separately based on Sørensen and Jaccard indices to visualize patterns of fish assemblages among the Luoxiao Mountains rivers (Legendre and Legendre 2012). PCA results were then analyzed using R 3.2.0 version (R Development Core Team 2014) and using the "ade4" package (Dray and Dufour 2007).
We performed Mantel tests and partial Mantel tests (Legendre and Legendre 2012) with 9999 permutations to assess the correlations (Spearman's method) between eight pairwise similarity matrices and the matrices of geographical drivers (difference in area, length, average precipitation, annual average runoff, average altitude, average gradient and average temperature among streams; Table 1) to explore the po- tential mechanisms that explained beta diversity patterns. The partial Mantel tests were used to remove the effect of covariation because an inter-correlation between matrices of difference in area, length, average precipitation, annual average runoff, average altitude, average gradient and average temperature was detected (P < 0.05). All the analyses were performed in R 3.2.0 (R Development Core Team 2014) using the packages BAT (Cardoso et al. 2015), BETAPART package (Baselga and Orme 2012) and VEGAN (Oksanen et al. 2015).

Fish species composition
The fish specimens sampled and identified in the Luoxiao Mountains were categorized into 113 species and 17 families ( Figure 2; Appendix 2). The number of Cypriniformes was the greatest, accounting for 68.1% of the total number of fish species, followed by Siluriformes and Perciformes, accounting for 14.2% each, and Beloniformes, accounting for 0.1% (Figure 2). In addition, according to the endangered categories of the Jiang et al. (2016), Least Concern fish species were the greatest, accounting for 77.9% of the catch (Appendix 3). Critically Endangered, Vulnerable, and Near Threatened fish species accounted for 7.1% (Appendix 3).
The PCA showed that fish composition similarity of LY, SS, SY, XH, and MS were similar based on BAS and POD frameworks; FS, JJ and SC were similar; HS and ML were similar; and YS was uniquely divided into other areas, respectively (Figure 3).   We found almost no significant effects of geographical drivers on overall beta diversity for the Luoxiao Mountains (Table 4). The correlation between BAS and POD frameworks and difference in length, average precipitation, mean temperature, and average gradient were not significant in the Luoxiao Mountains. The correlation between β sne (β rich ) and differences in area and annual average runoff was significant. The correlation between β sim (β -3 ) and difference in average altitude was also significant (Table 4).

Fish species composition
Studies on fish composition and diversity in streams is the basis for the conservation and management of stream fishes (Liu et al. 2017;Zhang et al. 2018). In this study, the fish specimens sampled and classified in the stream of the Luoxiao Mountains were categorized into 113 species. Compared with species numbers of the Shiwanda Mountains (102 species; Zhao and Zhang 2001), Wuyi Mountains (117 species; Song et al. 2017), and the Tibetan Plateau (114 species; Wu and Tan 1991), the fish abundance in the Luoxiao Mountains was also higher.

Beta diversity patterns
Abiotic and biotic factors and their ecological processes in different stream sizes varies substantially (Zhang et al. 2018). At least in streams, local species richness of fishes, habitat diversity and complexity often increase in large streams (Roberts and Hitt 2010;Zhang et al. 2018). Comparing alpha diversity and beta diversity at local and landscape scales is an important, yet little-understood, area of basic and applied ecological research (Kessler et al. 2009). However, most studies on fish diversity of streams have focused on alpha diversity, whereas fewer studies have investigated beta diversity (Tisseuil et al. 2013;Johnson and Angeler 2014). Knowledge of beta diversity patterns can go beyond the systematic conservation planning method that only considers the location of protected area in relation to natural physical and biological patterns (Margules and Pressey 2000;Wiersma and Urban 2005). The efficiency of protected areas not only relies on species richness, but also on how well the complementarity among sites increases biodiversity conservation (Howard et al. 1998;Bush et al. 2016;Socolar et al. 2016). In this study, as turnover brought the larger contribution to beta diversity, additional conservation efforts must target an increase in the number of protected areas, which should be spread across each one of the regions, to maximize the protection of species diversity.

Biogeographical processes
The modern freshwater fish fauna of Eurasia originated in the early Tertiary (Chen et al. 1986;Liu and Quan 1996;Zhang 2012). At the same time, the primitive species of the Danioninae and Barbinae became the main component of the fish fauna with the flattened land and the warming climate (Chen et al. 1986;Tang et al. 2001). During the dramatic changes of landscape and climate of the Eurasian continent in the late Oligocene and the end of the Pliocene, the primitive species component had been reduced rapidly (Chen et al. 1986;Tang et al. 2001). After the Quaternary ice age, only some offspring fishes of the old Tertiary Period remained (Chen et al. 1986;Tang et al. 2001). Moreover, Labeoninae, Gastromyzontidae, Balitoridae and Sisoridae were dominant during the uplift of the Tibetan Plateau (Chen et al. 1986;Yang et al. 1982;Tang et al. 2001). At the same time, a large area of alluvial plains appeared in eastern China, and special habitats were created under the influence of the East Asian monsoon (Chen et al. 1986;Zhang and Chen 1997). The cold-water fishes, such as Leuciscinae and Gobioninae became the endemic fishes of the river plain in East Asia (Hypophthalmichthyinae, Culterinae, Xenocyprininae, Acheilognathinae, Gobiobotinae) and the warmwater fishes the endemic fishes of Southeast Asia (Botiinae, Clariidae, Amblycipitidae, Belontiidae, Channidae, Mastacembelidae). Since then, these taxa have become the major faunal component in southern China (Chen et al. 1986). In this study, Culterinae, Gobioninae, and Acheilognathinae had a high species composition (Appendix 2). At the same time, the spatial turnover component is the main contributor of beta diversity in Hemiramphidae, Amblycipitidae, Catostomidae, Clariidae, Balitoridae, and Percichthyidae, indicating that it would be necessary to conserve habitats in the Luoxiao Mountains.

Threats to fish diversity
The headwater stream is a tributary of a larger river, which is often located in a mountainous area with high altitude. Compared with large rivers, it had relatively simple habitat structure, poor nutrition, obvious hydrological change, and low species diversity (Vannote et al. 1980;Grossman et al. 1990;Zhang et al. 2018). Therefore, the ecosystem of the stream is more fragile, its resistance to external disturbance and resilience is lower, and it would be more difficult to recover once it is damaged by humans. Fish, as the apex consumers of the stream, are very important to the stability and functioning of the stream ecosystems (Nogueira et al. 2010;Yan et al., 2011;Arthington et al. 2016;Liu et al. 2017). During the long evolution process, fishes have gradually adjusted their corresponding morphological characteristics, phenological rhythms, and life history countermeasures so that they could adapt to the unique natural environment of the stream (Lytle and Poff 2004;Osorio et al. 2011;Ren et al. 2016). However, due to habitat loss, water pollution, alien-species invasions, forest overcutting, climate change, overfishing etc., the fish biodiversity of most streams in China have been seriously threatened (Dudgeon et al. 2006;Allan and Castillo 2007). For example, numerous small dams in mountain streams were established (Huang et al. 2008;Hu et al. 2009). Dam constructions modified these small fast-flowing streams, which led to the decline of fish species adapted to rapid streams (Hu et al. 2009). In addition, a large number of fishing methods such as traps, gill nets, and electro-fishing has led to overfishing which has also caused a dramatic decline in fish biodiversity (Huang and Gong 2007;Zhang et al. 2010). Heavy metal pollution has affected the aquatic ecosystem in the Luoxiao Mountains (He et al. 1998). The contents of heavy metals have greatly exceeded the recommended standards (Xu et al. 2016). In this study, critically endangered (Myxocyprinus asiaticus), vulnerable (Leptobotia elongata, Pseudobagrus pratti, Liobagrus marginatus, Siniperca roulei), and near threatened (Onychostoma barbatulum, Siniperca obscura, Siniperca undulata) fish species accounted for 7.1% of the species recovered. At the same time, the PCA results showed that the fish composition among the streams sampled in the Luoxiao Mountains were similar. As turnover brought the larger contribution to beta diversity, additional conservation efforts must target an increase in the number of protected areas, which should be spread across each of the regions, to maximize the protection of species diversity.

Conservation implications
Freshwater fishes were thought to be the world's most threatened group of vertebrates after amphibians (Bruton 1995;Hiddink et al. 2008;Liu et al. 2017) and, without protection, 20% of the world's freshwater fishes may become extinct in the next 50 years (Moyle and Leidy 1992;Fu et al. 2003). Although endangered fish have raised public awareness, conservation strategies of fish biodiversity in China are concentrated on endangered species and economic fish (Fu et al. 2003;Liu et al. 2017). In addition, protected areas mainly occur in terrestrial conservation strategies, but freshwater habitats are commonly protected only incidentally as part of their inclusion within terrestrial reserves (Huang et al. 2013). For example, conservation areas of plants, animals, and wetlands in Jiangxi Province have been established, but there are very few freshwater protected areas nor are there any fish passage facilities in the rivers (Huang et al. 2013). In this study, species turnover component is the main pattern of beta diversity, implying that a larger number of protected areas would be necessary to conserve the regional biodiversity in the Luoxiao Mountains. Therefore, in order to protect fish biodiversity, the establishment of freshwater protected areas in the streams of the Luoxiao Mountains should be considered.