4.2 Environmental effects on fish assemblage
The present study showed fish assemblages in streams of the Ren River basin were structured by a combination of large-scale landscape factors (e.g., altitude and C-link) and local habitat features (e.g., channel width, water temperature and depth). Among which, landscape factors acted as the most important contributor in both wet and dry season. The importance of landscape position in structuring fish assemblages have been demonstrated by numerous studies (e.g., Fairchild et al. 1998; Smith and Kraft 2005; Li et al. 2012; He et al. 2017; Zhu et al. 2017). According to landscape perspective, streams should be regarded as connected networks with a definable “network geometry”, rather than a linear hierarchy most represented by stream order (Benda et al. 2004; Smith and Kraft 2005). Abiotic and biotic stream characteristics changed from low-order headwater streams to high-order downstream locations (Fausch et al. 1984; Smith and Kraft 2005). Specially, high altitude and low-order stream often refer to low water temperature, narrow channel, shallow water and low immigration (Murugavel and Pandian 2000.). Therefore, landscape position in stream network geometry can influence the distribution patterns of fish species and thus assemblage structure (Smith and Kraft 2005; Li et al. 2012). In this study, R .oxycephalus , a typical cold-water species, was restricted to headwaters and small tributaries with higher elevation, and absent from the lower locations. By contrast, species that adapted to warm-water and lentic environment (e.g., S. argentatus and H. leucisculus ) were distributed exclusively in the lower locations. The barrier caused by the extreme and harsh environmental conditions in high altitudes seems effectively prevented the dispersion and colonization of these fish species from lowland (Li et al. 2012).
Compared to above-mentioned landscape spatial factors, local habitat factors (e.g., channel width, water temperature and depth) played less important roles in determining fish assemblage in the Ren River basin. However, the influence of these factors increased somewhat in the dry season. In the wet season, water temperature and resource availabilities were not the limiting factors for warm-water fish species, such asS. argentatus , H. leucisculus and Z. platypus , because they were high enough to satisfy the ecological requirements of these species. However, As the water level and water temperature declined in winter, diversity and availability of habitat and food resources became limited (Liu et al. 2019). In this situation, these species tended to occupy larger and deeper habitats where possess warmer water temperature and richer food resources (Li et al. 2012).