3.3 Community similarity
Overall, both the taxonomic and functional similarities of fish assemblages differed significantly between the impoundments and free-flowing segments (P <0.001). Specifically, the taxonomic similarity of fish assemblages was significantly lower in impoundments than that in free-flowing segments, whereas the functional similarity of fish assemblages was generally higher in impoundments (Figure 4). Meanwhile, an opposite trend was detected between the changes in taxonomic and functional similarities, i.e. taxonomic differentiation (taxonomic ΔCS: −2.09% ± 21.89%) was accompanied by functional homogenization (functional ΔCS: 4.05% ± 13.03%) in stream fish assemblages.
Mantel tests showed that changes in taxonomic and functional similarity were positively correlated (r =0.33, P <0.001; Figure 5b). Furthermore, Chi-square tests showed that the observed percentages of taxonomic ΔCS and functional ΔCS (i.e. differentiation and homogenization) for pairwise assemblages were statistically significant (χ2 =140.1, df =1,P <0.001). Among the 1,378 pairwise assemblages, 64.73% (892 comparisons) showed uniform changes between taxonomic ΔCS and functional ΔCS, of which 36.14% pairs (quadrant I: 498 comparisons) presented both taxonomic and functional homogenization and 25.11% pairs (quadrant III: 394 comparisons) showed differentiation (Figure 5b, Table 3). However, 35.27% (486 comparisons) showed differences between taxonomic ΔCS and functional ΔCS. Specifically, 25.11% pairs (quadrant II: 346 comparisons) showed taxonomic differentiation but functional homogenization, and 10.16% pairs (quadrant IV: 140 comparisons) presented the opposite changes (Figure 5b, Table 3).