The role of the environment and connectivity with large rivers and streams on local fish diversity of tropical headwater streams

The aim of this investigation was to determine whether fish diversity in headwater streams is influenced by environment, connectivity with large rivers via source-sink dynamics, and/or by an interchange of individuals with other stream assemblages. We measured local diversity using four indices: species richness (SR), functional diversity (FD), taxonomic diversity (TD), and local contribution to beta diversity (LCBD). For each descriptor, we applied a variation partitioning framework based on a partial linear regression, considering environmental variables and distance of headwater streams to the main stem or pairwise distance among streams as predictors. The results indicated that water velocity was positively associated with SR, while FD is positively associated with elevation and channel width, and negatively with conductivity. All diversity indices were significantly associated with distance among streams. SR and TD were both related to spatial filters describing larger scales, while FD was related to smaller scales, and LCBD was related to both smaller and larger scales. These results indicate that differences in SR and TD are greater among distant assemblages, whereas FD and LCBD respond to an interplay between local and regional processes. The variation of the four indices was also explained by the shared component between environment and space but not by the distance of headwater streams to the main stem. Therefore, patterns of local fish diversity seem to be more related to patch dynamics or species sorting, rather than to source-sink dynamics. Strategies to protect assemblages of headwater streams in tropical regions should consider multiple sites rather than single sites to guarantee species dispersal and the preservation of environmental heterogeneity.