Spatial variation of stream fish composition explained by mainstem distance and environmental variables in the Madeira River basin, Amazon

Background/Question/Methods

Species composition and patterns of diversity of fish in streams are correlated to environmental characteristics, and also to the distance to mainstem rivers, which represents source areas for upstream colonization. The Madeira River basin (1,380,000 km²), in the Amazonian drainage, is one of the most diverse river systems of the world, having an estimated 700 fish species, and supposedly representing an important fish source to its tributaries. In that perspective, we investigate if the stream fish composition and richness are driven by the distance to the Madeira River and/or by environmental variables (landscape and local). Samplings were conducted in 23 stream reaches that were randomly selected in a tributary drainage of the Madeira River. Using a standardized sampling method with sieves and seines, fish were collected during a dry period. The relative contribution of predictor variables (riverine distance to the mainstem, four landscape, 19 local) on species composition and species richness was assessed by partial RDA and partial regression, respectively. Prior to analysis, a PCA (Principal Component Analysis) was used to retain variables having a contribution higher than 0.01 for the formation of the first two PCA axes.

Results/Conclusions

Overall, 112 fish species were registered. The variation in species composition is explained by the position of the streams in the basin (distance to mainstem per se) combined to the influence of variables that are spatially structured (R²_adj=0.247, p<0.001). The species richness was not associated to the streams position in the basin nor to the environment variables (p=0.9). Streams in the lower portions of the basin (closer to the main river) tended to have a species composition distinct to that of streams more distant from the main river. Two processes seem to be responsible for that differentiation: (i) contribution of the main river as a source of species, mainly influencing the streams in the lower portions of the basin; (ii) influence from environment conditions, which are different between streams of the lower and higher portions of the basin. Strategies of handling and managing the protection of aquatic biodiversity should consider a hydrographical basin as a heterogeneous system. More specifically, maintenance of aquatic diversity in similar hydrographical basins depends on actions directed towards the protection of streams as well as the main rivers of the basin.