There is a complex suite of local and landscape factors that influence species structure and patterns of diversity of streams fish assemblages. The Madeira River basin (1,380,000 km2), in the western Amazonian drainage, is one of the most diverse river systems of the world, having an estimated 1,000 fish species. In order to determine the factors driving the species composition and diversity in this basin, we investigate the relative influence of environmental and/or spatial variables (n=43) on stream fish assemblages. Samplings were conducted in 75 stream reaches that were randomly selected in a tributary drainage of the Madeira River. Using a standardized sampling method with sieves and seines, fish and variables were collected during dry periods of 2011 and 2012. To select predictor variables, a Principal Coordinate Analysis (PCoA) was conducted; the relative contribution of variables on fish composition (evaluated as genera) and species richness was determined by partial RDA and partial regression, respectively. We testing differences in beta diversity between streams, considering vegetation cover conditions at local scale (streams with riparian vegetation vs. those with bare soil in the riparian zone) and at landscape scale (streams located in the most forested basins vs. those located in cleared basins).
Results/Conclusions
Overall, 142 fish species of 90 genera were registered. The shared influence of environmental and spatial variables explained the greatest amount of variation in species composition (14%, p=0.002), followed by variation explained purely by stream position (8%, p<0.01). Streams in the lower portion of the basin are environmentally distinct and have different fish composition than streams more distant from mainstem. Additionally, we found that the environmental and spatial variables explained 28.5 and 12.6% of the variation in species richness. Beta diversity did not vary significantly between forested vs. cleared basins (Anova F=1.704, p=0.195), in contrast to the vegetative strata that exhibited significant differences (Anova F=4.544, p<0.01). These results emphasize the importance of local variables on composition, alpha, and beta diversity. In highly diverse basins, the maintenance of aquatic diversity depends on actions that protect tributaries as well the main channel. In this sense, main threats to Machado fish are the giant dams under construction in the Madeira River, which will limit upstream fish colonization, and the agriculture/pasture development on the medium and upper portions of the basin, that alters local stream conditions.