Effects of a major Amazonian river confluence on the distribution of floodplain forest avifauna

Abstract

Aim: Although ‘river color’ or water type is an important determinant of Amazonian floodplain biodiversity, the relevance of mixing distinct water types at river confluences to the distribution of terrestrial floodplain fauna has been largely overlooked. We investigated how the influx of a sediment-rich whitewater tributary affects the floodplain forest avifauna along the world's largest blackwater river. Location: Northwestern Brazilian Amazon. Taxon: Birds. Methods: We sampled floodplain avifauna and retrieved estimates of sediment concentration in the water (the main parameter in river-water classification) at 52 sites, along the Negro (blackwater) and Branco (whitewater) rivers, above and below their confluence. We compared species richness and composition using analyses of variance, ordinations, generalized linear models and indicator species analyses. Results: Bird species composition on the lower Rio Negro (below the confluence) was distinct from both the upper Negro (above the confluence) and the Branco, and species richness on the Negro increased below the confluence. Typical whitewater bird species occurring on the Branco were found exclusively or predominantly along the left side of the lower Rio Negro, where the Branco's muddy waters seem to be channelled. Overall avian compositional variation among sites was correlated with sediment concentration in the water, a determinant of floodplain forest structure. Main conclusions: The influx of the muddy waters of the Rio Branco into the Rio Negro promotes the co-occurrence of bird species that are segregated by river type upstream, increasing species richness at the landscape scale. Rather than just representing a potential blackwater barrier between whitewater systems, the lower Rio Negro comprises a unique biogeographical transition zone, with a mixed avifauna occurring in a mosaic of varied floodplain forest types. Our results suggest that confluences of large rivers of distinct water types represent a significant factor determining species distributional boundaries and geographic patterns of Amazonian floodplain biodiversity. © 2020 John Wiley & Sons Ltd