Effects of natural rapids and waterfalls on fish assemblage structure in the Madeira River (Amazon Basin)

Abstract – Habitat connectivity is considered a central factor shaping ecological communities, and the effects of waterway barriers such as natural waterfalls on fish movements are expected to produce differing assemblage structures in riverine ecosystems. Here, we evaluate the influence of a sequence of waterfalls on the compositional dissimilarity of fish assemblages along the Madeira River, the largest tributary of the Amazon River. We found significant differences in species composition between rivers stretches located upstream and downstream of Teotônio waterfall and, to a less extent, Jirau waterfall, independently of the hydrological period. After accounting for the relative roles of local and regional factors in explaining fish compositional dissimilarity, we still observe a significant effect of the waterfalls. We conclude that these waterfalls act as natural ecological barriers limiting fish dispersal processes and discuss aspects of these ecological filters and the potential effects of two dams currently under construction in the Madeira River.