Riparian vegetation structure and seasonality influence functional diversity more than taxonomic diversity of stream fish assemblages in the Colombian Amazon

The Amazon rainforest has experienced rapid land-use changes, including extensive deforestation of riparian areas, putting at risk among the most diverse freshwater fish assemblages on the planet. At nine study reaches distributed among three Colombian blackwater streams, we described and quantified how variability in riparian vegetation structure—during both dry and rainy seasons—influenced the taxonomic and functional diversity of fish assemblages. We observed 117 species across our study reaches, but taxonomic measures such as species richness and evenness were not different among low, intermediate, or high levels of riparian vegetation coverage. Fish assemblage composition was also not different by riparian vegetation, although we observed significant seasonal shifts—only 16% of species were observed in both rainy and dry seasons. Evenness and effective number of species derived from Simpson’s Index were 7 and 38% greater in the dry season than in the rainy season, respectively. Diet varied by riparian zone condition: 56% of fish stomach contents were of allochthonous origin in stream reaches with intact vegetation versus 37% in streams with altered riparian vegetation. We also observed that the relative proportion of detritivorous fishes was 13% greater in the rainy season, while the proportion of invertivores was 26% greater in the dry season. Both functional divergence and dispersion—measures of the variability of species’ traits—were higher in reaches with more intact vegetation during the rainy season. Our findings highlight the importance of the interaction of riparian vegetation and seasonality, and both longitudinal and lateral hydrological connectivity, for fish functional diversity of Amazonian streams.