Spatial and temporal heterogeneity foster high fish biodiversity in an Amazonian floodplain.

The high biodiversity of tropical ecosystems was one of the earliest ecological patterns to be reported, but the ecological processes that maintain this diversity remain unresolved. Here, we revisit Pianka's 1966 hypothesis that spatial heterogeneity contributes to tropical biodiversity. Using a comprehensive survey of a fish community in a central Amazonian floodplain, conducted over the 2003, 2012, and 2022 hydrological cycles, we ask if the high levels of environmental heterogeneity (both temporal and spatial) that characterize this ecosystem constrain the capacity of species to dominate local assemblages. We find rapid replacement of dominant species, with local species richness inversely related to dominance. Marked turnover in fish community structure and composition is also present, with different functional groups favored under changing environmental conditions. By showing that turnover in dominant species fuels species coexistence, we identify an ecological mechanism that helps explain how environmental heterogeneity can foster tropical biodiversity.