Patterns of endemism in Amazonian floodplain birds

The subdivision of Amazonia in large interfluvial areas of endemism (delimited by major rivers), based mostly on bird species distributions, has been a recurrent starting point to the understanding and conservation of the biome’s megadiversity. Yet, no areas of endemism or regionalization have been described for the well over 100 bird species that occupy floodplain habitats along the rivers, and thus are not expected to have ranges delimited by the rivers themselves. Here, through spatial analyses of updated range maps (based on a dataset with more than 80 thousand occurrence records), for a revised list of 182 floodplain specialized bird taxa, we identified ten areas of endemism and a complementary habitat-specific regionalization of the biome (with 13 regions). For the floodplain birds, Amazonian major rivers are segmented into distinct areas of endemism rather than these areas being delimited by the rivers. The well-established large interfluvial areas of endemism are appropriate for terra firme species but fail to account for taxa associated with floodplain habitats. Natural history traits and taxonomy of endemic species suggest that both ecological and historical processes have contributed to the patterns found. This new regionalization is consistent with the view of Amazonia as a mosaic of ecoregions and offers a complementary scheme for studies on the evolution and conservation of the floodplain component of its biodiversity.