Amazonian white-sand forests occur on quartzitic sandy soils, are distributed as an archipelago of habitat islands across the rain forests of Amazonia and contain many endemic plant species. Surprisingly, we found that only 23 percent of plant species in western Amazon white-sand forests are white-sand specialists, while the remaining species (77%) also occur in other habitat types. Overall, our analyses revealed: (1) somewhat unexpected composition similarity of white-sand forests with nearby non-white-sand forests; (2) phytogeographical connections among distant white-sand forests; and (3) a large proportion of western Amazon white-sand specialists occurring in floras of the western and central Guiana Shield region (7-43%). These results suggest that dispersal from both neighboring oligotrophic non-white-sand habitats and distant white-sand forests is fundamental in shaping western Amazonian white-sand forests' species composition and diversity. Although endemism in Amazonian white-sand forests may be lower than previously estimated, conservation of this unique and fragile environment should remain a priority. Such conservation will require the maintenance of regional dispersal processes that connect these archipelagos of habitat islands and other ecologically similar oligotrophic habitats across the Amazon and the Guiana Shield.