A general rule on the organization of biodiversity in Earth's biogeographical regions.

Life on Earth is a mosaic distributed across biogeographical regions. Their regional species pools have experienced distinct historical and eco-evolutionary pressures, leading to an expected context-dependent organization of biodiversity. Here we identify a general spatial organization within biogeographical regions of terrestrial and marine vertebrates, invertebrates and plants (more than 30,000 species). We detect seven types of areas in these biogeographical regions that reflect unique combinations of four fundamental aspects of biodiversity (species richness, range size, endemicity and biogeographical transitions). These areas form ordered layers from the core to the transition zones of the biogeographical regions, reflecting gradients in the biodiversity aspects, experiencing distinct environmental conditions, and exhibiting taxonomic dissimilarities due to nestedness. These findings suggest this ubiquitous organization is mainly driven by the action of two complementary environmental filters, one acting on species from regional hotspots and the other on species from permeable biogeographical boundaries. The influence of these regional filters extends across spatial scales and shapes global patterns of species richness. Regional biodiversity follows a universal core-to-transition organization governed by general forces operating across the tree of life and space.