Abstract

General explanations are lacking as for why biodiversity is extremely unevenly distributed at regional scales across the planet1. This question is exemplified by biodiversity hotspots, which hold >40% of all species yet cover only 17% of the Earth land surface without any obvious geographic pattern2. Regional differences in biodiversity have been ascribed to differences in the rates at which new species are generated, in times of colonisation and in rates of dispersal3, all of which are at least partially driven by the abiotic environment4. However, these hypotheses have rarely been tested in conjunction and at a global scale. Here we sought to uncover the processes shaping the disproportionate accumulation of species across hotpots of mammal and bird endemicity. Our approach involved analysing the tempo and mode of both diversification and dispersal within major biogeographic realms. We found that hotspots had fewer ancient lineages and generally more recent lineages than surrounding non-hotspot regions. This difference stemmed from contrasting macroevolutionary routes across biogeographic realms: hotspots in most tropical realms had higher rates of in situ cladogenesis whereas those in largely temperate realms received relatively more immigrant species from their surrounding regions. There was no support for the hypothesis that hotspots had more time to accumulate diversity as they were not consistently colonised earlier than non-hotspot regions, nor did these patterns arise merely due to differences in the sizes of hotspot and non-hotspot regions. Environmental differences that can promote higher rates of cladogenesis and immigration in hotspots than in surrounding regions, such as greater habitat diversity, topographic complexity and energy availability, may explain our results both within and among realms. Our study highlights how assessing historical differences in macroevolutionary patterns can help reveal why biodiversity varies so much across the globe.