Dispersal limitation promotes the diversification of the mammalian gut microbiota

The gut bacterial communities of mammals have profound effectson host fitness, but the processes that generate and maintain gutbacterial diversity remain poorly understood. We mapped compositionalvariation (i.e., ß-diversity) in the gut microbiotas of 136 pairsof wild mammalian species living throughout the Americas to assesshow the distribution of mammals across geographic space influencesthe diversification of their gut bacteria. Comparing the gutmicrobiotas of sympatric and allopatric mammalian populationsprovided insights into the flow of gut bacteria within and betweenmammalian communities, revealing that spatial limits on bacterialdispersal promote ß-diversity between the gut microbiotas of mammalianspecies. Each geographic locale displayed a unique gutmicrobiotacomposition that could not be fully explained by thediets and phylogenetic histories of the resident mammalian hosts,indicating that some gut bacteria are geographically restricted.Across the western hemisphere, the compositional overlap betweenthe gut microbiotas of allopatric mammalian populationsdecayed exponentially with the geographic distance separatingthe hosts. The relationship between geographic distances amonghosts and compositional differences among their gut microbiotaswas independent of dietary and phylogenetic divergence amonghosts. Within mammalian communities, we observed widespreadsharing of gut bacteria between predator–prey host-species pairs,indicating horizontal transfer of gut bacteria through mammalianfood chains. Collectively, these results indicate that compositionaldifferences between the gut microbiotas of mammalian taxa aregenerated and maintained by limits to bacterial dispersal imposedby physical distance between hosts.