Understanding the factors that generate and maintain biodiversity are foundational in ecology and evolution and critical for conservation. Despite generations of research and hundreds of proposed theories a cohesive and mechanistic understanding is still lacking. Moreover, existing theory derives strongly from studies of plants and vertebrates. The strongest correlates with global- and regional-scale biodiversity underscore the importance of climatic drivers, although the roles of biotic interactions remain weakly tested at large scales and in complex systems. We use data from comprehensive lichen inventories of 208 one-hectare plots throughout the southern Appalachian Mountains of southeastern North America to examine that how abiotic and biotic factors relate to species richness in this diverse and highly threatened system. Lichen richness strongly declined with increasing anthropogenic disturbance, as has been found in prior studies. After controlling for disturbance, we detected a mid-elevational peak in species richness that was repeated across separate latitudinal bands, but we detected no trend in latitudinal species richness. The strongest impacts on lichen diversity were biotic interactions, both internal and external to lichen symbioses.