Abstract Saturation is the idea that a community is effectively filled with species, such that no more can be added without extinctions. This concept has important implications for many areas of ecology, such as species richness, community assembly, invasive species and climate change. Here, we illustrate how biogeography can be used to test for community saturation, when combined with data on local species richness, phylogeny and climate. We focus on a clade of frogs (Terrarana) and the impact of the Great American Biotic Interchange on patterns of local richness in Lower Middle America and adjacent regions. We analyse data on species richness at 83 sites and a time‐calibrated phylogeny for 363 species. We find no evidence for saturation, and show instead that biotic interchange dramatically increased local richness in the region. We suggest that historical biogeography offers thousands of similar long‐term natural experiments that can be used to test for saturation.