Why are there so few endemics in island bryophytes? Macaronesian flora answers

Among land plants, bryophytes exhibit the lowest rates of endemism on oceanic islands. This pattern has primarily been interpreted in terms of intense gene flow preventing speciation from the closest continental sources, so that endemism would have to originate from geographically remote sources through chance long-distance dispersal. Here, we test this hypothesis through a meta-analysis of species distribution patterns and phylogeographic analyses in the Macaronesian bryophyte flora. Our results reveal a striking mismatch between the strong similarity of the Macaronesian and Mediterranean floras, and the largely extra-Mediterranean origin of endemism that is, at first sight, compatible with this hypothesis. This hypothesis is, however, at odds with the significant genetic differentiation systematically found among populations disjunct between Macaronesia and the Mediterranean. Our reconstruction of the biogeographic origin of non-endemic species based on ABC analyses in a coalescent framework reveal a complex history of colonization of the archipelago from multiple continental sources, but also of continental colonization from island ancestors. Altogether, our results thus suggest that Macaronesian bryophyte flora originated from different sources depending on the variation of wind connectivity through time, triggering endemic speciation from various sources. While speciation rates may thus not have necessarily been lower than in other land plant lineages, the low rates of endemism among island bryophyte floras result from subsequent dispersal events towards continental areas.