Aim: To provide insights into the assembly and structure of biotic communities by exploring several processes of island biodiversity assembly: in situ speciation, immigration and ecological processes.
Location: The Mascarene Islands.
Methods: The phylogenetic relationships of 211 out of 232 Mascarene leptosporangiate fern species were reconstructed in relation to a global phylogeny of 400 leptosporangiate fern species to assess the importance of immigration versus in situ speciation and any deviation from randomness. Correlations between speciation and habit (terrestrial or epiphytic/lithophytic) were tested.
Results: We inferred at least 197 colonization events onto the islands, accounting for > 80% of their leptosporangiate fern diversity. The phylogenetic structure of the assemblage of colonists was mostly overdispersed but aggregated at the tips of the phylogeny. Several families were more diverse on the islands than expected by their global richness. Among the 46 endemic species, up to 65% were inferred to have originated by cladogenesis, which in turn was found to be significantly related to a terrestrial habit.
Main conclusions: Immigration has played a major role in the assembly of the fern flora on the Mascarene Islands. The overdispersion pattern supports the hypothesis of a dominant role of competitive interactions in the flora assembly, and we propose that most available niches were filled by species arriving via long-distance dispersal, preventing further immigration of closely related taxa. The over-representation of some families may reflect better colonization capacities, preadaptation to the Mascarene ecological niches, or ecological differentiation. Frequent colonization by long-distance dispersal has probably hampered in situ speciation in some habitats such as the epiphytic realm. Conversely, cladogenetic speciation appears to occur more frequently in terrestrial habitats.