Résumé
The study of island biodiversity has inspired many advances in evolutionary biology. However, whether patterns of microorganism diversity are influenced by insularity is poorly understood. In particular, microorganisms that live in symbiotic association, such as the microbiota that inhabit the gastrointestinal tract of bigger animals, are subjected to demographic and coevolutionary processes that may add complexity to the common expectation of impoverished diversity on oceanic islands. Here, we explore this topic by studying the cultivable gut bacteria of two sister species of birds, from Sao Tome island and nearby mainland Gabon, the endemic Sao Tome thrush Turdus olivaceofuscus and the African thrush Turdus pelios. We found no differences in the diversity of cultivable gut bacteria between these thrushes, suggesting that, unlike what is commonly found for macrofauna, insularity might not represent a strong constraint for gut bacterial diversity. Although further research on complete gut bacterial communities and a broader range of species and areas is needed, our initial results suggest that the cultivable gut microbial community may bypass the diversity loss associated with island colonization. This could arise from intrinsic factors such as their large population sizes within hosts and low rates of extinction. Furthermore, as gut communities are composed mainly by mutualistic bacteria, diversifying selection (against an impoverished bacterial community), may counteract the diversity loss brought about by the stochastic and demographic effects of the founder process.