Tropical bryophyte floras: a homogeneous assemblage of highly mobile species? Insights from their spatial patterns of beta diversity

The biotic poverty of tropical Africa has been interpreted in terms of massive extinctions caused by historical climate shifts. While recolonisation has been hampered in tropical angiosperms that are characterized by low dispersal capacities, tropical bryophytes exhibit high dispersal syndromes. Using tropical mosses as a model, we test here the hypotheses that (i) species-turnover is higher within than among continents and (ii) in tropical Africa, extant distribution patterns are better explained by variation in environmental conditions (species turnover) than by the ordered loss of species along gradients of recolonisation from putative refugia (species nestedness). Global beta diversity was slightly, but significantly higher among than within tropical regions, suggesting that geographic isolation rather than regional environmental variation shapes macroecological patterns of moss species distributions. Species turnover was, however, systematically higher than nestedness within each tropical region, and nestedness in tropical Africa was not significantly higher than in other tropical regions. The results therefore do not support the hypothesis of an ordered loss of species due to delays in the recolonisation of formerly savanna areas from refugia. Rather, the low species richness in Africa is interpreted in terms of shifts towards specialized, but species-poor communities along sharp ecological gradients.