The geographical diversification of Furnariides: the role of forest versus open habitats in driving species richness gradients

Aim: Explaining species richness gradients in space and time requires understanding the evolutionary processes that ultimately alter the number of species. Here we examine species richness differences between primary habitats (forest versus open) for Furnariides birds, a Neotropical endemic bird clade, to test three major historical hypotheses – diversification rate, out of the tropics and tropical niche conservatism – and assess the role of evolutionary processes in driving the Furnariides species richness gradient. Location: Neotropics. Methods: We used phylogenetic and spatial data to tests the historical hypotheses. First, we used GeoSSE and Bayesian Analysis of Macroevolutionary Mixture models to evaluate differential diversification and dispersal rates between habitats. Second, we quantify the root distance of each species and examined the phylogenetic structure of the richness gradient and the correlation between total species richness and the richness of early-diverged and recently originated species. Results: Furnariides species richness is higher in forest than in open habitats. However, we found higher speciation, extinction, and dispersal rates in open when compared to forest habitats, resulting in a higher diversification rate in open habitats and higher dispersal rate out of open habitats than into them. The phylogenetic structure of the richness gradient showed strong spatial pattern, with early diverged species richness peaking in forest habitats and driving the overall Furnariides gradient. Main conclusions: The Furnariides species richness gradient results from the joint effect of differential rates of macroevolutionary processes. Our findings highlight dispersal and extinction as dominant forces driving richness differences between habitats, through the addition and extirpation of species from open to forest habitats. Differences in species richness between habitats support niche conservatism of forest habitat preferences of Furnariides species. We suggest that open habitats are effective evolutionary arenas and a key to the maintenance of bird diversity in forest habitats over evolutionary time.