Niche conservatism drives a global discrepancy in palm species richness between seasonally dry and moist habitats

Aim Rapid global environmental change predicts increasingly seasonal climate in the tropics, causing expansion of seasonally dry habitats and leading to shifts in species distribution and potential extinction. Here, we use a macroevolutionary framework to understand the processes driving palm diversity patterns between moist and seasonally dry tropical habitats. We hypothesize that the discrepancy in species richness between habitats is explained by higher speciation rates in moist habitat and that niche conservatism prevents frequent shifts between habitats. Location Global. Time period Last 100 Ma. Major taxa studied Arecaceae. Methods We used trait-dependent diversification models to test whether different habitats affect palm speciation rates. Furthermore, palm assemblages were divided into three regions (Africa, Australasia and Neotropics) to test for niche conservatism and evaluate phylogenetic dissimilarity. Results We found no relationship between speciation rate and habitat type. We detected phylogenetic signal for habitats at both global and continental scales, indicating that closely related species were more similar than expected by chance. Colonization of seasonally dry habitats occurred c. 60 Ma, yet most clades only diversified after c. 30 Ma. The high phylogenetic dissimilarity between habitat types at both global and continental scales was driven by high lineage turnover, at least for Africa and the Neotropics. Main conclusions We found a lack of differential speciation rates in setting the seasonally dry and moist palm-richness discrepancy. However, over evolutionary history most palm lineages fail to colonize seasonally dry habitats owing to a tendency to retain ancestral habitat. Indeed, seasonally dry palm assemblages are the result of the diversification of particular lineages. Likewise, long-term dry periods appear to induce shifts in taxonomic and functional diversity, and we emphasize that the expansion of dry habitats might also imply a loss of palm clades, hence a reduction in phylogenetic diversity.