Understanding historical and current patterns of species richness of babblers along a 5000-m subtropical elevational gradient.

Aim To understand the causes of historical and current elevational richness patterns of Leiothrichinae babblers, a diverse and mostly endemic group of birds. Location A 5000-m elevational gradient in the Hengduan Mountains, China. Methods By means of a dated phylogenetic tree and reconstructed ancestral states, we estimated elevation-specific diversification rate, applied a new method to estimate colonization frequency and age and, for the first time, modelled historical species richness patterns that take account of temporal patterns of palaeotemperature. As explanations for current richness patterns, we assessed area, geometric constraints, temperature, precipitation, seasonality and productivity. Results The current elevational pattern of species richness is a hump-shaped curve with a peak at about 1000-2500 m. The reconstructed palaeopatterns of species richness suggest that babblers, as a clade, first occupied the Hengduan Mountains at low to mid-elevations, although the method of ancestral state reconstruction cannot conclusively reject origins outside the current elevational distribution of the group. Diversification rates varied little along the elevational gradient, and thus cannot explain the richness pattern, but historical colonization frequency and colonization age were highly correlated with present-day species richness. Seasonality and productivity had greater power than area and geometric constraints in explaining the present-day richness pattern of babblers along the elevational gradient. Conclusions Historical and modern factors have both played important roles in shaping species richness patterns. Reconstructed historical richness patterns suggest that babblers first diversified in the Hengduan Mountains at low to mid elevations, but richness patterns almost certainly shifted substantially under changing climates of the past 10 Myr. The current richness patterns of babblers are associated with seasonality and productivity, but they are also a product of historical evolutionary and ecological dynamics. The methods we introduce for assessing historical colonization rates and past patterns of richness offer promise for understanding other elevational richness gradients. [ABSTRACT FROM AUTHOR]