More ancient influence of the Nanling Range on bird occurrence
Despite the mountains’ current, limited effect on gene flow in some passerine species, our examination of the occurrence of all species of birds in the Nanling region indicates substantial differences between areas north, south, and in the range itself. These differences suggest the mountains have had a strong historical effect. The chi-square tests of family variation indicated the significant differences between South Nanling and North Nanling, which support Cheng and Chang’s (1956) division of Chinese zoogeographic regions that used the Nanling Mountains as the boundary between the Central China and South China.
Statistical tests of family divergence based on occurrence records across the mountains indicate that pronounced taxonomic divergence exists among the three areas compared (Supporting information). For example, both South Nanling and the Nanling mountains themselves hold more members of the Leiothrichidae when compared to North Naling, and the Nanling Mountains showed exceeding abundance in species in Phasianidae, Phylloscopidae and Leiothrichidae compared to south and north of the mountain range. Phasianids are mainly terrestrial forest species (Tobias et al. 2020). Members of Phylloscopidae are small leaf warblers known to have experienced recent evolutional radiation since c. 11.7MYA (Alström et al. 2018). Leiothrichidae is also a recent group, originating c. 14.73 MYA in the mid-Miocene (Cai et al. 2020). In the mid-Miocene, climatic conditions became humid and more tropical (Guo et al. 2008), giving rise to more broad-leafed vegetation worldwide and general diversification of tropical and subtropical bird groups (Jacques et al. 2011). In addition to the Leiothrichidae, multiple other babbler families originated in the mid-Miocene (Cai et al. 2020). Muscipcapidae species are also more numerous in South Nanling compared to North Nanling. All The muscicapids are a diverse group of old-world flycatchers, assumed to originate c. 34 MYA, (Sangster et al. 2010, Kumar et al. 2017), at the end of Eocene. The end of the Eocene and beginning of the Oligocene was a major transition time for birds, as the extensive tropics in northern latitudes shifted southward and many older tropical lineages suffered extinction and were eventually replaced by more modern groups (Mayr 2013, Oliveros et al. 2019, Sheldon et al. 2015). Except for the the phasianids, all taxa are recently formed, indicating the current bird community has been shaped after this period. Habitat differences in north and south of the mountains clearly help maintain or accentuate those differences. South of the Nanling Range, there is more subtropical forest, where an enhancement of forest inhabitants would be expected.
The Nanling Range was formed in the Cretaceous and remained stable thereafter. Its long existence likely allowed ancestral montane lineages of birds to be conserved during the Oligocene extinction by virtue of the mountain’s landscape heterogeneity in the face of changing climate. The uplift of mountains would also have led to differentiated climatic conditions between the coast and southcentral China, and stimulating adaptive and allopatric diversification (Hoorn et al. 2018). Being in the subtropical monsoon climate, Nanling could also drive climatic divergence by slowing down or block seasonal winds, forming differentiate climatic environment in southern China. The results of this study indicate that that the development of bird diversity in southern China had been directly or indirectly shaped by the existence of the Nanling Mountains.s