INTRODUCTION
The variation in landscape configuration and dispersal ability of organisms could affect both the population structures (Hubbell 2001) and community structures (Soininen et al. 2007). Mountains, with heterogeneous landscape, play multiple roles in the evolutionary process (e.g., Hoorn et al. 2018, Rahbek et al. 2019a, b). The diverse habitats may provide refugia during periods of climatic change, and their elevational variation may provide both conduits and barriers to dispersal, connecting or disconnecting populations. In terms of dispersal ability of birds, the tropical and subtropical species tend to be sedentary despite that some taxa are considered highly vagile and capable of remarkable dispersal (Baker et al. 2001, Ramos et al. 2016). Variation in dispersal capabilities among species is substantial depending on their habitat requirements and migratory habits (Burney and Brumfield 2009), and tropical and subtropical forest species appear less mobile than temperate species (Janzen 1967, Moore et al. 2008). It is now widely acknowledged that tropical birds tend to live sedentary lives to adapt to the tropical environment, while being sensitive to the heterogeneity and dynamics of the environment changes (e.g., Turcotte and Desrochers 2003, Stratford and Robinson 2005). Studies in tropical Amazonian forest, for example, have found that understory birds rarely cross roads (Laurance 2004, Develey and Stouffer 2008) and even avoid crossing forest gaps (Van Houtan et al. 2007).
The Nanling Range of subtropical southcentral China (c . 23.63°-27.23°N and 109.72°-116.68°E) comprises low to medium elevation granitic hills and mountains (0-2000 m above sea level, m.a.s.l.; Supporting information). It is a biodiversity hotspot, renowned for its endemic plants and animals, whose conservation importance is growing because of its position at the center of an area of expansive human growth (Tang et al. 2006, Mi et al. 2021). The mountain range was formed in the late Cretaceous (Chen et al. 2002, Li et al. 2014) and subsequently served as a key refuge for animal and plant taxa, helping maintain biodiversity during the dramatic climatic shifts of the Quaternary ice ages (López-Pujol et al. 2011b, Dong et al. 2017). As a result, the mountains comprise a particularly rich area of paleo-endemism and are often regarded as an “evolutionary museum” (López-Pujol et al. 2011b, a; Hu et al. 2021, Mi et al. 2021). They are also thought to act as a barrier between the fauna of central China and southern China based on the composition of species in and around the range (Cheng and Chang 1956) and, as such, maybe important to fomenting current phylogeographic diversity. Although the Nanling Range’s historical role as a refuge is well studied and seems relatively certain (Cheng and Chang 1956, López-Pujol et al. 2011b, a; Hu et al. 2021, Mi et al. 2021), little is known about its role as a geographic barrier to birds.
One method of detecting and assessing the effectiveness of barriers to dispersal is to examine the degree of genetic connectivity between geographically separate populations. Many studies have examined genetic divergence in mountain birds (Spellman and Klicka 2006, Wu et al. 2012, Qu et al. 2015), but they have focused mostly on demographic history and rarely have considered the effect of landscape variation on the potential movement of birds. Great mountain range with large altitude span, such as in the Andes Mountains (e. g., Chaves et al. 2011, Hazzi et al. 2018) and the Himalayan Mounatins (e.g., Päckert et al. 2012, White 2016), have led to significant divergence in avian lineages. While in the moderate elevation range of Nanling, the divergence, as opposed to the diversity, would be a more subtle question to address.
In this study, we take a two-pronged approach to investigate the extent to which the Nanling Mountains have acted in structuring bird populations in the Nanling region by examining in two different scales. (1) The phylogeography of five generalist forest bird species that reside in and around the mountains. With population genetic methods, studies could reveal the recent demographic changes caused by a barrier, e.g., the Nanling Mountains. (2) The distribution patterns of all bird species inhabiting the region. On the broad scale, it could illustrate how significant regional differences are and in which groups the differentiation nested. Together, the two approaches provide insight to the efficacy of the Nanling Mountains as an impediment to gene flow among populations and a causal factor in species distribution. They show that the Nanling mountains do, to some extent, act as a barrier. However, the extent of their influence is complicated by variations in the montane landscape and the idiosyncrasies of individual species’ dispersal characteristics. On the population level, the mountains have enhanced the expansion of generalist bird populations by serving as a forest buffer zone and corridor. But among higher taxa the mountains have acted as a significant north-south barrier to constituent species. The variable influence of the Nanling Mountains on the distribution of avian taxa at different taxonomic levels suggests the mountains’ role as a barrier is not only multidimensional now, but has changed— possibly many times—over their long history.