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.