4 Discussion
Species richness, including that of rodents, is characterized by spatial
heterogeneity (Gaston, 2000). The results of this study indicate that
there is a wide variety of rodents, including endemic species, in the
Hengduan Mountains and surrounding areas of Southwest China. The
Hengduan Mountain area is considered an ecological corridor between the
Palaearctic and Oriental realms, connecting the northern and southern
fauna. It is an important center for the origin of species (Wu et al.,
2016). This may explain why southwest China has the highest level of
endemism. Southwest China is rich in plant resources and has a complex
topography and a high degree of habitat heterogeneity. The area’s
climate ranges from tropical to temperate and is dominated by high
temperatures and abundant precipitation (Liu et al., 2015; Shrestha et
al., 2017). These findings may contribute to the diversity of rodents in
China. In comparison, the extreme climatic conditions of the
Qinghai-Tibet Plateau, Tarim Basin, and northern China may limit the
distribution of species intolerant to climatic factors (Hu et al., 2017;
Ding et al., 2019). The results show that the border areas of China’s
Yunnan, Sichuan, and Tibet autonomous regions, especially the Qionglai,
Yunling, Minshan, and Gaoligong mountain systems in the Hengduan
Mountains, have the highest richness values of all studied areas due to
their diverse topography, hydrothermal conditions, and habitats. The
distribution pattern of rodent species richness may be explained by a
combination of habitat heterogeneity, climate seasonality, energy-water,
and human factors (Figure 3).
The close relationship between species richness and habitat
heterogeneity and the variance partitioning results showed that habitat
heterogeneity is the most influential predictor variable for describing
the species richness of all and non-endemic rodent species in China. It
is also an important factor in the richness distribution pattern of
endemic species. Habitat heterogeneity is considered the most critical
factor in shaping biological distribution patterns. Habitat variation
creates a microhabitat for species reproduction by creating dramatic
changes in climate and habitat differentiation on a small scale,
enabling species to coexist locally (Carmignotto et al., 2022).
Southwest China has a variety of hydrothermal conditions and habitats
that result from a combination of topographic changes caused by
elevation differentiation (Huang et al., 2011; Liu et al., 2015). As a
result, the site had the highest richness values for rodent species. In
this study, we used the elevation range as one of the main predictors of
habitat heterogeneity, arguably the best indicator of topographic
variation. Meanwhile, southwest China is also the richest region in
terms of plant species (Shrestha et al., 2017; Dakhil et al., 2019; Sun
et al., 2020; Pandey et al., 2020). Abundant plant resources provide a
large amount of food for rodents and a suitable shelter for unfavorable
climatic conditions (Barreto, et al., 2019). In addition, the
Qinghai-Tibet Plateau’s uplift has profoundly changed the geomorphology
of mainland China, mountains, ravines, and canyons, resulting in huge
mountain height differences. Quaternary ice sheet intrusion drives
rodent migration from higher to lower elevations, and the cyclical
effects of warming cause rodents to return to higher elevations (Barreto
et al., 2019). Faunas that cannot migrate can only adapt to the
differentiation of low- and medium-altitude environments, thus causing
high endemic species richness in rodents. In addition, highly
heterogeneous tropical and subtropical mountain ranges may be cradles of
biodiversity and thus dominate in terms of species richness and
concentrations of endemic species with narrow distributions.
The results showed that climatic seasonality is the dominant factor in
the distribution patterns of endemic species of rodents in China.
Conversely, it explained the least of all the species and non-endemic
species richness patterns. This suggested that seasonal temperature
changes have a profound effect on endemic species richness (Penjor et
al., 2022). Previous research has also identified a significant role of
climate seasonality in maintaining patterns of species richness in
endemic species of small mammals (Amori et al., 2011; Wu et al., 2013;
Hu et al., 2017). The temperature in China fluctuates widely, with more
extreme temperatures in the north than in the south. This might be an
important factor in northern China’s restricted distribution of endemic
species. Our findings support the theory of tropical niche conservation
and the inability of endemic rodent species to adapt to northern
environments. Their ability to migrate from south to north is consistent
with our findings (Romdal et al., 2013). This seasonality in southwest
China creates stable climatic conditions for endemic species to survive
under harsh conditions. Araújo et al. (2008) and Dakhil et al. (2019)
reported the significant role of climate stability during the warmest
season of the Quaternary glaciation. Climatic stability is an ecological
indicator of the range stability of subtropical fauna in the
high-altitude regions of southwest China, including the eastern Tibetan
Plateau (Thuiller, 2004; Huang et al., 2011). Due to climate variability
and the unsystematic variation in daily maximum and minimum
temperatures, modifying the thermal environment experienced by rodents
to increase the body’s tolerance level enables them to be widely
distributed geographically.
Our results also revealed the importance of anthropogenic variables in
the distribution patterns of total and non-endemic species. However, its
effect on endemic species richness was minimal. Thus, it can be expected
that anthropogenic activities will significantly impact the distribution
of rodents. Anthropogenic activities can drive the contraction and
expansion of species. Many species have lost significant areas of
distribution owing to increased land use and other human activities
(Ceballos & Ehrlich, 2002; Di Marco & Santini, 2015). Human activities
have also contributed to the range expansion of many other species, and
many regional native species have experienced increased range through
human-mediated dispersal and the ability to thrive in anthropogenic
landscapes (Li et al., 2015). However, endemic species have more
specialized habitat requirements, are concentrated in specific
geographic areas, and may deviate ecologically from a wide range of
species (Tomašových & Jablonski, 2016). Therefore, the anthropogenic
impact on rodent endemics is limited. However, endemic species are
confined to limited areas, and their habitats are often fragile and
highly susceptible to reduction or even disappearance by external
factors. Therefore, we cannot ignore human-interference scenarios. In
southwest China, anthropogenic disturbances, such as natural resource
exploitation, tourism, and land use change, are gradually increasing,
accelerating the impact of human activities on wildlife. Therefore,
protecting this center of species richness and endemism is a big
challenge.
On a large scale, energy-water did not have a significant direct
relationship with the distribution pattern of rodent species richness in
China. This finding differs slightly from Hu et al. (2017) and Wu et al.
(2013) at the local scale. The spatial pattern of species richness and
the importance of biotic and abiotic predictor variables in influencing
species richness may depend to a large extent on the spatial scale of
the sampling unit (Qian & Kissling, 2010). In some cases, species
richness patterns and potential drivers can be reasonably explained by
ecological requirements and the evolutionary history of species grouping
(Wu et al., 2013). Rodents are phytophagous or omnivorous; therefore,
the pattern of species richness may be more closely related to plant
species richness (Hawkins & Pausas, 2004). At a large spatial scale,
the direct impact of energy-water on plants is often more significant
(Zhang et al., 2015). Although the effect of energy-water on the
distribution pattern of rodent species richness is not very significant
at large scales, on a smaller scale, rodent dispersal behavior is
temperature oriented, and species may spread more rapidly when
temperature changes (Wu et al., 2013). Therefore, temperature and
precipitation may strongly influence rodent species richness at a
regional scale.
In summary, we against using all species or non-endemic species richness
as proxies for endemic species richness. This is consistent with the
results of previous studies (Orme et al., 2005; Isik, 2011). The reasons
for the formation of endemic and widespread species differ. Endemic
species formation is often closely related to environmental and
anthropogenic changes. Some endemic species may have been widely
distributed; however, due to climate change during the geological and
historical period, their habitats have shrunk or even disappeared,
forcing them to confine their distribution to a narrow range; or newly
evolved species have not yet spread to a larger geographic area; or
destruction of the environment limits their ranges. In addition, we
found that the reasons for determining the distribution of non-endemic
and endemic species richness differed.
Although this study contributes to a more nuanced understanding of
rodent species richness patterns and their drivers in China, it has
several limitations. For example, there are 271 species of rodents in
China (Wei et al., 2021), however, only the 237 species with relatively
complete data were analyzed in this study, which does not provide a
complete picture of the distribution pattern of rodents in China. The
variables used in MaxEnt partially overlap with the factors involved in
the regression model, which will inevitably affect the regression
results. In addition, the species distribution model assumes that
species distribution is influenced only by environmental variables,
without interactions between organisms and biological dispersal
constraints. In practice, species distribution is also influenced by
biological factors such as competition, predation, and disease. A more
sophisticated and integrated SDM for species range prediction is
required to consider these factors, which is an important direction for
future model development (Kearney & Porter, 2009).