4.2 | Processes of community assembly
Community assembly occurs via the combined influence of evolutionary and ecological processes. Whereas evolutionary processes relate to the origins of species, such as speciation and extinction, ecological processes address how species are assembled, for example, via habitat filtering or competition exclusion. With rapid advances in molecular biology, the phylogenetic structures of communities can be better assessed (Faith, 1992; Webb, Ackerly, McPeek, & Donoghue, 2002). The analysis of community phylogenetic structure can facilitate the determination of the ecological and evolutionary processes that regulate community assembly at different scales (Webb, Ackerly, McPeek, & Donoghue, 2002). Evolutionary processes such as rapid in situ speciation, niche conservatism, and dispersal limitation can lead to phylogenetic clustering (Lu et al., 2018). In comparison, evolutionary processes such as niche evolution, convergent evolution, and colonization may lead to phylogenetic overdispersion within communities (Allen & Gillooly, 2006). For ecological processes, habitat filtering and competition exclusion can result in non-random community phylogenetic structures (Webb, Ackerly, McPeek, & Donoghue, 2002). Habitat filtering which can lead to phylogenetic clustering refers to the selection of certain traits of species in a community (Wiens & Graham, 2005), whereas competition exclusion can result in phylogenetic dispersion (Burns & Strauss, 2011).
Overall, the SES-PD results illustrated the floristic history of the counties; 27 county-level communities had young flora, while only one county-level community contained ancient flora (Banma County). Therefore, the TR, PD, and SES-PD results suggested that the eastern parts of the Kunlun Mountains were the center for biodiversity conservation; and that the KMF was comprised primarily of young flora, which was consistent with the findings of a previous study (Lu et al., 2018).
In the western and the middle parts of the Kunlun Mountains, the negative NRIs in 10 county-level communities indicated that these communities were the result of phylogenetic dispersion, however, significant differences were observed in only three negative NRIs (P < 0.05), whereas the other negative NRIs had no significant differences. The positive NRIs in 10 county-level communities indicated that these communities showed phylogenetic clustering, however, these positive NRIs were not significantly different. The positive NTIs in 20 county-level communities indicated that these communities were phylogenetic clustering, however, two NTIs were no significant differences, whereas the remaining NTIs did show significant differences (P < 0.05).
In the eastern parts of the Kunlun Mountains, the NRI and NTI of Banma County were negative, and the two indexes were statistically significant (P < 0.05). The result suggested that the community in Banma County was phylogenetic overdispersion. The NRI and NTI of other counties were positive which indicated that these communities were phylogenetic clustering, however, significant differences were detected between three NRIs (P < 0.05) and six NTIs (P < 0.05).
NRI and NTI analyses can help to reveal the ecological and evolutionary processes in a community. First, the latest research shows that China is identified as having five phytogeographical regions, including the Paleotropic, Holartic, East Asiatic, and Tethyan regions, as well as the QTP. The relationships among the regions have been inferred as follows: (Paleotropic [East Asiatic + Holarctic] + [Tethyan + QTP]) (Ye et al., 2019). The Kunlun Mountains are in the transition zone between the Tethyan region and the QTP. Therefore, the current plants of the Kunlun Mountains may have originated from multiple phytogeographical regions. Second, the current environment in the Kunlun Mountains is dry and cold, particularly in the western and middle parts (Zheng, 1999). In addition, both the Qaidam and the Tarim Basins belong to the Tethyan region (Ye et al., 2019), and they are contiguous with the Kunlun Mountains. The plants that are well adapted to drought stress and cold conditions may have originated from the Tethyan regions and alpine flora of other regions. However, the phylogenetic relationships among the species are distant, because they may have originated from different phytogeographical regions. Third, the mechanisms driving biodiversity patterns after glacial events are the expansion of seed plants from adjacent glacial refugia into the Kunlun Mountains.
In the western and the middle parts of the Kunlun Mountains, the negative NRIs in 10 county-level communities may indicate that the current species were from multiple phytogeographical regions, named the Tethyan regions and the alpine flora of the Hengduan Mountains according to geographical location. The positive NRIs in the 10 county-level communities indicated that most of the species were from relatively homogeneous phytogeographical regions, for example, the species in the western parts may have mainly originated from Mountains of Central Asia, the species on the northern slopes of the middle parts may have mainly originated from the Tethyan regions, and the species on the southern slopes of the middle parts may have mainly originated from the alpine flora of the Hengduan Mountains. Therefore, recolonization of species from different phytogeographical regions may have dominated the evolutionary processes of the current communities in the 20 county-level communities. The NTI values were positive in 20 counties, which indicated that habitat filtering presented a strong explanation for the ecological processes of communities in the western and middle parts of the Kunlun Mountains.
In the eastern parts of the Kunlun Mountains, the NRI and NTI of Banma County were negative, which suggested that the community in Banma County results from species that originated from the alpine flora and forest flora of the Hengduan Mountains. The positive NRI and NTI values in seven county-level communities indicated that the species in these communities originated from the alpine flora of the Hengduan Mountains, and that the environmental conditions in the area selected for species possessing similar traits.
In conclusion, both species recolonization and habitat filtering may contribute to current plant communities of the Kunlun Mountains via ecological and evolutionary processes, and habitat filtering may play a critical role in an ecological processes.