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.