4.1 Stochastic processes dominated community assembly under
natural grassland degradation
Both deterministic and stochastic
community assembly processes play important roles in shaping community
composition and structure in natural ecological systems (Chase, 2010;
Chase & Myers, 2011; Zhou et al., 2013). Therefore, elucidating the
assembly processes of microbial community could enhance the
understanding of the responses of communities to grassland degradation.
However, numerous studies that emphasized the importance of
environmental filtration have focused on determining the dominant
environmental factors driving and structuring bacterial community
structure (Feng et al., 2017; Knelman et al., 2019; Yu, Wu, et al.,
2019; Liu, Graham, et al., 2020), with the assembly processes in
bacterial communities in the course of grassland degradation and the
community response strategies remaining poorly understood. Therefore, in
the present study, the authors investigated the turnover and assembly
strategies of bacterial communities in response to shifts in resource
conditions under grassland degradation. Overall, the authors observed
that bacterial community assembly processes in degraded grassland
systems were dominated by stochastic processes (Figure 4A and
4B) .
Numerous researchers have reported that as environmental change is
prolonged, deterministic processes may become more important due to the
accumulation of environmental selection; however, the above often occurs
after stochastic processes have dominated the initial phase (Ferrenberg
et al., 2013; Zhang et al., 2016; Liu, Graham, et al., 2020). Therefore,
the stochastic processes observed in the present study could be
associated with the short term and limited spatial scales applied for
the sampled grasslands under degradation, and continuous environmental
change could alter the dominant community assembly processes(Figure 4A and 4B) . In addition, degree of change in
environmental variables is a key factor influencing community assembly
process at the regional scale (Feng et al., 2018). In the present study
on a degraded grassland with low human interference, the mild changes in
environmental variables could be another major reason for the dominance
of stochastic processes in community assembly processes (Liu, Graham, et
al., 2020).
Stochastic processes represent undetermined ecological assembly
processes that yield community diversity patterns that are random (Zhou
& Ning, 2017). Stochastic processes (e.g., colonization and
extinction) are as important as or more important than deterministic
processes (e.g., species selection and species interactions) in
structuring natural microbial community (Chase & Myers, 2011).Although numerous studies have highlighted the importance of stochastic
factors in driving the structure and functions of communities (Zhou et
al., 2008; Caruso et al., 2011; Chase
& Myers, 2011; Stegen et al.,
2012; Zhou et al., 2013; Zhou
&
Ning, 2017), the dominance of
stochastic process in community assembly makes it extremely challenging
to explore the underlying community dynamics because of the
unpredictable compositional variation (Zhou et al., 2013; Zhang et al.,
2016). Consequently, we investigated community composition dynamics in
grasslands with different levels of degradation in an attempt to
elucidate the corresponding mechanisms of bacterial community assembly.