3.4 Disentangling the community assembly processes of abundant
and rare taxa
The values of βNTI varied partly from -2 to +2 for abundant taxa, while
rare taxa exhibited βNTI values higher than 2 (Fig. 4a). Null model
analysis revealed that stochastic processes, especially dispersal
limitation (DL, 84%) were the most important processes responsible for
the community assembly of abundant taxa, followed by heterogeneous
selection (HeS, 15%) (Fig. 4b). By contrast, the deterministic
processes were largely responsible for the community assembly of rare
taxa (79%), of which the greatest contributor was HeS (77%, Fig. 4b).
Taken together, these results suggest that rare taxa are more
environmentally constrained and less limited by dispersal than their
abundant counterparts.
Mantel tests were performed to examine the relationships between βNTI
and the variation of environmental variables in order to infer the
relative influences of deterministic and stochastic assembly processes in
the abundant and rare subcommunities. Results of Mantel tests
demonstrated that forest area and TN were the most important variables
associated with βNTI of the abundant (r = 0.116, P = 0.005) and
rare (r = 0.171, P = 0.002) subcommunities, respectively (Table
1). Furthermore, the changes in forest area and TN were significantly
correlated with pairwise comparisons of βNTI of the abundant
(R2 = 0.029, P < 0.001, Fig. S5a) and
rare (R2 = 0.041, P < 0.001, Fig.
S5b) subcommunities, respectively. Despite the βNTI values of the
abundant subcommunity being significantly related to differences in
forest area, the distribution of βNTI values were mainly in the region
of stochastic assembly (−2 < βNTI < +2). Yet the
distribution of βNTI values of the rare subcommunity were mainly in the
region of HeS (βNTI > +2) with the changes in sediment TN
content.