4.5 Roles of abundant and rare taxa in mediating interspecific interactions
Network analysis can improve our understanding on the potential roles of abundant and rare taxa in the complex interaction webs of fluvial sediments. Our results illustrated that the co-occurrence network had non-randomly connected properties and closely related taxa tended to be interconnected and clustered together within the network, suggesting that taxonomic relatedness plays a key role in determining the network modular structure. This non-random pattern may reflect the effects of environmental filtering and niche differentiation among species in the Yarlung Tsangpo River sediments (Hu et al., 2017; Ju and Zhang, 2015).
Our results revealed that rare taxa rarely coexisted with abundant taxa within the fluvial sediment network (Fig. 5a). This result may be explained by the fact that abundant and rare taxa occupy different ecological niches and respond differently to environmental variables (Jiao et al., 2017). The topological properties of the network can also reflect interactions between species (Xue et al., 2018). Previous research has revealed that abundant taxa, with higher degree and betweenness centrality values, are more connected with each other and more often located in central positions, than rare taxa within the network (Jiao et al., 2017; Zhang et al., 2021a). Our results showed that the values of degree and betweenness centrality for abundant taxa were significantly higher compared with rare taxa, implying that abundant taxa may play a more important role in maintaining the co-occurrence network than rare taxa (Röttjers and Faust, 2018). However, rare species might be keystone taxa. In our study, five rare OTUs were identified as underlying keystone taxa in the co-occurrence network (Table S5 and Fig. S6). Considering the importance of keystone taxa in mediating interspecific interactions (Sun et al., 2021), our results thus indicated the potential roles of those rare keystone bacteria in maintaining ecosystem stability in sediments of the Yarlung Tsangpo River. Our results also showed that intermediate taxa had the most interactions with both abundant and rare taxa, and more than half of the module hubs and connectors belonged to intermediate taxa, indicating that intermediate taxa may act as bridges between rare and abundant taxa within the co-occurrence network.