It is well documented that endophytic fungal communities are highly diverse and important for plant fitness, and much new research is focused on teasing apart the factors that contribute to shaping these communities. Several studies have shown that host species identity and tissue type (root vs. leaf) play a large role in determining endophyte community structure. However, few studies have looked at how abiotic conditions, such as soil moisture, may influence communities inside plant roots. Furthermore, while many studies investigate the network architecture of plant-endophyte interactions, few assess how that structure may change across the landscape. Here, we test how an environmental gradient, soil moisture, influences the network structure and composition of endophytic root microbes. Our study system is a high-elevation alpine ecosystem that exhibits considerable variability in soil moisture across the landscape. To test for differences in network structure, we characterized fungal (ITS) communities, constructed bipartite networks, and evaluated the network architecture as measured by specialization, modularity and nestedness. To test for differences in community composition we conducted PERMANOVA to assess taxonomic overlap between low and high soil moisture. We hypothesize that across the soil moisture gradient both network architecture and community composition will change.
Results/Conclusions
We find that network architecture does not differ between high and low moisture plant-fungal networks. Across the soil moisture gradient, the networks exhibited high modularity, indicating that the communities are highly organized into clusters with little interaction between the clusters. In addition, both networks exhibited relatively low nestedness, even lower than would be expected by chance, meaning that taxa with few connections did not tend to have a subset of the connections that taxa with more connections had. Nestedness is considered an important attribute of mutualistic interactions, so this might suggest that many of the plant-fungal interactions at both high and low soil moisture are not mutualistic. Further, as is commonly found in networks of symbiotic relationships, both networks exhibited moderate interaction specialization, suggesting that fungal endophytes in this system are specialized to one or a few plant species hosts. Despite network complexity not changing over the soil moisture gradient, fungal community composition did change. Also, at high soil moisture there was greater fungal richness than at low soil moisture. Overall, results indicate that even though species composition may turn over and richness may change over environmental gradients, network structure may be conserved despite the presence of different interacting organisms.