Symmetric assembly and disassembly of an ecological network

Abstract The position of species in interaction networks can predict their extinction probability. However, the processes whereby these network roles emerge and persist, then decay during ecosystem development are unknown. Here we study networks of plant and arbuscular mycorrhizal fungal (AMF) communities along a 120,000 yr soil chronosequence, with two distinct phases: progressive (i.e. assembly, where plant richness and biomass increase) and retrogressive (i.e. disassembly, where plant richness and biomass decline with declining nutrients). We find that the order of interaction formation during progression mirrors that of interaction extinction during retrogression, and that interactions in sites moving forwards or backwards from the (12,000 yr) site of peak biomass were nested subsets of the interactions at that site. Network assembly and disassembly were symmetrical, self-reinforcing processes that together generated key attributes of network architecture. Plant species that had high AMF-partner overlap with others in the community (measured as ‘closeness centrality’) were best able to attract and retain new AMF partners, and AMF species with high partner overlap were better able to retain their interactions with plants. In contrast, interaction generalism (‘node degree’) per se was a poor predictor of partner attraction during assembly or retention during disassembly. These results could be used to predict extinction sequences based on community assembly or network roles, identify focal points for invasions, and suggest trajectories for ecosystem restoration.