Network structure variation across scales offers clues to the macroevolutionary persistence of specialized mutualisms

Highly intimate (symbiotic) mutualisms, such as ant-myrmecophyte and brood pollination mutualisms, are characterized by reciprocally specialized and modular network architecture and constitute evolutionary paradoxes. Few examples of these mutualisms are known, fewer are species-rich, and theory suggests that they should be vulnerable to extinction. However, no studies have examined their structure at both multiple local sites and regional scales. Here, we examine these phenomena using a brood pollination mutualism between leafflower trees (Phyllanthaceae: Glochidion) and leafflower moths (Lepidoptera: Gracillariidae: Epicephala) across eight sites in tropical and subtropical China. We find that significant reciprocal specialization is conserved across local and regional scales in these networks, but we also find local-regional discrepancies in whether networks are significantly modular, resulting in adjacent local networks that differ in architecture. Interaction turnover is almost entirely due to species turnover rather than interaction rewiring among widely distributed species. Unexpectedly, we find that two regional networks are significantly nested, while none of the local networks are. Theory suggests that nestedness may confer robustness to species loss but also should not evolve in symbiotic mutualisms. The finding that some regional networks are nested in this system suggests a possible explanation for the macroevolutionary persistence of such specialized mutualisms.