Benoît Perez‐Lamarque

Characterizing and understanding the processes that shape the structure of ecological networks, which represent who interacts with whom in a community, has many implications in ecology, evolutionary biology and conservation. A highly debated question is whether and how the structure of a bipartite ecological network differs between antagonistic (e.g. herbivory) and mutualistic (e.g. pollination) interaction types. Here, we tackle this question by using a multiscale characterization of network structure, machine learning tools, and a large database of empirical and simulated bipartite networks. Contrary to previous studies focusing on global structural metrics, such as nestedness and modularity, which concluded that antagonistic and mutualistic networks cannot be told apart from only their structure, we find that they can be told apart by combining a meso‐scale characterization of their structure and supervised machine learning. Motif frequencies appear particularly informative, with an over‐representation of densely connected motifs in antagonistic networks and of motifs with asymmetrical specialization in mutualistic networks. These structural properties can be used to predict the type of interaction with relatively good confidence. Beyond this classical mutualism/antagonism dichotomy, we also find significant structural uniqueness linked to specific ecologies (e.g. pollination, parasitism). Our results clarify structural differences between antagonistic and mutualistic networks and suggest the investigation of the structural uniqueness of specific ecologies as a promising approach for characterizing interactions beyond the coarse antagonistic/mutualistic dichotomy.

In soils, plants and fungi can form complex mycorrhizal networks allowing nutrient transfers between plant individuals and species. It is less clear, however, whether such networks exist on the bark of trees where epiphytic plant communities thrive in rainforests. Previous work showed that tropical epiphytic orchids especially, harbour symbiotic fungi in their roots, but the structure and determinants of the resulting networks remain unknown at the tree scale. We tested the hypothesis that epiphytic orchids rooted in the same area on the bark share mycorrhizal fungi, regardless of their species (i.e. spatial determinant). For this purpose, we selected the trunk of six trees of two common species in a rainforest and sampled orchid roots, protocorms and surrounding bark. We identified mycorrhizal fungi including Tulasnellaceae using high‐throughput sequencing of the ITS2 marker, and reconstructed orchid–fungus bipartite networks for each tree to analyse their structure and the spatial turnover of this symbiosis. We found that epiphytic orchid communities form antinested and highly modular networks with mycorrhizal fungi spread on the bark. As expected, modules of interactions are explained by their spatial structure, with nearby roots sharing fungi, but also by the orchid species involved. These results reveal the presence of shared mycelial networks in epiphytic habitats, whose roles in the resilience and facilitation of epiphytic plant communities need to be assessed. Synthesis . Tropical tree barks are densely colonized by certain mycorrhizal fungi that can form symbioses in nearby adult and young orchids simultaneously. These mycorrhizal networks may allow water and nutrient transfers to alleviate the stressful conditions of the epiphytic habitats.