Structure and robustness of an epiphyte–phorophyte commensalistic network in a neotropical inselberg

Ecological networks describe the interactions between species, the underlying structure of communities, the function and stability of ecosystems. To date, network analyses have been extensively applied to understand mutualistic and antagonistic interactions, but few have examined commensal interactions, particularly in neotropical regions. The inselbergs of southeastern Brazil are considered one of the three most important regions in the world in terms of terrestrial species diversity and endemism but are poorly studied. In this study, we constructed the first epiphyte–phorophyte commensalistic network in a Brazilian inselberg and examined its structure and robustness to simulated species loss. A total of 138 phorophyte individuals belonging to eight species were observed in 20 2 m × 50 m transects, interacting with 5039 individuals of vascular epiphytes belonging to 85 species. The epiphyte–phorophyte network structure exhibited a low degree of specialization (H2′), low connectance and robustness; when the most connected phorophyte species were sequentially removed the number of secondary extinctions was high, based on robustness metrics. One generalist phorophyte, Pseudobombax sp. nov., was particularly important, hosting a high number of epiphyte species. A single phorophyte individual of Pseudobombax supported 46% of the total richness of the epiphyte community studied. Our results demonstrate that the richness and abundance of epiphytes were correlated with phorophyte size (as measured by the diameter at breast height (DBH)), probably due to increased habitat area and the time available for colonization. We conclude that phorophyte size and species identity are important factors for predicting the structure of epiphyte–phorophyte interaction networks. We discuss the conservation implications of phorophyte loss and secondary extinctions of epiphytes in Atlantic Forest inselbergs.