Commensalistic epiphyte-phorophyte networks in woody vegetation of tropical inselbergs: Patterns of organization and structure

The properties of ecological networks help to understand the patterns of commensal interactions between epiphytes and phorophytes as well as the importance of each species in the structure of the interactions in the community. Consequently, understanding how these interactions occur is extremely important, especially on inselbergs, one of the most threatened ecosystems in the world. In this study, we used connectivity, specialization, nestedness and robustness to investigate the horizontal structure and modularity of the vertical structure in the epiphyte-phorophyte networks in woody vegetation on two inselbergs in the Atlantic Forest in south-eastern Brazil. We found 113 phorophyte individuals in 13 species interacting with 4412 epiphyte individuals in 30 species, with a predominance of bromeliads, orchids and cactus. The networks had a nested horizontal structure, poor connectivity and low specialization. Using a modelling approach, these systems had low robustness when the phorophytes with more interactions were eliminated, being a fragile system in regard to local species loss, with 67% of the species restricted to Pseudobombax, a generalist phorophyte. There was little vertical modularity since many species of epiphytes were found in two or more modules, that we used to describe the vertical phorophyte zones. Highest species richness was found in the lower crown and on surface roots (mainly accidental epiphytes) while abundance had a different pattern, being highest in the upper crown, especially for the bromeliad Tillandsia loliacea. Our results suggest that both size (measured as DBH) as well as the species of phorophyte are important factors in the structuring of epiphyte-phorophyte networks on neotropical inselbergs woody vegetation.