Specialization increases in a frugivorous bird–plant network from an isolated montane forest remnant

The structure of mutualist interactions is still poorly understood in isolated remnants of montane forests. We tried to answer whether a plant–frugivore network from an isolated remnant could maintain its functional structure, despite the absence of certain species and traits. We hypothesized that a network of frugivore birds and plants from an isolated remnant would be less specialized and modular, and would show higher degree of nestedness than one from a continuous forest area. Functionally distinctive species in the networks were also identified based on analyses of the fourth-corner matrices, which cross plant and bird traits weighted by their interaction frequencies. The structure of studied networks showed a high similarity in which most interactions were performed by small generalist bird species on plant species carrying small unprotected fruits. Both networks showed similar levels of niche partitioning, although specialization and modularity increased in the isolated remnant network. Networks also showed a strong correlation between fruit weights and bird masses reflecting the functional distinctiveness of large frugivorous birds and plants with the heaviest fruits. Several large bird species were recorded in the studied networks although they contributed with a low percentage of interactions. Therefore, networks from isolated forest remnants seem resilient to the reduction of species richness due to the persistence of tolerant bird species and plant species carrying small fruits.