Drivers of species' role in avian seed-dispersal mutualistic networks

The mutualistic interaction between frugivore birds and the fruiting plants they disperse presents an asymmetric interaction pattern, with some species having a more important role (i.e. being essential) for maintaining the structure and functioning of the interaction network. The identification of the biological characteristics of these species is of major importance for the understanding and conservation of seed-dispersal interactions. In this study, I use a network approach and avian seed-dispersal networks from 23 different geographical areas to test five hypotheses about species characteristics determining the structure of the assemblage. I expected (i) large birds to forage on a large number of fruits and (ii) large fruits to be dispersed by few bird species (because of morphological constraints), and (iii) highly energetic fruits to be dispersed by more bird species (in accordance with optimal foraging theory). Besides the number of interacting partners, I also expected (iv) large birds and (v) small energetic fruits to be important for the maintenance of the structure of the interactions in seed-dispersal networks. As species that are closely related are more likely to be similar to each other, I performed phylogenetically corrected analyses to account for this data dependence. Although bird size was not associated to species important in the maintenance of the structure of the seed-dispersal community, I identified that bird species whose diet was strongly dependent on fruits were important for the structure of the network. Regarding the plants, I found that large fruits were dispersed by fewer species, but the most important attribute to predict the role of a fruit was its energy content (higher energy, more bird species dispersing the plant, but low-energy fruits being of conservation concern because they are dispersed by specific species). The results of this study suggest that the role of the species in seed-dispersal assemblages seems to be determined by the role of the species as consumers (frugivory degree for animals) or by their nutritional inputs (energy content for fruits) rather than by morphological constrains.