1. Species differ in their resource use and their interactions with other species and, consequently, they fulfil different functional roles in ecological processes. Species with specialized functional roles (specialists) are considered important for communities because they often interact with species with which few other species interact, thereby contributing complementary functional roles to ecological processes. However, the contribution of specialists could be low if they only interact with a small range of interaction partners. In contrast, species with unspecialized functional roles (generalists) often do not fulfil complementary roles but their contribution to ecological processes could be high because they interact with a large range of species.
2. To investigate the importance of the functional roles of specialists versus generalists, we tested the relationship between species' degree of specialization and their contribution to functional-role diversity for frugivorous birds in Andean seed-dispersal networks. We used two measures for the specialization of birds—one based on the size, and one based on the position of their interaction niche—and measured their effect on the birds' contribution to functional-role diversity and their functional complementarity, a measure of how much a species' functional role is complementary to those of the other species.
3. In all networks, there were similar log-normal distributions of species' contributions to functional-role diversity and functional complementarity. Contribution to functional-role diversity and functional complementarity increased with both increasing niche-position specialization and increasing niche size, indicating that the composition of functional roles in the networks was determined by an interplay between specialization and generalization. There was a negative interaction between niche-position specialization and niche size in both models, which showed that the positive effect of niche-position specialization on functional-role diversity and functional complementarity was stronger for species with a small niche size, and vice versa.
4. Our results show that there is a continuum from specialized to generalized functional roles in species communities, and that both specialists and generalists fulfil important functional roles in ecological processes. Combining interaction networks with functional traits, as exemplified in this study, provides insight into the importance of an interplay of redundancy and complementarity in species' functional roles for ecosystem functioning.