ANDEAN frugivory: data on plant–bird interactions and functional traits of plant and bird species from montane forests along the Andes

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. 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. 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. 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.