Abstract:
Abundance and trait-driven processes have both been identified as potential mechanisms in determining the occurrence of species
interactions. However, little is known about how these two mechanisms interact to determine the relative frequencies of interactions
between species, and thereby species-specific contributions to ecological functions. Here, we evaluate the effect of both
species’ abundance and trait-matching on the occurrence of plant-bird seed dispersal interactions in the Cantabrian Range (northern
Spain). For two years at fourteen plots, we independently sampled the abundance and diversity of fleshy-fruited plants and frugivores,
as well as the consumption of fruits by birds. We quantified trait-matching by applying a food-web approach based on the
log-ratios of species traits relevant to seed dispersal and traits related to fruit-handling and foraging-stratum. We fitted multi-level
models incorporating phylogenetic relatedness to identify phylogenetically independent effects of species abundance and traitmatching
on interaction frequencies. Fitted models showed that species abundances of both plants and birds always had strong
positive effects on interaction frequencies. Trait-matching effects associated with fruit-handling were weak, but consistent across
years, whereas those derived from foraging stratum varied across years, according to strong interannual changes in species abundance.
Our findings reveal that both species abundance and functional traits are required for a mechanistic understanding of species
interactions, as well as for predicting species roles in ecosystems under global change.
