Abundance and trait-matching both shape interaction frequencies between plants and birds in seed-dispersal networks

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 trait-matching 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.The research was funded by MinECo/FEDER grants CGL2011-28430 and CGL2015-68963-C2-2-R to D.G., and BES2012-052863 and BES-2016-078260 to I.D. and R.P., respectively. R.P. received an Alumni-Grant from Senckenberg University. I.D. is currently funded by the Balearic Government. J.R.P. was supported by BIOINTFOREST funded by “Obra Social la Caixa” and “Fundación Caja Navarra”, under the agreement LCF/PR/PR13/51080004 in the framework of UPNA's “Captación de Talento” program.Peer reviewe