Seed dispersal effectiveness by frugivorous birds: Identifying functional equivalent species in bird assemblages

Seed dispersal by birds is a pivotal ecosystem function worldwide; yet, the rapid and ongoing global biodiversity loss poses a major threat to this essential ecosystem service. Seed dispersal effectiveness (SDE) is a key outcome of the interaction, represented by the product of seed dispersal quantity (number of seed dispersed) and quality (probability of recruitment of dispersed seeds). Therefore, identifying functional equivalent species in terms of SDE should become a key issue for bird conservation, since the effects of local extinctions on seed dispersal services may be weakened by remaining equivalent species. However, a method to quantitatively identify functional equivalent species in frugivorous bird assemblages is still lacking. To estimate SDE overlap between seed dispersers and assess whether two species may be functionally equivalent, I apply a novel nonparametric niche overlap index based on kernel functions and null models to test its statistical significance. For each bird species, I account for intraspecific variation in seed dispersal effectiveness to obtain a distribution of seed dispersal effectiveness values, an often neglected source of variation in seed dispersal assemblages. Non-significant differences in seed dispersal effectiveness overlap support the hypothesis that two species are functional equivalent, thus playing similar functional roles. The model proposed is applicable to any other quantity or quality component and is independent from the method or sampling design used to quantify SDE. The identification of functional equivalent species in seed dispersal assemblages adds to the theoretical framework of seed dispersal effectiveness and offers new insights into the ecology of the seed dispersal service provided by birds.