Unpacking frugivory: trait‐mediated bird responses explain fruit removal in a tree population

Abstract

Frugivory and seed dispersal by birds represents a major ecosystem service across temperate and tropical regions, playing a crucial role in maintaining tree diversity and vegetation recovery in human-altered landscapes. Frugivore assemblages typically encompass multiple species with distinct ecological traits, including dietary preferences and foraging behaviors. This complexity makes it challenging to predict fruit removal patterns of the entire assemblage. Here, we used a hierarchical model of species communities to assess how the combined effects of fruit removal by individual bird species translate into fruit removal by the bird assemblage in a single tree species population. We determined the relative contributions of tree, fruit and seed traits (tree height, number of fruits, fruit diameter, seed mass and sugar concentration), bird species traits (gape width and fruit-handling behavior), spatial context (spatial location of trees) and potential biotic interactions to fruit removal at the species and bird assemblage levels. We found variable species-specific responses to fruit traits, as expected, but increased sugar concentration and decreased seed mass both increased fruit removal at the bird assemblage level. We also found a strong relationship between fruit handling behavior and sugar concentration, with pulp consumers (species feeding on the pulp and discarding the seed) targeting fruits with higher sugar concentrations than gulpers (species swallowing fruits whole). As a result, trees bearing fruits with high sugar concentration were more likely to attract pulp consumers, which in turn translated into increased fruit removal. In addition, tree size was positively related to frugivore richness. We detected no evidence of residual interspecific associations. Our results suggest that, despite the prevalence of generalist species in frugivore assemblages with contrasting preferences for fruit traits, trait-mediated bird responses to fruit traits may result in predictable patterns of fruit removal.