Frugivory Specialization in Birds and Fruit Chemistry Structure Mutualistic Networks across the Neotropics

Abstract

The interaction between fruit chemistry and the physiological traits of frugivores is expected to shape the structure of mutualistic seed dispersal networks, but it has been understudied compared with the role of morphological trait matching in structuring interaction patterns. For instance, highly frugivorous birds (i.e., birds that have fruits as the main component of their diets), which characteristically have fast gut passage times, are expected to avoid feeding on lipid-rich fruits because of the long gut retention times associated with lipid digestion. Here, we compiled data from 84 studies conducted in the Neotropics that used focal plant methods to record 35,815 feeding visits made by 317 bird species (155 genera in 28 families) to 165 plant species (82 genera in 48 families). We investigated the relationship between the degree of frugivory of birds (i.e., how much of their diet is composed of fruit) at the genus level and their visits to plant genera that vary in fruit lipid content. We used a hierarchical modeling of species communities approach that accounted for the effects of differences in body size, bird and plant phylogeny, and spatial location of study sites. We found that birds with a low degree of frugivory (e.g., predominantly insectivores) tend to have the highest increase in visitation rates as fruits become more lipid rich, while birds that are more frugivorous tend to increase visits at a lower rate or even decrease visitation rates as lipids increase in fruits. This balance between degree of frugivory and visitation rates to lipid-poor and lipid-rich fruits provides a mechanism to explain specialized dispersal systems and the occurrence of certain physiological nutritional filters, ultimately helping us to understand communitywide interaction patterns between birds and plants.

Abundance predominates over niche factors as determinant of the frequency of interactions between frugivorous birds and plants

“…Our results challenge this notion by showing that niche factors rarely considered in trait-matching studies (e.g., mass ratio and lipid content of fruits) also matter for the frequency with which species interact with each other (Pizo et al, 2021). Therefore, rather than redundant with respect to seed dispersal services, the so-called generalist bird species using second-growth Cerrado might well be complementary if we consider their affinities to fruits with different mass ratios and nutrient profiles.…”

Seed dispersers shape the pulp nutrients of fleshy-fruited plants

“…By contrast, migratory birds ingest an excess of fruit pulp sugar to enhance body-fat syntheses prior to migration [15,55]. Predominantly frugivorous birds consume a lower proportion of lipid-rich fruits, instead preferring fruits rich with easily absorbed sugars (especially hexose sugars) through rapid gut-transit times [27]. These species (e.g.…”

“…Lipids contain more energy per gram than other fuel sources [3]; however, it is costly for parent plants to include high lipid concentrations in fruit pulp because of the direct energy and complex indirect synthesis pathway involved [26]. Both bird- and mammal-dispersed fruits will contain sufficient sugar concentrations to appeal to the disperser to meet its dietary energy demands, as sugars can be absorbed easily and quickly to refuel the consumer [27]. Both bird- and mammal-dispersed fruits will contain similar proportions of protein, as both need protein to (re-)generate structural tissues and support physiological activity; however, protein involves a complex synthesis pathway for plants, although less costly than for lipid synthesis [26].…”

Mechanistic models of seed dispersal by animals

“…In general, frugivores are not specialized on a single plant species but mix different fruits within their diets (Murphy 1994, Whelan et al 1998, Dzul‐Cauich et al 2018). The way in which animals decide to forage on different plant species has important consequences for community assembly and species persistence (Jordano 2000, Carlo and Morales 2016, Morán‐López et al 2018a, b2018b, Pizo et al 2021). Also, understanding fruit choices by frugivores can help to predict interacting network rewiring after species loss (Morán‐López et al 2020a).…”