Weaker Plant-Frugivore Trait Matching Towards the Tropics and on Islands

Abstract

Biotic interactions play an important role in species diversification and maintenance and, thus, are regarded as the architecture of biodiversity. Since Darwin and Wallace, biologists have debated whether biotic interactions are stronger towards the tropics and on continents, when compared to temperate regions and islands. Here, based on 354 avian frugivory networks accounting for 22,199 interactions between 1247 bird species and 2126 plant species, we quantified trait matching strength, which reflects interaction strength and specificity, across gradients of latitude and insularity globally. We found that matching between beak size and fruit size was significantly stronger towards the poles and on continents, when compared with the tropics and on islands. As underlining ecological factors, trait matching was stronger with a larger proportion of frugivory (measured as the mean proportion of fruits in bird diets) and network-level mean beak size, and with a smaller proportion of fleshy-fruited species (measured as the proportion of fleshy-fruited plant species in the botanical country where the network was located). These findings suggest that the latitudinal and insular patterns in trait matching are driven by biotic factors that may relate to trait co-evolution between interacting species and optimal foraging for bird species.

Keywords: avian frugivore; biogeography; biotic interaction; co‐evolution; endozoochory; seed dispersal.

FIGURE 1

Geographic and climatic ranges of frugivory networks in this study. (a) Global locations of frugivory networks and trait matching strength. The inset shows the distributions of continental (yellow) and insular (grey) networks along latitudes. (b) Distribution of networks in Whittaker biomes according to the mean annual temperature and precipitation of sampled sites. (c) Proportion of continental and insular networks in major biogeographic realms.

FIGURE 2

Geographic patterns of trait matching in avian frugivory networks. Yellow symbols illustrate continental networks, and grey symbols illustrate insular networks. (a) Trait matching strength along absolute latitudes. The solid line represents model prediction (continental networks as the reference level), and grey‐shaded area represents the 95% confidential interval. (b) Trait matching strength on continents vs. islands. Violin plots represent the distribution of trait matching strength. Dots and error bars show the mean value and 95% confidential interval, respectively. Detailed model results are given in Table S9.

FIGURE 3

Effects of ecological factors on trait matching strength. (a) Model‐averaged coefficients of ecological factors. Points and error bars represent mean values and 95% confidential interval of the coefficients. (b) The relative importance of ecological factors in explaining variances in trait matching strength. The proportion of variance is calculated as the individual contribution of each factor to the total variances (marginal R ²) of the linear mixed‐effects model with trait matching strength as the response variable and all ecological factors as fixed effects. Detailed model results are given in Tables S11 and S12.

FIGURE 4

Results of the piecewise structural equation model. (a) Final model showing direct and indirect effects of latitude, insularity and main ecological factors on trait matching strength. Solid arrows indicate significant positive (blue) and negative (red) effects, respectively (p < 0.05). Dashed arrows indicate non‐significant effects. Double‐headed arrows indicate correlation between factors. Values along each path are standardised model coefficients of the piecewise structural equation model. R ² shows the explained variance of the response variable. (b) Summary of direct, indirect and total effects of latitudes, insularity, and ecological factors on trait matching strength. Bars and error bars represent mean values and their 95% confidential intervals. Detailed model results are given in Tables S13 and S14.