At a loss for birds: insularity increases asymmetry in seed-dispersal networks

Aim The decline of species richness with insularity is a well-known pattern in ecology and biogeography. Despite recent progress, our understanding of the effects of insularity on other components of biodiversity, e.g. the diversity of species interactions, is still poorly developed. In this study, we investigated the effect of insularity on the structure of seed-dispersal networks on a global scale. Location World-wide. Methods We compiled a large dataset of seed-dispersal networks (n = 28). We (1) compared the diversity (interaction evenness, complementary specialization) and asymmetry (web asymmetry, interaction strength asymmetry) of interactions between island and mainland networks, and (2) tested the effects of island area and isolation on the four network metrics. Results Neither measure of interaction diversity differed between islands and the mainland. In contrast, island networks were more asymmetric than mainland networks, due to a paucity of animal frugivores, especially birds. Both measures of network asymmetries were closely associated with island isolation, but were unrelated to island area. Patterns were consistent in analyses with and without non-native plant and animal species. Main conclusions Because of the highly asymmetric structure of seed-dispersal networks on islands, plant species on islands are highly dependent on particular animal species. These asymmetries probably arise from low rates of colonization and high rates of human-caused extinction of frugivorous animals on isolated islands. Although interaction evenness and specialization were similar between islands and the mainland, additional work is needed to test whether high asymmetries will make island networks more vulnerable to future change and functional collapse.