Defensive mutualisms affect plant and ant island biogeography at a global scale

Aim: Mutualism can shape biogeographic patterns at macroecological scales. Recently, Delavaux et al. (2024) found that plant mutualisms with mycorrhizae, nitrogen-fixing bacteria, and animal pollinators limit island colonization, weakening the latitudinal diversity gradient (LDG) on oceanic islands. Here, we revisit their analysis and examine whether ants and plants engaged in a common defensive mutualism mediated by extrafloral nectaries (EFN) are similarly under-represented on oceanic islands, and whether this defensive ant-plant mutualism also weakens the LDG on islands. Location: Global. Time period: Present-day species distributions and traits. Major taxa studied: Vascular plants and ants. Methods: We used global trait and occurrence databases to compare ant and plant species richness between oceanic islands and their likely source mainlands for taxa that do and do not interact mutualistically via EFNs. When analyzing the factors that determine whether a plant species occurs on oceanic islands, we also included the mutualism types studied by Delavaux et al. (2024), namely biotic pollination, mycorrhizal fungi, and nitrogen-fixing bacteria, and added plant habit, life history, and phylogeny as covariates. Results: Both plants with EFNs and EFN-visiting ants are significantly over-represented on islands. The species richness of EFN-visiting ants and EFN-bearing plants also positively covary across islands. In ants, engaging in mutualisms mediated by EFNs significantly strengthens, rather than weakens, the LDG on islands, while for plants, EFNs have no effect or strengthen the LDG on islands, depending on data filtering. After accounting for plant habit, life history, and phylogeny, only biotic pollination significantly limits plant colonization of islands, whereas mycorrhizae and nitrogen-fixing bacteria now have positive or non-significant effects, respectively, on island colonization. Main conclusions: EFNs facilitate rather than limit plant and ant colonization on islands. Mutualism does not ubiquitously limit island colonization, and some mutualism types strengthen rather than dampen the LDG on islands.