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

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 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 mutualism also weakens the LDG on islands. 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. The species richness of EFN-visiting ants and EFN-bearing plants positively covary across islands and plants with EFNs and EFN-visiting ants are significantly over-represented on islands, equally so across latitudes. Thus, EFNs facilitate rather than limit colonization on islands, and not all mutualisms dampen the LDG on islands.