Mutualisms weaken the latitudinal diversity gradient among oceanic islands

The latitudinal diversity gradient (LDG) dominates global patterns of diversity, but the factors underlying the LDG remain elusive. Here, we use a unique global dataset to show that vascular plants on oceanic islands exhibit a weakened LDG and explore potential mechanisms to explain why. Our results show that traditional physical drivers of island biogeography – namely area and isolation – contribute to the difference between island and mainland diversity at a given latitude (i.e., the island species deficit), as smaller and more distant islands experience reduced colonization. However, plant species with mutualists are underrepresented on islands, and we find that this plant mutualism filter explains more variation in the island species deficit than abiotic factors. In particular, plant species that require animal pollinators or microbial mutualists like arbuscular mycorrhizal fungi contribute disproportionately to the island species deficit near the equator, with decreasing contributions with distance from the equator. As such, plant mutualist filters on species richness are particularly strong at low latitudes where mainland richness is highest, weakening the LDG of oceanic islands. These results provide empirical evidence that mutualisms, habitat heterogeneity, and dispersal are key to the maintenance of high tropical plant diversity and mediate the biogeographic patterns of plant diversity on Earth.