Ant-plant mutualism contribute to island species deficit at the global scale

Vascular plants on oceanic islands exhibit a weakened latitudinal diversity gradient (LDG), with certain mutualisms (e.g., plant-pollinator interactions) exerting mitigating effects on oceanic islands species deficit. However, the role of ant-plant mutualism in shaping this species deficit remains poorly understood. Here we used the Global Inventory of Floras and Traits (GIFT) database to explore the potential mechanisms of underlying ant-plant mutualism on oceanic island species deficit. Our results showed that ant-plant mutualism exhibited a significant positive effect on the ocean-mainland plant diversity difference at a given latitude (that is, the oceanic island species deficit). In addition, the strength of ant-plant mutualism filter could explain 52.14% of the total variation in species deficit which indicated this biotic filter is more influential for oceanic island plant diversity than the traditional physical drivers of island biogeography (i.e., area and isolation). While non-myrmecophytic plants contribute more to the absolute species deficit (due to their larger mainland species pool), the proportional species deficit of myrmecophytic plants is significantly higher than that of non-myrmecophytic plants. This finding confirms that ant-plant mutualism acts as a selective biotic filter, reducing the colonization success of myrmecophytic plants on oceanic islands and thereby contributing to the weakened LDG on islands. These results provide empirical support for the role of ant-plant mutualism in maintaining high tropical plant diversity and shaping global plant biogeographic patterns.