Ant mutualists as a biotic interaction filter of flowering plant colonization on islands

Aim: Oceanic island floras are well-known for their disharmonic assemblages, with certain taxa and functional groups being over- or underrepresented compared to their source pools, due to effects of dispersal, environmental filtering, and biotic interactions. However, the role of biotic interactions in generating this disharmony remains poorly explored. Location: Global islands. Time Period: Present-day species distribution. Major Taxa Studied: Flowering plants bearing extrafloral nectaries (EFNs) and domatia, and their ant parters. Methods: We compiled plant distributions from the Global Inventory of Floras and Traits (GIFT) and ant distributions from the Global Ant Biodiversity Informatics database (GABI) for islands worldwide. We then assembled a dataset of ant genera known to interact mutualistically with plants bearing EFNs or domatia. Using this dataset, we quantified the representational disharmony of ant-associated plants and evaluated whether their ant partners act as a biotic interaction filter for the representation of ant-associated plants on islands. Results: We found that domatium-bearing plants are generally underrepresented, whereas extrafloral nectary (EFN)-bearing plants are overrepresented, likely reflecting differences in their relationship with ant partners. Besides the effects of island characteristics, the representation of both domatium- and EFN-bearing plants shows a strong yet conditional relationship with the diversity of their interacting ant partners. Specifically, this biotic interaction filter is more pronounced on larger, less isolated islands. Main Conclusions: Our findings underscore that mutualistic interactions are a key driver of island plant assembly and that their filtering effect depends on island characteristics. We conclude that explicitly integrating biotic interactions and their context is essential to advance our understanding of island biogeography.