Coinvasional disruptions to island pollinator networks

Biological invasions can adversely affect pollinator diversity by threatening the maintenance of animal-pollinated plant communities. Although most studies have examined single invasive species, accelerating species introductions driven by global trade highlight the need to understand how multiple co-occurring invaders jointly influence pollination networks. We leveraged the relative simplicity of the Ogasawara archipelago to assess the combined impacts of multiple invaders on pollination networks. We surveyed floral and pollinator communities and quantified plant-pollinator interactions across four islands with varying degrees of invasion. We characterized invasion degree based on the abundance of three invasive taxa: green anoles (Anolis carolinensis), a predator; western honey bees (Apis mellifera), a floral competitor; and several flowering plants. Structural equation models were used to quantify direct and indirect effects of these invaders on effective plant-pollinator partners (niche breadth or variety of partners a species interacts with in a given community) and the degree of specialization (d', niche partitioning or selectivity of a species) of plant and pollinator communities. All invaders strongly affected pollinator diversity and, to a lesser extent, plant diversity. Anoles had the strongest negative effect on pollinator diversity, triggering cascading effects across the trophic chain. Directly or indirectly, all invaders negatively affected plant resource use. Honey bee abundance had a direct negative effect on plant effective partners and specialization, and anole abundance and the proportion of invasive plants indirectly affected plant resource use through a reduction in pollinator diversity. Degree of invasion and network structure mediated whether honey bees contributed to functional compensation or facilitated invasional meltdown. Our findings demonstrated that co-occurring invaders generated synergistic pressures on pollinators and plants, underscoring the importance of evaluating invasions in a multistressor context. Integrative approaches are essential for developing conservation strategies in vulnerable ecosystems, particularly oceanic islands, where biological invasions are the principal driver of biodiversity loss.