Disruption of pollination services by invasive pollinator species.
Plant-pollinator interactions and associated pollination services are essential for crop production and the integrity of terrestrial ecosystem services. Introduced pollinators, in particular social bees such as honeybees and bumblebees, have become invaders in many regions of the world, strongly affecting the pollination of native, cultivated, and non-native plants. These effects can be direct, when invaders interact with local flowering plants, or indirect, when invaders modify the interaction of native pollinators with flowering plants. Direct effects on pollination depend on whether the plant benefits from the flower visits are greater than their costs, a relationship that can be density dependent. Shifts from mutualism to antagonism occur when invasive pollinators reach extremely high densities, because the interaction costs exceed the benefits. Indirect effects depend on whether pollinator invaders alter the benefit-cost ratio of native pollinator visits, displace them, or trigger reductions in native pollinator diversity. Through a literature review, we found that the impacts of invasive pollinators on pollination were predominantly negative for native plants, mixed for crops, and positive for invasive plants. Furthermore, they can synergistically interact with other stressors on pollination such as climate change and habitat disturbance. Although invasive pollinators can back up pollination of some native plants in highly disturbed habitats, and some crops in intensively modified agro-ecosystems, they cannot replace the role of a diverse pollinator assemblage for wild plant reproduction and crop yield. Hence, managing agro-ecosystems for enhancing wild pollinator diversity, and avoiding further introductions of non-native pollinators, are realistic cost-effective measures for the provision and stability of pollination services.