Exotic insect pollinators and native pollination systems.
Insect pollinators have been relocated by humans for millennia and are, thus, among the world's earliest intentional exotic introductions. The introduction of managed bees for crop pollination services remains, to this day, a common and growing practice worldwide and the number of different bee species that are used commercially is increasing. Being generalists and frequently social, these exotic species have the potential to have a wide range of impacts on native bees and plants. Thus, understanding the consequences of introduced species on native pollinator systems is a priority. We generated a global database and evaluated the impacts of the two main groups of invasive bees, Apis mellifera and Bombus spp., on their pollination services to native flora and impacts on native pollinators. In a meta-analysis, we found that per-visit pollination efficiency of exotic pollinators was, on average, 55% less efficient than native pollinators when visiting flowers of native species. In contrast to per-visit pollination efficiency, our meta-analysis showed that visitation frequency by exotic pollinators was, on average, 80% higher than native pollinators. The higher visitation frequency of exotic pollinators overcame deficiencies in pollen removal and transfer resulting in seed/fruit set levels similar to native pollinators. Also, evidence showed that exotic pollinators can displace native insect and bird pollinators. However, the direct effects of exotic insect pollinators on native pollination systems can be context dependent, ranging from mutualism to antagonism.