Invasive pollinators can disrupt native pollination mutualisms. We investigated the impact of the invasion of the European bumble bee Bombus terrestris in NW Patagonia, Argentina, on the pollination mutualism between the native legume Vicia nigricans and its main pollinator, the native bumble bee B. dahlbomii, and its consequences on plant reproduction. We assessed visitation frequency by both bumble bees and reproductive output of V. nigricans across space (in 12 sites) and time (over five seasons, from 1999 to 2012), before and after invasion in 2006. In 2012, we studied the behavior, visitation frequency, and nectar consumption by both bumble bees, the nectar consumed, the quantity and quality of pollen deposited and the reproductive output of V. nigricans in a subset of five sites across a gradient of B. terrestris invasion. Six years after invasion, B. terrestris became the most frequent visitor and nectar robber of V. nigricans flowers, while visits by B. dahlbomii dropped by 50%, fruit set by 43% and seeds per fruit by 32%. This detrimental effect on plant reproduction resulted from the high frequency of nectar robbing by B. terrestris, the nectar depletion, and the concomitant reduction of legitimate visits by the native bumble bee, which in turn lead to a decreased deposition of high quality pollen. Our study demonstrates that biological invasions can alter ecological interactions by replacing a plant-pollinator mutualism by a plant-nectar robber antagonism, hence disrupting plant reproduction and undermining long-term population viability. This conclusion raises the urgent need to regulate the international trade of non-native pollinators.
