Oecologia DOI 10.1007/s00442-008-1039-5 123 PLANT-ANIMAL INTERACTIONS - ORIGINAL PAPER Displacement of a native by an alien bumblebee: lower pollinator eYciency overcome by overwhelmingly higher visitation frequency JoseWn A. Madjidian · Carolina L. Morales · Henrik G. Smith Received: 14 September 2007 / Accepted: 3 April 2008  Springer-Verlag 2008 Abstract Biological invasions might constitute a major threat to mutualisms. Introduced pollinators might competi- tively displace their native counterparts, which in turn aVects the pollination of native plants, if native and alien visitors diVer in pollinator eVectiveness. Since its invasion in 1994 into south-west Argentina, the introduced European bumblebee Bombus ruderatus has continuously increased in abundance, along with a simultaneous decrease in the abundance of the native Bombus dahlbomii. The latter is the only native bumblebee species of the temperate forests of southern South America, and the main pollinator of the endemic herb Alstroemeria aurea. In order to evaluate the impact of the ongoing displacement of the native by the alien bumblebee, we compared the pollinator eVectiveness (i.e., the combination of pollinator eYciency per visit and visitation frequency) between both bumblebee species, as well as related pollinator traits that might account for potential diVerences in pollinator eYciency. Native Bom- bus dahlbomii, which has a larger body and spent more time per Xower, was the more eYcient pollinator compared to Bombus ruderatus, both in terms of quantity and quality of pollen deposited per visit. However, Bombus ruderatus was a much more frequent Xower visitor than Bombus dahlbomii. As a consequence, Bombus ruderatus is nowa- days a more eVective pollinator of A. aurea than its native congener. Despite the lack of evidence of an increase in seed set at the population level, comparisons with historical records of Bombus dahlbomii abundances prior to Bombus ruderatus’ invasion suggest that the overall pollination intensity of A. aurea might in fact have risen as a conse- quence of this invasion. Field experiments like these, that incorporate the natural variation in abundance of native and alien species, are powerful means to demonstrate that the consequences of invasions are more complex than previous manipulated and controlled experiments have suggested. Keywords Alstroemeria aurea · Bombus dahlbomii · Bombus ruderatus · Introduced species · Pollinator eVectiveness Introduction During the last centuries, introduction of alien species to new ecosystems has become increasingly common and is today of major international conservation concern (Witten- berg and Cock 2001). A range of complex ecological prob- lems may arise when an invasive alien species is introduced to a new environment, e.g., by threatening native species through predation and/or competition (Christian 2001; Hol- way et al. 2002). Also, co-evolutionary processes in which native species are involved may be aVected, in particular mutualistic interactions such as the ones between pollina- tors and plants (Barthell et al. 2001; Traveset and Richard- son 2006). Alien plants and pollinators may modify the structure of plant-pollinator webs due to the increase in alien mutualists in the webs. This process involves the Communicated by Florian Schiestl. J. A. Madjidian · H. G. Smith Department of Ecology, Plant Ecology and Systematics, Lund University, Lund, Sweden C. L. Morales Laboratorio Ecotono, Universidad Nacional del Comahue, Bariloche, Argentina J. A. Madjidian (&) Plant Ecology and Systematics, Ecology Building, 223 62 Lund, Sweden e-mail: joseWn.madjidian@ekol.lu.se