Displacement of a native by an alien bumblebee: lower pollinator efficiency overcome by overwhelmingly higher visitation frequency

Abstract

Biological invasions might constitute a major threat to mutualisms. Introduced pollinators might competitively displace their native counterparts, which in turn affects the pollination of native plants, if native and alien visitors differ in pollinator effectiveness. 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 effectiveness (i.e., the combination of pollinator efficiency per visit and visitation frequency) between both bumblebee species, as well as related pollinator traits that might account for potential differences in pollinator efficiency. Native Bombus dahlbomii, which has a larger body and spent more time per flower, was the more efficient 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 flower visitor than Bombus dahlbomii. As a consequence, Bombus ruderatus is nowadays a more effective 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 consequence 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.

Impacts of alien bees on native plant-pollinator relationships: A review with special emphasis on plant reproduction

“…The introductions were intended to improve red clover pollination. Bombus ruderatus was introduced to Chile (from New Zealand) in 1982 for the same reason and reached Argentina by 1994, where it affected native interactions between Alstroemelia aurea and B. dahlbomii (Madjidian et al, 2008).…”

“…Only three studies have investigated alien bumblebee impacts on native plant reproduction (Kenta et al, 2007;Dohzono et al, 2008;Madjidian et al, 2008). In these cases, alien bumblebees interacted with congeneric native bumblebees and affected bumblebee-pollinated plants, because they tend to share plants that have complex floral morphology specialized for bumblebee-pollination (Suzuki et al, 2007).…”

“…Seed production of five plant species decreased only in treatment (2), in which the aliens failed to access the floral mechanism correctly. However, the relative abundances of native and alien bumblebees were controlled in this experiment, and extrapolation to field circumstances is difficult because natural variation in pollinator abundances was not taken into account (Kenta et al, 2007;Madjidian et al, 2008).…”

“…Another important field study was performed in Argentina (Madjidian et al, 2008; Table 1). The native plant Alstroemeria aurea was originally pollinated only by native Bombus dahlbomii.…”

“…Introduced honeybees and bumblebees compete with native pollinators and reduce the fitness of some native plant species (Gross and Mackay, 1998;Celebrezze and Paton, 2004;Dupont et al, 2004;Dohzono et al, 2008). However, in other cases, plant reproduction is unaffected (Vaughton, 1992;Dupont et al, 2004) or even enhanced by the presence of introduced honeybees and bumblebees (Gross, 2001;Madjidian et al, 2008). To understand the variability in the effects of bee introductions, it is necessary to identify the precise mechanisms by which these invasive pollinators affect plant reproduction.…”