Bombus dahlbomii

Abstract: Summary: The Palaearctic Bombus ruderatus (in 1982/1983) and Bombus terrestris (1998) have both been introduced into South America (Chile) for pollination purposes. We here report on the results of sampling campaigns in 2004, and 2010-2012 showing that both species have established and massively expanded their range. Bombus terrestris, in particular, has spread by some 200 km year-1 and had reached the Atlantic coast in Argentina by the end of 2011. Both species, and especially B. terrestris, are infected by protozoan parasites that seem to spread along with the imported hosts and spillover to native species. Genetic analyses by polymorphic microsatellite loci suggest that the host population of B. terrestris is genetically diverse, as expected from a large invading founder population, and structured through isolation by distance. Genetically, the populations of the trypanosomatid parasite, Crithidia bombi, sampled in 2004 are less diverse, and distinct from the ones sampled later. Current C. bombi populations are highly heterozygous and also structured through isolation by distance correlating with the genetic distances of B. terrestris, suggesting the latter's expansion to be a main structuring factor for the parasite. Remarkably, wherever B. terrestris spreads, the native Bombus dahlbomii disappears although the reasons remain unclear. Our ecological and genetic data suggest a major invasion event that is currently unfolding in southern South America with disastrous consequences for the native bumblebee species. © 2013 British Ecological Society.

Abstract: The separation between staminate and pistillate phases in protandrous, long- lived hermaphroditic flowers results in a seasonal shift in sex ratio from a pure male to a pure female flower population. In theory, temporal sex ratio associated with dichogamy may affect pollination, promoting the evolution of sexual specialization in plants when pollinator visitation does not limit seed output. Alstroemeria aurea, a perennial herb native to the temperate forests of southern South America, bears strongly protandrous flowers. Over a three-year period, I assessed the influence of seasonal changes in flower sex ratio and pollinator abundance on the pollination dynamics of this plant species. Workers of Bombus dahlbomii accounted for >90% of all visits to A. aurea flowers and were the most efficient pollinators on a per-visit basis. As the season progressed, pollen loads carried by bumble bees decreased by almost one order of magnitude, and the amount of pollen deposited per visit decreased as well from >40 to <4 pollen grains from the beginning to the end of the flowering season. As flowering patches became more female, the number of male-phase flowers visited uninterruptedly during each bumble bee foraging bout decreased, while the number of sequential visits to female-phase flowers increased. A laboratory experiment of manipulating dried bumble bees showed that the number of male- and female-phase flowers visited in uninterrupted sequence strongly influences the amount of pollen deposited on a flower's stigma. Pollen deposition in this experiment closely mimicked the relationship between sex ratio and the observed seasonal decline in per-visit pollen deposition in the field. Because overall rate of bumble bee visitation increased monotonically over the flowering season, total pollen receipt per flower also increased during the first half of the season. Nevertheless, pollen deposition decreased during the second half of the season when vis- itation rates saturated pollen removal, and low pollen availability associated with an in- creasingly female-biased sex ratio apparently became the limiting factor in pollen transfer. Despite pronounced seasonal changes in pollinator and pollination levels, comparisons of seed output between open- and hand-pollinated flowers showed no evidence that seed production was ever limited by total pollen receipt. Because flower sex ratio influenced pollinator visitation and pollination patterns, and pollinator availability did not limit seed output, protandry should favor the evolution of female and male specialists flowering at the tails of the flowering season. Neither unisexuality nor any other form of sexual poly- morphism is found in A. aurea, however. Likewise, I found no evidence that plants might be compensating for a seasonal shift in sex ratio through differential allocation into pollen and ovule production within flowers or to relative duration of male and female sexual phases. These results suggest that a seasonal shift in operational sex ratio within a di- chogamous species can profoundly affect pollination without necessarily driving the evo- lution of floral polymorphism.

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.