Bottom-up effects of an invasive plant on native bumble bee pollinators

Background/Question/Methods

Invasive plants can dramatically alter native plant communities via both direct competitive effects as well as indirect competitive effects mediated through changes in pollination services. However, the repercussions of plant invasions for native pollinator populations have rarely been explored. An increase in the abundance of the Eurasian forb yellow toadflax, Linaria vulgaris, in the western United States has been concurrent with a marked shift in the foraging behavior of some native bumble bee species, with workers demonstrating a preference for this invasive plant over native flowering species. To assess the bottom-up effects of L. vulgaris on native pollinators, we compared the colony density and genetic diversity of bumble bees at sites with and without L. vulgaris in southwestern Colorado.  We sampled workers of two bumble bee species (Bombus bifarius and B. appositus) that commonly forage on L. vulgaris at six paired sites in 2010 and twelve paired sites in 2011. We collected a middle leg from each netted worker for molecular analysis. We also measured flower abundance and richness of native plants and flower abundance of L. vulgaris. We genotyped up to 20 microsatellite loci in approximately 2100 workers to evaluate kinship and colony density at each site.

Results/Conclusions

Data suggest that sites with L. vulgaris have significantly higher bee abundance than sites without L. vulgaris. Moreover, bees from significantly more colonies forage in sites with vs. without L. vulgaris, and the estimated effective population sizes of bumble bees at invaded sites are significantly higher than at uninvaded sites in the first field season. However, abundance of the two focal bee species varied substantially between our field seasons and between sites with and without L. vulgaris; these differences in abundance may result in different patterns of bumble bee population dynamics between field seasons.  Our current results suggest that pollinators can benefit from foraging on L. vulgaris, and the presence of an invasive plant may therefore play an important role in maintaining the size, number and genetic diversity of bumble bee colonies.  Recent studies have suggested that reduced genetic diversity may be linked to declines in bumble bee populations; thus, invasive plants may surprisingly provide a critical safeguard for native pollinators.