Invasive plants and native pollinators: How do alien floral rewards affect bumble bee fitness?

Background/Question/Methods Changes in the composition of plant communities directly affect the density and diversity of pollinators, particularly if the quantity or quality of floral rewards changes as well. While the effect of invasive plants on native plant-pollinator interactions and native plant performance has been documented, the impact of alien floral rewards on native pollinator performance has not been explored. In this study, we examined the effect of consuming pollen and nectar from an invasive plant on estimates of native pollinator fitness. Linaria vulgaris is an invasive perennial weed found throughout North America and is frequently visited by native bumble bees. In the Colorado Rocky Mountains, L. vulgaris has become a preferred food source for the native bumble bee Bombus appositus. First, we compared the nectar and pollen quality of L. vulgaris to several native plants that are significant components of bumble bee diet. Second, we created bumble bee microcolonies and fed them pollen and nectar that was a) from L. vulgaris alone, b) from several native flowers, and c) a 50:50 mixture of L. vulgaris and native plants. We then estimated bumble bee fitness by monitoring microcolony development and offspring viability. Results/Conclusions Results indicate that L. vulgaris has similar per-flower nectar production rates compared to native plants, but its nectar has a higher average sucrose concentration, making it richer in calories. Further, L. vulgaris produces more pollen with significantly higher protein content on a per-flower basis than its native counterparts. Pollen and nectar from L. vulgaris should therefore translate into improved microcolony fitness, however factors such as digestibility or toxicity of these foreign diet components may counteract their nutritional benefits. Understanding the fitness effects of including invasive plants in pollinator diets will provide insight into factors that affect pollinator performance. Bumble bees, like many other pollinators, are currently in decline; determining the effect of an invasive plant on such critical native pollinators will not only inform us about the shape of future plant communities that rely on pollinators but may improve our ability to create effective management plans for pollinators.