D3478: Non-native Bombus impatiens broadens dietary niches of native Bombus spp. in the Pacific Northwest

Invasive species often have strong competitive effects on native congeners. Currently, non-native Bombus impatiens Cresson, native to eastern North America, is rapidly expanding its introduced range in the Pacific Northwest region of North America, yet its potential competitive effect on foraging by native congeners remains unknown. We surveyed bumble bee communities and floral visitation across a gradient of B. impatiens invasion in western British Columbia, Canada, and built plant-pollinator interaction networks based on floral visitation. We then explored the influence of B. impatiens on native Bombus dietary composition by extracting species-level network metrics for native Bombus that reflect dietary specialization, and relating these metrics to B. impatiens forager abundance.

Mean dietary specialization (Bluthgen’s d’) for native Bombus species decreased in response to increasing abundance of foraging B. impatiens. Dietary specialization showed little response to other attributes of floral-Bombus community composition, such as abundance or richness of native Bombus and floral resources. Our results suggest that native Bombus spp. broaden their dietary niches when foraging in the presence of abundant B. impatiens, possibly as a response to displacement from preferred resources. This behavior has the potential to affect both the nutritional quality of native Bombus diets and the effort expended on foraging.