Range-wide niche models of three declining North American bumble bees: The closely related Bombus pensylvanicus and B. sonorus and their parasite B. variabilis (Hymenoptera: Apidae)

Bumble bees are important pollinators that provide critical ecosystem services; however, one-quarter of bumble bee species in the USA are declining and listed by the IUCN as Vulnerable, Endangered, or Critically Endangered. Their decline has been attributed to the synergistic effects of several phenomena, including climate change, pesticide use, disease, and habitat loss. The US Fish and Wildlife Service is currently working to determine whether the status of Bombus pensylvanicus sensu lato, which includes the American bumble bee [B. pensylvanicus (DeGeer)] and Sonoran bumble bee [B. sonorus Say] as well as that of their Critically Endangered parasite, the variable cuckoo bumble bee [B. variabilis (Cresson)] warrants the federal-level protections afforded by the Endangered Species Act. Regardless of the outcome of this review, we suggest high-resolution, continental-scale habitat distribution models are needed to fine-tune conservation plans aimed at stemming further losses. A feature selection algorithm was employed to derive an ensemble of 15 fine–scale (1 km), top–performing MaxEnt habitat distribution models that were derived from optimally–sized, decorrelated variable subsets using presence data obtained from numerous sources and over 525 environmental predictor variables. Our ensemble models accurately predicted the distributions of interest (Area Under the Curve [AUC]=0.83–0.92). We found that all but the most liberal projection of each species’ distribution indicates their ranges have contracted significantly. The relatively frequent appearance and mean permutation importance of climatic- and certain vegetation index-based variables suggest that climate change is putting pressure on these bumble bees and their vegetation.