Habitat Suitability Modeling of Bombus affinis under Climate Change

Abstract

The North American bumble bee Bombus affinis (B. affinis) is an important wild pollinator that has experienced severe population declines since the late 20th century. However, spatially explicit assessments of how climate change and human activities jointly shape its distribution remain limited, constraining forward-looking conservation planning. Here, we used a maximum entropy model (MaxEnt), integrating species occurrence records with multidimensional environmental variables, to characterize the historical distribution of B. affinis and project changes in potential habitat suitability under different climate scenarios for the mid- and late 21st century. The results show that mean temperature of the warmest quarter was the dominant predictor of its distribution, contributing 59.0% to the model. Moreover, our model indicated a highly restricted historical distribution, with medium- and high-suitability habitats (suitability > 0.7) accounting for only 7.9% of the study area and concentrated along the North Atlantic coast, the Great Lakes region, and the Appalachian Mountains. By the late 21st century, the SSP585 high-emission pathway would cut both the size and the quality of suitable habitats markedly; the range would also contract northward, shifting away from the south-central and northeastern United States toward higher latitudes and finally into southern Canada. This work gives a quantitative evaluation of B. affinis habitat suitability in both historical and future periods, points to the key role that global emission cuts play in easing the species' survival pressure, and supplies scientific support for identifying cross-border priority conservation zones and building a climate-resilient conservation framework.