PS 32-201 - Up, up, and away: Mountain tops are the last refuge for Bombus affinis in its southern range

Abstract: The rusty patched bumble bee (Bombus affinis) has seen significant population declines over the past few decades. Listed as endangered under the Endangered Species Act, its populations once ranged across much of the Eastern United States, but are now only found in pockets of the Upper Midwest and the mountains of Virginia and West Virginia. The causes of this decline remain largely unknown. With an emphasis on the southern portion of its range, we generated habitat suitability models to examine environmental variables that may predict where the species is still found and where to search for additional populations. We developed most-predictive models using currently known populations in the southern Appalachians and compared areas of high-quality habitat based on past and present environmental conditions. This approach may reveal which factors have changed over time and suggest potential causes of decline. We also conducted surveys on bumble bee populations at 40 sites around or near high probability B. affinis habitat to see how these populations change with changes in elevations and local plant communities.
Habitat suitability mapping found temperature variables as the strongest predictors of B. affinis habitat in the southern Appalachian portion of its range. Our models estimated 15.9% of the study area to be high-quality habitat based on past environmental data, but only 1.1% based on current conditions. High quality habitat in the southern region shrunk to primarily high-elevation areas, where cooler temperatures persist as average temperatures across the historic range increased. Extrapolating northward, this model successfully predicted the species persistence in the upper midwest but also predicted its persistence in the northeast, where it may no longer occur. Thus, factors other than temperature likely influence the species overall distribution. Surveys within projected high quality habitat yielded two distinctive bee and plant communities: B. impatiens dominated sites largely comprising roadside weeds regardless of elevation and B. vagans dominated sites comprising primarily native forest flowers at higher elevations. Thus, if the primary habitat of B. affinis tracks B. vagans in its southern range, then it may be restricted by both temperature and habitat mediated interactions with common, non-forest related bumble bees. Other factors not studied here that should be explored across its whole range include parasites and pesticide exposure.