Postdoctoral researcher New Mexico State University Las Cruces, New Mexico
Species’ geographic ranges may be determined by physiological tolerance to environmental conditions. Identifying real-world conditions critical for survival, however, remains a difficult challenge, especially for cryptic portions of the life cycle. For Bombus queens, overwintering survival may be range limiting where wintertime temperatures are below their supercooling point despite the protections of shallow underground hibernacula. We tested this hypothesis using Bombus impatiens, an agriculturally important bumble bee species historically native to the eastern USA but introduced across the USA. In this study, we (i) inferred the distribution of supercooling point for Bombus impatiens queens, based upon measurements of individual workers, and (ii) inferred the distribution of below-ground temperatures as a function of depth throughout the winter at each of 388 locations across the United States and into Canada. Together these measurements allow inference of the survivorship probability as a function of hibernaculum depth across the species’ native and introduced range. Near the northern boundary of the species range the probability of survivorship declines, an indication that below-ground environmental temperatures could limit the northernmost range boundary. Not only does this study directly investigate how a potentially important environmental condition can affect the extent of a species range, it illustrates how one might assess whether other environmental conditions are limiting a species' geographic range. Importantly, uncertainty in individual response to the environment and stochasticity in the environment itself are fully accommodated by the analysis. Consequently, our approach will be particularly useful for studying how climate change affects the geographic ranges of species.