Historical changes in bumble bee body size and range shift of declining species

Bumble bees are declining worldwide, their vital ecosystem services are diminishing and underlying mechanisms are species specific and multifaceted. This has sparked an increase in long-term assessments of historical collections that provide valuable information about population trends and shifts in distributions. However, museums specimens also contain important ecological information, including rarely measured morphological traits. Trait-based assessments of museums specimens provide additional information on underlying mechanisms of population trends, by tracking changes over time. Here, we used museum specimens of four Bombus species, spanning a timeframe of 125 years to: (i) compare body size of declining and increasing species, (ii) assess intra-specific trends over the last century, and (iii) investigate shifts in geographical distribution over time. We found that declining Bombus species were larger than increasing ones. All four species were smaller in current time than a century ago. Intra-specific size declines were more pronounced for larger-bodied species. With our sampling, declining and increasing species showed an upward shift in elevation, and declining species showed an additional geographic shift in recent times as compared to historic records. Intra-specific body size declines may represent species adaptation to unfavorable environmental conditions, and may be a useful metric to complement traditional species vulnerability assessments. We highlight the utility of incorporating trait-based assessments into future studies investigating species declines.