Thermal physiology of queens reveals insights into climate vulnerability in two co-occurring native bumble bees (Bombus)

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Poore, Colton Leroy
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Riddell, Eric A
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Bumble bees (Bombus) are economically and ecologically important pollinators found worldwide. However, recent evidence has revealed widespread bumble bee declines in North America, partially driven by the effects of climate change. Existing models that forecast the effects of climate change on Bombus populations often fail to include an underlying mechanism, which limits the potential of their predictive power. By studying the physiology of bumble bees at high temperatures, we may be able to use individual physiological responses to explain population dynamics across Bombus ranges as a result of climate warming. Also, by measuring the ability of bumble bees to acclimate to high temperatures, we might be able to determine differences in climate resiliency across species of Bombus. In this study, we compared the thermal physiology of queen bumble bees from B. impatiens and B. auricomus, two co-occurring species native to Iowa, with a focus on their ability to acclimate various physiological traits in response to warming temperatures. Using flow-through respirometry, we evaluated differences in metabolic rates between both species at ecologically-relevant temperatures. Additionally, we quantified rates of water loss in both species to investigate whether individuals would exhibit strategies by which to reduce water loss at high temperatures. Lastly, we studied the thermal tolerances of queens across both species to examine differences in their critical thermal minima, critical thermal maxima, and breadth of thermal tolerance, as well as in their survival rates over multiple time periods following thermal tolerance trials. Overall, the results indicate significant species differences in metabolic rates at high temperatures between B. impatiens and B. auricomus, as well as significant differences between species in regards to their critical thermal minima and maxima. However, neither species acclimated to the treatments in any of the studied physiological traits, which might indicate that bumble bees might be limited in their ability to respond to future warming. We also failed to document any strategy to reduce water loss rates at high temperatures, as water loss rates were primarily driven by the evaporative demand of air as well as by body size. Taken together, these results have important implications for understanding how native bumble bees might be affected by future climate warming.
Ecology, bumble bees, climate change, ecophysiology, respirometry, thermal tolerance, water loss