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Sarah A. Waybright and Michael E. Dillon: Read the article
Waybright & Dillon combine new data on physiology with continental scale ground temperature data to develop landscapes of mortality risk for overwintering ectotherms and apply the approach to predict mortality risk for overwintering bumble bee queens
In the story of Goldilocks and the Three Bears, a young girl stumbles upon a cabin in the woods and proceeds to make her way through the home, finding the perfect bowl of porridge, the comfiest chair, and the most sleep-worthy bed, much to the chagrin of the furry homeowners. Like Goldilocks, some organisms can tolerate only a narrow range of conditions that dictate their survival. These conditions become especially important during periods of limited resources and greater environmental stress, like winter. If they become too cold, they run the risk of freezing; if they become too hot, their metabolism may become overactive and run through the precious lipid storage they’ve worked to build up in the months prior. In particular, animals whose body temperature tracks that of the environment (ectotherms) are especially sensitive to varying environmental conditions. With global environmental change accelerating at a rapid and unpredictable pace, understanding organisms’ unique responses to a landscape of thermal conditions can help us predict their risk of mortality under future conditions.
In a recent American Naturalist publication, “Soilscapes of mortality risk suggest a Goldilocks effect for overwintering ectotherms” (Waybright and Dillon 2024), the authors propose a framework for predicting the survival landscape of any overwintering ectotherm based on physiological traits, historical temperature records, and future climate change scenarios. To test this model, they examined the life history strategies of one of the greatest contributors to global pollination and a royally critical member of the hive, the Queen Bumble Bee. Bumble bees spend the warmer part of the year buzzing around from flower to flower, collecting nectar and moving pollen. During this warmer season, they regulate their own body temperature, classifying them as a heterotherm. However, in temperate and polar regions of the world, bees will enter a dormant period during the winter where they reduce their energy usage and survive in a semi-dormant state until the temperature warms and the flowers begin to bloom. In this time, they become ectothermic, meaning their temperature matches that of the surrounding environment. To do so, the queen bees must crawl into the soil to find the perfect depth for overwintering. At shallow depths, temperatures can be too cold and increase the risk of freezing. At deep depths, temperatures may be too warm, leading to premature exhaustion of fat stores. While bees are certainly at risk due to changes in global temperature, research predicts bumble bees to be at high risk due to climate change based on their responses to growing season conditions. To address this, the authors set out to understand how changes in winter conditions might be contributing to these dramatic declines.
The authors characterized queen bee responses to various temperatures and the winter temperatures at various soil depths. Through measurements of dormant queen bee metabolic rates, the authors were able to determine how much of the fat stores are used by a queen bee across a range of temperatures. Measuring the freezing temperature threshold for queen bees would help to determine the probability of freezing, allowing the authors to further examine whether queen bees were being “pinched” between soil that was either too hot or too cold. Based on soil temperatures across a range of latitudes and elevations, they determined there is a limited range, typically between 10 cm and 20 cm, where temperatures are “just right” for these ectotherms. This suggests a narrow Goldilocks zone for queen bees. Historic records have shown queen bees overwintering anywhere between 2 and 20 cm, suggesting they may already be seizing these more desirable zones. Additionally, there is a great deal of spatial variation in using this model framework to predict Goldilocks zones and requires taking into account ecological variables (you can read the full paper to learn more about this model framework and its limitations!). Based on climate predictions, the findings of this study suggest overall warmer soil temperatures may reduce the risk of freezing, but increase the risk of exhausting fat stores. However, increased seasonal temperature swings combined with increased daily fluctuations in temperature will lead to an overall increased freezing risk.
The results from this study can be used to understand the risk for queen bees under current and future climate scenarios. It also provides a valuable framework for investigating the vulnerability of dormant ectotherms during an understudied period of life. Even further, this study pairs existing data on soil landscapes with laboratory measurements of basic physiological traits, both underscoring the need for more research on ectotherm physiology during dormancy and demonstrating a clear pathway to continue similar research.
Haleigh Yang is a 2nd-year PhD student in the Ecology and Evolutionary Biology department at UC Santa Cruz. She is studying the kelp forest dynamics in Southeastern Alaska and interested in understanding how consumers respond to seasonal changes in resources.
