Before the brink: considering sublethal impacts of climate change on stingless bee flight performance

Vulnerability to climate change is often predicted using species critical thermal limits (CT_MAX), the temperature at which an organism experiences a loss of physiological function. However, climate change will impact species physiological traits, behaviour, reproduction, and phenology before their critical thermal limits are reached. Thermal performance curves (TPCs) are one way to evaluate how temperature impacts ecologically relevant traits before species reach their upper thermal limits. Comparing species TPCs to the climatic conditions they experience through their geographic ranges can provide insights into how vulnerable species are to further climate change and how their ranges might be altered. We assessed how flight performance - an important trait for escaping predators, dispersing, and finding resources and mates - is affected by temperature in two agriculturally important species of native stingless bees, Austroplebeia australis and Tetragonula carbonaria. A. australis has a broad arid/tropical geographic range and T. carbonaria has a narrower coastal subtropical range. We tested the thermal flight performance of both species at seven distinct temperatures between 18 and 42 °C. A. australis had a broader TPC and higher thermal optima than T. carbonaria reflecting the broader range of environmental temperatures and the hotter average environmental conditions they experience across their range. However, while A. australis could maintain flight performance at hotter temperatures, a larger proportion of their range is hotter than their thermal optima, suggesting their geographic range might constrict prior to the range of T. carbonaria.