COS 88-2 - The abiotic factors governing ground-nesting bee emergence in the context of climate change.

Abstract: Globally, one of the most prominent biological responses to climate change is a shift in the timing of an organism’s life-history events, or their phenology. Phenology is tied to an organism’s growth, survival and reproduction and is often cued by the environment. To predict how an organism will respond to a changing environment, an understanding of the environmental cues of phenology is necessary, especially for species that provide critical ecosystem services, such as bees. This study focuses on the most diverse yet understudied group of bees, solitary ground-nesting bees, and how they interact with their soil environment to respond to seasonal cues. Specifically, we ask, how do soil moisture and soil temperature affect ground-nesting bee emergence? To approach this question, the emergence of three early season bee species at the Rocky Mountain Biological Laboratory in Gothic, Colorado were analyzed. Soil moisture and soil temperature were manipulated in a factorial design at 3-6 known bee aggregations in 2021 and 2022 (number varies by year). We mimicked historic, pre-climate change conditions by cooling the soil and increasing soil moisture content. We hypothesized that soil moisture content might affect emergence timing via effects on soil temperature. Emergence traps were used to determine mean emergence date for bees at each plot.

Our experimental manipulations cooled soil temperature by approximately 3-4˚C and increased soil moisture by approximately 30% compared to the control treatments. There was a positive effect of soil temperature on bee emergence in 2021 (R² = 0.6, p < 0.001), with bees emerging earlier under cooler temperatures, and a negative effect of soil moisture content on bee emergence in 2022, with bees emerging earlier under drier conditions (R² = 0.5, p = 0.01). However, there was no effect of soil temperature in 2022 (R² = 0.46, p = 0.13), or soil moisture content in 2021 (R² = 0.4, p = 0.86). We therefore do not find evidence that soil moisture affects bee emergence indirectly via temperature but instead has direct effects on the timing of bee emergence. The results of this study aim to improve our understanding of how ground-nesting bees interact with their soil environment, and how these interactions will affect pollinator vulnerability to climate change.