The potential for phenological mismatch between a perennial herb and its ground-nesting bee pollinator

Climate change may alter the timing of flowering and pollinator activity to differing degrees, resulting in phenological mismatches between mutualistic partners. Assessing the potential for such mismatches requires an understanding of the environmental factors that cue flowering and pollinator activity. Biological context is key to determining specific impacts of climate change, and therefore it is important to study mutualisms with pollinators of different nesting biologies. Our study focused on the phenology of two mutualists native to the coastal dunes of northwestern California: the silky beach pea (Lathyrus littoralis) and its main pollinator, the ground-nesting solitary silver bee (Habropoda miserabilis). We measured the current phenological overlap between the two species and took advantage of local fine-scale spatial variation in the timing of flowering and bee nesting activity to develop predictive models of flowering and flight period timing based on variation in soil temperature and moisture. Temperature best predicted both flowering and bee activity, although soil moisture influenced the timing as well. Comparison of linear regression slopes of phenology against temperature suggests that bee nesting time is more sensitive to differences in seasonal maximum temperatures, and may advance more rapidly than flowering with temperature increases. Although the current phenological overlap between the two species is high, this differential response to temperature could result in a decrease in overlap with climate warming. Our results highlight that nesting biology may be critical in determining impacts of climate change on pollination mutualisms, as ground-nesting bees may respond differently than other bee species. In addition, this work reveals the utility of studying bee species that nest in aggregations for understanding ground-nesting bee phenology.