Soil type and floral phenology interact to structure wild bee communities in serpentine grasslands

Much recent examination of bee-plant communities and diversity has focused on the role of anthropogenic drivers (e.g. agriculture, forest management, forest fires). But very few studies address whether underlying environmental factors contribute to expected community variation at local to landscape scales. We studied whether serpentine soils – an environmental filter known to structure plant communities – also structures wild bee communities in California’s coastal mountain ranges. We sampled flower and bee communities of meadows located on serpentine and non-serpentine soil types. We asked whether soil type affected (1) the floral community phenology and composition, and (2) the phenology and composition of wild bees visiting those flowers. We also evaluated how community composition of both plants and bees changed over time, and between different soil types. We found that floral communities in serpentine meadows were consistently more diverse and abundant than in non-serpentine meadows and remained more abundant later into the spring/summer. Similarly, bee communities in serpentine grasslands were richer and more abundant than in their non-serpentine counterparts. Notably, the difference in bee abundances between soil types was most pronounced at the beginning and end of the spring flowering season, when the floral abundance differences between soil types were greatest. The composition of floral communities differed between serpentine and non-serpentine meadows, but that signal did not extend to bee communities. Although bees were more abundant in serpentine, bee community composition did not differ. Serpentine soils serve as resource islands by supporting late-blooming floral communities, supporting a more diverse late-season bee community.