Local nesting resources rather than floral food alter ground-nesting bee communities in grazed habitats

Abstract

O_LIUnderstanding how the availability of both trophic and non-trophic resources impacts key ecosystem service providers is crucial for informing conservation efforts. Non-trophic resources such as shelter can play critical roles in ecosystem function and resilience by affecting predation and pathogen rates, reproductive success, and interspecific interactions. However, non-trophic resources are often overlooked. This holds true for insect pollinators; impacts of shelter on community dynamics and pollination remain poorly understood. C_LIO_LIBecause the majority of bee species spend most of their lives underground, we explored whether nesting habitat characteristics are key drivers of ground-nesting bee abundance, diversity, and community composition. We additionally compared impacts of aboveground versus belowground nesting habitat characteristics, as the latter have received significantly less attention. Nesting habitat characteristics we considered included bare ground cover; and soil texture, compaction, and depth. We also included floral abundance and richness. We focused on rangelands, natural or semi-natural grasslands that are managed for livestock grazing, where management can dramatically alter trophic and non-trophic resources. Thus, we also investigated impacts of grazing management on bee habitat. C_LIO_LIWe found that belowground, but not aboveground, nesting habitat characteristics were important for ground-nesting bee assemblages. Ground-nesting bee abundance and richness were highest at sites with sandier and less compacted soils. C_LIO_LIOur results suggest that livestock grazing management may indirectly impact ground-nesting bee abundance through impacts on floral abundance. Sites with longer grazing rest periods had higher floral abundance, and sites with higher floral abundance supported more ground-nesting bees (but not more bee species). C_LIO_LIPolicy implications: Our study highlights the importance of considering non-trophic resources in conservation and restoration management. Managers should be cautious in using an aboveground measure such as bare ground as an indicator of bee habitat quality. Rather, we recommend that belowground habitat characteristics such as soil texture and compaction be included in habitat quality assessments. Evidence-based rangeland conservation and restoration planning may need to manage for ground-nesting bees preferred levels of soil compaction. Additionally, management-insensitive habitat characteristics such as soil texture may be important considerations for site prioritization in pollinator habitat conservation and restoration efforts. C_LI