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

Understanding 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 space, shelter, or mates 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 non-trophic resources on community dynamics and pollination remain poorly understood.

We investigated the importance of food (flowers) and shelter (nests) for ground-nesting bees. Because the large majority of bee species spend most of their lives underground, we explored whether nesting habitat availability is a key driver of bee abundance, diversity, and community composition. We additionally compared impacts of aboveground characteristics that describe nesting habitat with the belowground characteristics that have received significantly less attention. Habitat characteristics we considered included floral abundance; bare ground cover; vegetative biomass; and soil texture, compaction, and depth.

We found that belowground, but not aboveground, nesting habitat characteristics were important for ground-nesting bee assemblages. Ground-nesting bee abundance and richness was highest at sites with sandier and more compacted soils. Floral food availability also increased bee abundance. No habitat characteristics affected bee community composition.

Policy implications: Our study highlights that the availability of high-quality nesting habitat can strongly impact ground-nesting bee assemblages, and that measuring belowground characteristics of nesting habitat may be critical for understanding bee community dynamics. Thus, data-driven conservation planning should consider management-insensitive site characteristics, such as soil texture. Because land and funding are limited, focusing on belowground site characteristics when prioritizing locations for conservation actions can facilitate conservation success.