LB 9-72 - Bottom-up impacts of rotational grazing disturbance on ground-nesting bee assemblages: do they dig it?

Determining the indirect effects of disturbance is essential to understanding how ecological communities change over time. This is especially true of grasslands, which depend on periodic disturbance. Grazing is a prominent disturbance in many grasslands and can exert powerful top-down effects on plant communities. However, its bottom-up impacts on grassland consumers such as pollinators are poorly understood. Grazing disturbance has the potential to impact ground-nesting bees on two main fronts: via changes to foraging resources (flowers) and nesting resources (soil substrate, ground cover, and canopy cover). While the importance of foraging resources for bees is well studied, the importance of soil nesting resources has often been overlooked. Ground-nesting bees are important to study in this context, as they comprise about 90% of grassland bee species and provide key pollination services. We examined the bottom-up impacts of grazing on ground-nesting bee assemblages at nine active ranch sites that have been in rotational grazing for either years or decades. We sampled bee assemblages to assess the impact of time in rotational grazing, and measured soil and floral resources to determine how these resources mediate grazing impacts on ground-nesting bee abundance and diversity. Time in rotational grazing did not significantly influence ground-nesting bee assemblages, but did impact availability of bare ground. Certain site characteristics unaffected by grazing were important for ground-nesting bee assemblages. Sites with shallower and sandier soils tended to have more ground-nesting bees, although this was largely due to increased abundance of certain gregarious species. Sites with those soil types also tended to have greater bee richness. Sites with less dense groundcover and higher abundance and diversity of forbs/flowers tended to have higher ground-nesting bee abundance and richness. Mean soil compaction, soil organic matter, slope-aspect, canopy height, humidity and temperature did not significantly affect site ground-nesting bee abundance or diversity. Our findings suggest that the bottom-up impacts of rotational grazing disturbance are relatively unimportant for ground-nesting bee assemblages compared to the availability of key habitat resources such as sandy soil, more easily accessible ground, and abundant and diverse flowers. All of these key resources were unaffected by time in rotational grazing on the timescales we compared. Our results suggest that rotational grazing may be compatible with grassland bee conservation, and highlight the need for determining the mechanisms through which grazing disturbance impacts diverse pollinators with different foraging and nesting resource needs.