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

Background/Question/Methods

Determining the indirect effects of disturbance is essential to understanding changes in ecological communities. 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). 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 bees. We then assessed whether floral- or soil-mediated grazing impacts were more influential for ground-nesting bee assemblages.

Results/Conclusions

Time in rotational grazing did not significantly influence ground-nesting bee assemblages. However, certain site characteristics unaffected by grazing did. We found more bees at sites with shallower soils. This pattern may be explained by the fact that shallower soils also tended to be sandier and have more bare ground, which ground-nesting bees generally prefer. Soil depth was not related to time in rotational grazing. Mean soil compaction and organic matter, slope-aspect, canopy height, humidity and temperature did not significantly affect site ground-nesting bee abundance. 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 bare sandy soil, which are unaffected by time in rotational grazing. Analyses of ground-nesting bee diversity and comparisons of the importance of floral resource availability versus nesting resource availability may find that additional factors are important for ground-nesting bee assemblages. Our results suggest that rotational livestock 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.