The effects of livestock grazing on flowering plant and pollinator communities in shrubsteppe ecosystems

Background/Question/Methods

Livestock grazing is a prevalent grassland disturbance with negative impacts on the biodiversity of many plants and animals. Although pollination is an essential ecosystem service, few studies have examined how grazing affects pollinator communities of North American grasslands. We investigated how pollinators and plants of shrubsteppe ecosystems in the southern Okanagan, British Columbia, were impacted by low intensity, spring cattle grazing. We surveyed flowering plants and pollinators, via pan-trapping and hand-netting directly off of flowers, in four grazed and four ungrazed shrubsteppe sites over the course of a five month flowering season. The effects of grazing on flowering plant and pollinator abundance, richness and community composition were investigated using generalized linear mixed models and multivariate analyses. We also compared properties of interaction networks (generalization, asymmetry and nestedness) between treatments to evaluate community resilience. Since habitat features other than floral resources, such as vegetation height and bare soil availability, can impact pollinator populations, we also analyzed the impacts of grazing on vegetation structure. Additionally, two shrubsteppe types – antelope-brush and big sagebrush – were sampled so we also analyzed the influence of habitat type on plant and pollinator communities. 

Results/Conclusions

We collected 8,797 putative pollinators, comprising over 250 species, and sampled 54 flowering plant species across all sites. Livestock grazing increased the percent cover of shrubs and bare soil and decreased the maximum height of both the grass and forb layers. However, grazing did not impact flowering plant or pollinator abundance, richness or community composition. Instead, the composition of floral and pollinator communities differed between the two habitats sampled, antelope-brush and big sagebrush shrubsteppe (P=0.0002), likely due to environmental differences associated with elevation. Similarly, plant-pollinator network properties did not differ with grazing, but network-level generalization was higher in big sagebrush than the more endangered antelope-brush shrubsteppe (P=0.0452). Our results suggest that although grazing does influence aspects of shrubsteppe vegetation structure, these ecosystems, when managed responsibly, can remain reservoirs of flowering plant and pollinator diversity.