Grazing and Grazers Support Pollinators’ Critical Role in Ecology 

GRAZED PASTURES HOST TWO TO THREE TIMES AS MANY NATIVE BEES AS NON-GRAZED PASTURES

While most people are familiar with the classic European honey beehives, most native bee species in western rangelands excavate nests under small patches of bare ground, where they raise their young. In fact, bare ground is a habitat requirement for 70% of these solitary ground-nesting bees.

New WLFW-supported research explored how grazing impacts these, and other, critical pollinators, including the supportive role grazing animals like cattle have on maintaining reproductive habitat for ground-nesting bees.

Pollination is critical to the world’s food supplies. Across the globe, up to $550 billion worth of food supplies rely on pollination and more than 80% of economically important crops are pollinated by insects. Flowering plants that don’t produce food for humans also rely on pollination, and many of those plants provide food for animals that humans eat. In addition to their critical role in pollination, pollinators also serve as food for imperiled grassland songbirds, game birds, and even megafauna like grizzly bears.

Unfortunately, several factors are impacting global pollinator populations, including habitat loss due to development and conversion of native landscapes to row-crop agriculture, toxicity to pesticides, disease, and a changing climate.

Native bees (hereafter ‘bees’) are the primary pollinators in rangeland ecosystems, which cover one-quarter of the earth’s surface and support half of global livestock production. In the continental U.S., rangelands cover 35% of the land area and contribute food, fiber, carbon storage, and other ecosystem services that benefit the entire nation.

It has long been believed that the availability of native flowering plants is a primary determinant of the composition and abundance of bees and other pollinators. But other factors, including the availability of bare ground and the amount of dead and decaying litter on the ground, also impact bee abundance and diversity.

WLFW-supported research from Hayes Goosey, a researcher from Montana State University, examined native pollinator abundance in rangelands with three different grazing regimes – pastures in a wildlife refuge that had been idled (left ungrazed for more than a decade), grazed pastures enrolled in a managed-grazing program through the USDA-NRCS’s Sage Grouse Initiative, and grazed pastures with owner-controlled grazing regimes.

Goosey’s team analyzed the amount of bare ground, the amount of litter on the ground, vegetation cover, and the abundance of bees in each of the grazing systems.  They found that ground-nesting bee abundance in grazed lands, regardless of whether they were enrolled in a managed-grazing program or were owner-controlled, was 2-3 times higher than in idled lands.

Goosey attributed this finding to the amount of small patches of bare ground and lack of accumulated litter in grazed pastures versus idled pastures. Bare ground is an important component of intact, functioning rangeland systems that evolved with grazers, like bison. When herbivores are removed from rangeland systems, bare ground decreases and litter accumulation increases.  

Goosey’s analysis showed that bare ground covered nearly twice as much of the pasture in managed and un-managed pastures than in idled pastures (15% vs. 7%). Litter accumulation was half on grazed pastures than in the idled pastures (12% vs. 24%).

Because ground-nesting bees rely on bare ground for nesting and reproduction, this correlation highlights the critical role that accessible, bare ground plays for native rangeland pollinators like ground-nesting bees.

The team found that the vegetative cover of flowering forbs, forbs, grasses, lichen, and prickly pear was similar regardless of treatment. These resources are what bees rely on for nectar, but this research suggests that floral availability is less constraining on bee abundance than the availability of suitable reproductive habitat.

This research provides support for maintaining the structural components of bare ground and reduced litter through grazing, particularly when compared to other alternative land-uses like residential or industrial development or crop agriculture.

Maintaining healthy and productive native rangelands is the primary goal of WLFW’s efforts in the sagebrush and Great Plains grasslands biomes. Threats like land-use conversion, invasive annual grass invasions, and conifer encroachment threaten rangeland health and grazing economies. Maintaining viable grazing operations, on both public and private lands, is a key tool for maintaining native bee populations and the pollination services they provide.

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POLLINATOR RESPONSE TO LIVESTOCK GRAZING: IMPLICATIONS FOR RANGELAND CONSERVATION IN SAGEBRUSH ECOSYSTEMS

Abstract: World food supplies rely on pollination, making this plant–animal relationship a highly valued ecosystem service. Bees pollinate flowering plants in rangelands that constitute up to half of global terrestrial vegetation. Livestock grazing is the most widespread rangeland use and can affect insect pollinators through herbivory.

We examined management effects on bee abundance and other insect pollinators on grazed and idle sagebrush rangelands in central Montana, USA. From 2016 to 2018, we sampled pollinators on lands enrolled in rest-rotation grazing, unenrolled grazing lands, and geographically separate idle lands without grazing for over a decade.

Bare ground covered twice as much area (15% vs. 7) with half the litter (12% vs. 24) on grazed than idle regardless of enrollment. Bee pollinators were 2–3 times more prevalent in grazed than idle in 2016–2017. In 2018, bees were similar among grazed and idled during an unseasonably wet and cool summer that depressed pollinator catches; captures of secondary pollinators was similar among treatments 2 of 3 study years.

Ground-nesting bees (94.6% of total bee abundance) were driven by periodic grazing that maintained bare ground and kept litter accumulations in check. In contrast, idle provided fewer nesting opportunities for bees that were mostly solitary, ground-nesting genera requiring unvegetated spaces for reproduction.

Managed lands supported higher bee abundance that evolved with bison grazing on the eastern edge of the sagebrush ecosystem. Our findings suggest that periodic disturbance may enhance pollinator habitat, and that rangelands may benefit from periodic grazing by livestock.

Citation: Hayes B Goosey, Gabrielle E Blanchette, David E Naugle, “Pollinator response to livestock grazing: implications for rangeland conservation in sagebrush ecosystems,” Journal of Insect Science, Volume 24, Issue 4, July 2024.

Permanent URL: https://doi.org/10.1093/jisesa/ieae069

Acknowledgements: The authors appreciate the constructive comments from 3 anonymous reviewers who facilitated considerable improvements to the manuscript. The authors thank the participating landowners who graciously allowed them access to their property to complete this project. The authors would also like to thank Casey Delphia for her help with pollinator identification, data curation, and thoughtful reviews of figures and text. Funding for this project was partly provided by the USDA Conservation Effects Assessment Project-Wildlife Component and the Pittman-Robertson Wildlife Restoration Act through the United States Department of the Interior, Fish and Wildlife Service.