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’ 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.
