Riparian Restoration and Targeted Grazing for Bees

Introduction

Image Details

Without pollinators, most flowering plants either cannot reproduce or have significantly lower reproductive success. On U.S. range and pasture lands, native bees provide the majority of pollination services for these plants. The seeds and fruits of these plants support herbivores such as birds, rodents, bears, and many other organisms. Therefore, rangeland ecosystems rely, to some degree, on healthy populations of native bees.

Ranching is an important economic activity in U.S. pasture and rangelands. Grazing can have negative impacts on native bees by disturbing the soil, where 70-80% of native bees nest, or by eating or trampling plants that bees use for pollen and nectar. 

Delaying grazing in important native bee habitat until after bloom richness and abundance peaks  is known as “phenologically targeted grazing.” This management strategy aims to keep livestock in areas that are of lower habitat value for native bees when other, more valuable habitat areas are in peak bloom and then rotating livestock into higher quality bee habitat after peak bloom. Native bees can then use the flowers in undisturbed areas as food resources without competing with livestock. 

Planting woody shrubs is an important aspect of riparian riparian
Definition of riparian habitat or riparian areas.

Learn more about riparian restoration to help stabilize soils and provide habitat for animals who depend on riparian corridors. These shrubs are also important sources of pollen and nectar for native bees and can enhance bee food resources when planted as part of ongoing riparian restoration efforts.

Researchers from Oregon State University evaluated two sites in eastern Oregon, a prairie managed by The Nature Conservancy and a forested riparian area managed by the U.S. Forest Service. Researchers compared the effects of late season grazing (after peak bloom) on native bees at both the prairie and riparian sites. Additionally, researchers captured bees from specific woody shrubs at the riparian site to understand which bees were using specific shrubs throughout the season.

Key Issues Addressed

Native bee communities worldwide face many challenges including habitat degradation (Potts et al. 2010). Although some studies have shown that livestock grazing negatively impacts native bees by degrading nesting habitat, reducing floral resources, and altering the foraging behavior of bees, the results in the literature are varied. Results have shown both beneficial and detrimental effects of grazing on native bees depending on the habitat type, composition of the bee community, timing, and intensity of grazing. In Oregon rangelands, grazing during peak bloom (defined here as “early season”) has been shown to reduce the richness and abundance of some native bee taxa (Kimoto et al. 2012) This study addresses the  effects of late season grazing, a management strategy purported to be more sustainable than continuous grazing, on native bees. This is the first study to formally investigate the effects of late season grazing on bees in riparian meadow and bunchgrass prairie systems in eastern Oregon.

Planting woody shrubs for habitat enhancement and soil stabilization is a common practice in riparian restoration, however, the benefits to native bees is not well known. Understanding which woody shrubs are most beneficial to which species of bee is critical if enhancing native bee habitat is to be one of the restoration goals for riparian restoration projects. For the current Case Study, researchers at OSU directly sampled native bee communities on specific flowering shrub species in restored riparian areas to understand which species of shrubs were most beneficial to the native bee community over the course of two years.

Project Goals

• Clarify the relationship between delayed grazing and the abundance and diversity of native bee communities by tracking bee and blooming plant community responses to delayed grazing
• Understand interactions between woody shrub species and native bees to inform restoration practices that seek to improve native bee habitat in the Inland Pacific Northwest
• See whether the plant & native bee communities in prairies or forested riparian areas are more vulnerable to grazing
• Utilize the results of the study to inform conservation and restoration of native bee habitat in grasslands and riparian corridors
• Contribute to knowledge of the distribution and habits of the native bee fauna of eastern Oregon and their interactions with native flowering plants

Project Highlights

Break for Bees: Grazing at moderate intensities after peak bloom did not have strong effects on native bees in prairies or riparian meadows where range riders kept cattle from overusing riparian areas. 

• New Knowledge about Old Projects: Bees were sampled from woody shrubs planted in 2012 as part of a riparian restoration project that planted over 50,000 woody shrubs. Sampling at this old restoration site provided new information about the benefit of riparian restoration and flowering woody shrubs to native bees.
• Thousands of Bees and Thousands of Flowers: Researchers surveyed bees and plants once a month from April to September. They counted a total of 68,568 flowers on 156 different species of plants and 7,128 bees from 172 different bee taxa! Each bee that was collected on a flower was identified to species to understand what time of year individual bee species were using flowers on specific woody shrubs.
• Shrub-Specific Netting: Throughout the season, researchers used nets to catch bees from woody shrubs to find out which species of native bee were using specific shrub species. Black hawthorn (Crataegus douglasii), was found to support a high diversity of native bee species. Wax currant (Ribes cereum), supported more “picky” bees, like the Pacific digger bee, that can show strong preferences for wax currant in this study system.

Lessons Learned

Image Details

Late season grazing did not adversely impact bee communities in the prairie or the riparian meadow as long as range riders prevented cattle from overusing the riparian area. In the riparian meadow, late season grazing reduced the abundance (total number) of blooms but not the richness (number of species) or diversity (combines abundance and richness) of blooms. Similarly, late season grazing had no strong negative effects on the abundance, richness or diversity of native bees at the riparian meadow. Surprisingly, in the prairie, late season grazing actually slightly increased bee abundance compared to ungrazed sites.

Blooming plant abundance and diversity is an effective and simple indicator of high quality bee habitat. Identifying areas with dense and diverse blooms allows land managers to identify important bee habitat without directly sampling bees.

Grazing cattle on old agricultural fields that have low abundance and diversity of blooms while peak bloom is occurring on higher quality bee habitat can be an effective strategy to reduce the negative impacts of grazing on native bees. Waiting as little as three or four days for peak bloom periods to end before moving cattle into an area may be enough to reduce the negative impacts of grazing on native bees.

Willows (Salix spp.) and wax currant (Ribes cereum) were found to be particularly valuable early season bee forage while black hawthorn (Crataegus douglasii) and mallow ninebark (Physocarpus malvaceus) were important later in the season.

Certain shrubs were important for possible specialist bees. The Pacific digger bee (Anthophora pacifica), for example, was found almost exclusively on wax currant. Several other specialist and rare bees were found on black hawthorn, mallow ninebark and willow. 

To reduce competition between ungulates, such as cattle and elk, and native bees, restoration practitioners can consider planting shrubs that are less palatable to cattle. Two species of low palatability that support many species of bees are wax currant and black hawthorn. These two species have complimentary bloom times and are used by 50 bee species observed during the study period.

Next Steps

• Publish this research in scientific journals and extension publications.
• This research highlighted several important research to be explored in the future: What is the effect of landscape variables (e.g., floral abundance) on pan trap efficiency? What are the effects of grazing on individual bee taxa of conservation concern? What are the grazing effects on annual vs perennial plants and how does this influence native bees? What can we learn from longer term studies looking at the effects of grazing on native native bees? What are the interactions between grazing and other disturbances (e.g., fire)?
• Expanding partnerships between land managers, private ranchers, and Citizen Science initiatives (such as Oregon Bee Atlas and Pacific Northwest Bumblebee Atlas) can reveal region-specific information about associations between specific plants and bee species.
• Evaluate abundance and diversity of native bee communities before and after restoration riparian restoration using woody, flowering shrubs to understand which woody flowering shrubs are most beneficial to native bees.
• Further assess specific interactions between bees and their chosen plants: Molecular techniques, such as DNA metabarcoding, are one potential tool for understanding apparent specialist interactions.
• Provide assistance to land managers to identify valuable native bee habitat and establish grazing plans such that critical bee habitat is grazed later in the season.

Funding Partners

• USDA NIFA NNF (Grant #549031)
• OSU Branch Experiment Station Internship Program
• U.S. Forest Service
• Oregon Department of Forestry

Resources

• Potts et al. (2010) “Global pollinator declines: trends, impacts and drivers.” Trends in Ecology & Evolution 25:345-353. https://doi.org/10.1016/j.tree.2010.01.007
• Kimoto et al. (2012) “Short-term responses of native bees to livestock and implications for managing ecosystem services in grasslands.” Ecosphere 3:88-107. https://doi.org/10.1890/ES12-00118.1

Shapira et al. (2019) “Rangeland sharing by cattle and bees: moderate grazing does not impair bee communities and resource availability.” Ecological Applications 30:3. https://doi.org/10.1002/eap.2066

Contacts 

• Scott Mitchell, Oregon State University: scott.mitchell@oregonstate.edu
• Sandra DeBano, Oregon State University: sandy.debano@oregonstate.edu

Suggested Citation

Léger, A., and Mitchell, S. (2021). “Riparian restoration and targeted grazing help native bees in eastern Oregon.” CART. Retrieved from https://www.fws.gov/project/riparian-restoration-and-targeted-grazing-bees.