Assessing Impacts of Grazing Management on Pollinator Conservation in Rangeland

View the project report

Commodities

• Animals: bees, bovine

Practices

• Animal Production: grazing management
• Crop Production: pollinator habitat, pollinator health

To determine how grazing management contributes to pollinator conservation, it is necessary to (1) quantify pollinator communities on ranchland and (2) determine relationships among grazing management, specific food and shelter resources, and pollinators. There is great opportunity for sustainability-oriented grazing strategies to promote beneficial insects. Determining pollinator occurrence and the underlying mechanisms, in relation to specific management strategies, is essential to meet this goal. It is also critical to determine how grazing management simultaneously affects pollinators and livestock.

Quantifying pollinator communities is an important first step for determining management impacts. Despite national and state calls for increased pollinator monitoring (Texas Parks and Wildlife Department 2012; Pollinator Health Task Force 2015), and the large geographic extent of rangelands (Havstad et al. 2007), there is still scant research on pollinators in rangelands. Indeed, the Texas Conservation Action Plan identifies northern Texas (where the proposed study will occur) as a region of high priority for increasing knowledge of pollinator distributions (Texas Parks and Wildlife Department 2012). Quantification provides a baseline for determining future population changes.

Effective management of rangelands for pollinator conservation also requires understanding the ecological processes that link grazing to pollinators, via pollinators’ resources. This mechanistic understanding is essential for balancing conservation and production goals. To achieve this, it is necessary to quantify food and shelter resources, and determine which of these habitat elements most strongly impact diverse pollinators. Research on grazing management sustainability, and current habitat monitoring done by some ranchers including the Dixon Water Foundation (a rancher collaborator on this project), is insufficient. Current monitoring focuses on habitat elements that support cattle production or reduce erosion, such as plant biomass and ground cover by grasses (Teague et al. 2011; McDonald et al. 2019; Harmel et al. 2021). In contrast, important pollinator resources include flowering forbs and small patches of bare ground (Harmon-Threatt 2020). Further, because pollinators vary in their resource requirements, effective pollinator conservation requires maintaining heterogeneous habitat with diverse food and shelter resources (Fuhlendorf & Engle 2001). Thus, for pollinator conservation, it is critical to quantify flower availability and diversity, ground cover categories including bare ground, and ground cover diversity. This includes measuring flower availability and diversity directly, and not simply estimating from forb biomass.

Simultaneously measuring floral and shelter resources, and the pollinator community, under different grazing management strategies will enable determining which habitat elements are most important for pollinator conservation in rangelands. Complex ecological processes underlie sustainable ranching. The approach proposed here enables determining which specific habitat elements (e.g., floral diversity, bare ground, grass cover), or habitat diversity itself, best support diverse pollinator communities. This approach also is critical for identifying habitat quality indicators that can best predict pollination services, for rangeland habitat quality assessment. This project will additionally add valuable knowledge of what constitutes shelter habitat for diverse pollinators, a key knowledge gap (Carvalheiro et al. 2021).

Sustainable ranchlands must meet the needs of both pollinators and livestock. While complete assessment of animal production outcomes is beyond the scope of this grant, forage production is one of the strongest determinants of livestock performance and is easily quantified. Thus, this project will determine how grazing management impacts on pollinators compare to impacts on plant biomass. This will provide the first assessment of how sustainability-oriented grazing management affects both pollinators and livestock.

Study sites: We will quantify pollinators and their resources at ranches in North Texas with different grazing management (Fig. 1). These include (a) nine sites maintained through high-intensity rotational grazing by the Dixon Water Foundation, (b) a continually grazed control site the Foundation will establish in 2023, (c) a continually grazed control site owned by Todd and Stephanie Underwood, and (d) a site that Mark Chapin (Westfork Ranch) manages through low-intensity rotational grazing. All sites are in the Cross Timbers ecoregion, consist mainly of open grassland, and have similar soil types (clay to clay-loam). At each site, the rancher cooperator will follow their typical management; this project does not necessitate any management changes. By working on active cattle ranches, we will fill a pressing need for research on sustainable grazing that is conducted on working lands (Teague et al. 2011). We selected the high-intensity rotational grazing sites in 2021, in cooperation with Dixon Water Foundation staff. Initial surveys in 2021 and 2022 confirmed that ground cover and plant communities differ among sites. This provides sufficient habitat variation to assess impacts of specific food and shelter habitat elements on pollinators. We selected all other sites in late 2022.

At each site we established (high-intensity rotational grazing sites) or will establish (all other sites) eight 90m transects that radiate out from a central point. Sampling along these transects enables capturing spatial heterogeneity within a site. This project will sample pollinators, flowering plants, ground cover, and forage production (biomass) using methods successfully piloted in 2021 and 2022. We will sample pollinators and vegetation during each of Texas’ main bloom periods (spring, summer, fall), and biomass in the dormant season (December), in 2023 and 2024. Sampling for two years will improve results, because insect and plant communities show inter-annual variation.

Insect sampling: We will sample pollinators through passive trapping (pan traps, and blue vane traps that target larger bees, deployed for 24 hours per site per season) and active netting of insects on flowers (Fig. 2 a,b). We will use an aerial net to collect flower-visiting insects during one 45-minute observation period in the morning and one 45-minute period in the afternoon at each site. For each netting sample, the observer will walk along each transect and collect all bees, wasps, butterflies, beetles, and flies observed in contact with plant reproductive tissues. All insect sampling will occur on days with favorable weather conditions (clear skies, temperature by 10AM > 17˚C, wind speed < 8.89 m/s). Pilot data collection in 2021 and 2022 indicates that insect activity is high under these conditions. All insects will be pinned and identified in the lab, to calculate abundance and diversity (e.g., richness) for each site.

Resource sampling: We will measure food and shelter resources via a quadrat method commonly used by pollination ecologists (Fig. 2c). We will place a 1x1m quadrat at six locations along each transect: 15m, 30m, 45m, 60m, 75m, and 90m from the center of the site (thus, 48 quadrats per site per season). In each quadrat we will identify all plant species currently in flower, count the number of flowers of each species, measure vegetation cover (separately quantifying green grass, green forbs, senesced vegetation, and open canopy), and measure ground cover (separately quantifying bare ground, litter, stems, rocks, and wood such as tree trunks). We will use these data to estimate total floral abundance and richness, relative cover by different vegetation types, and the proportion of the ground that is suitable for different insect shelter types (ground, litter, etc.) at each site. We will voucher all flowering plant species at each site by uploading detailed photos to an iNaturalist project that the lab maintains. We developed this project in collaboration with Dixon Water Foundation staff, as a tool both for our research and to demonstrate to stakeholders the plants found on Foundation property. Other rancher cooperators will decide whether we make public the vouchers from their properties; Mark Chapin has expressed an interest in doing so.

Biomass measurement: We will measure biomass by clipping all vegetation within a standard area, drying the vegetation, and weighing it. We will place a 0.5x0.5m quadrat 100m from the site center along four transects per site (N, E, S, W in 2023; NE, SE, SW, NW in 2024 to avoid carryover effects). At each location we will cut and collect all vegetation occurring within the box defined by the quadrat. To better estimate potential pollinator shelter resources we will separately collect standing matter (connected to the ground) and loose litter from within each quadrat. In the lab we will weigh each fresh sample, then dry each sample in an existing drying oven in the Lichtenberg lab, and measure dry weight. The total biomass weight from each site will measure forage production.

Relationship determination: Sampling will yield data on the pollinator community, flowers (food), shelter resources and biomass at each site. We will first determine whether the pollinator communities or available resources systematically differ among grazing management strategies. This will indicate whether each strategy can promote pollinators. We will then use linear regression and multivariate indicator analyses to determine which types of resources affect pollinator community diversity, and specific pollinator species. This will identify habitat elements that are most important for pollinator conservation, and that are useful indicators of habitat quality. Because sustainable ranching must support both livestock and diverse wild animals, we will also compare pollinator communities and important habitat elements to forage production at each site. This will provide an initial assessment of whether there are strong trade-offs between livestock production and biodiversity conservation.

Figures: Lichtenberg_SSARE22_figures