Final report for OS23-162
Project Information
The purpose of this project is to quantify the contributions to pollinator conservation of different grazing management strategies. Pollinators are critical for healthy rangelands. They enable production of food for both people and livestock, and help maintain healthy water and soil (Black et al. 2011). While bees are the most important pollinators in many grasslands, other insects including beetles, flies, butterflies, and moths make significant contributions (Hanberry et al. 2021; Lichtenberg et al. 2024). Rangelands cover almost a third of the US land area with largely natural land (Havstad et al. 2007). Thus, they have strong potential to support pollinators by providing critical food (e.g., pollen, nectar, larval host plants) and shelter (e.g., bare ground for ground nesting species, woody cavities, pithy stems) habitat. Indeed, state and national initiatives target grassland management as critical for pollinator conservation (e.g., Pollinator Health Task Force 2015; USDA 2021). However, extensive overgrazing, and conversion to row crops or developed areas, have degraded rangeland habitat.
One management strategy that aims to increase grazing sustainability is rotational grazing. This strategically incorporates periods of rest to enable plant regeneration, and distributes cattle impacts across the entire landscape rather than a small number of preferred locations. Compared to continuous grazing, rotational grazing can reduce soil compaction, decrease erosion, increase nutrients available for plants, and improve cattle forage availability and quality (Teague et al. 2011; Byrnes et al. 2018; Harmel et al. 2021). Two common rotation categories are high and low frequency. The former is often referred to as high-intensity, low-rotation or adaptive multi-paddock grazing. High-frequency rotation seeks to mimic historic bison grazing by frequent rotation of cattle herds among many smaller paddocks. Each paddock receives several months of rest after a short period (often several days) of intense grazing. Low-frequency rotation includes the Merrill system (three herds rotated among four pastures), the switchback system (one herd, two pastures), and many variations that involve rotating among pastures every few months. Research to date on the sustainability of rotational grazing has focused almost entirely on cattle forage and production. However, rangelands as functional ecosystems must support various ecosystem functions and services, in addition to meeting production goals.
There is a particular need to study grazing impacts on pollinators. The properties of ranchland that are most sensitive to management – vegetation cover and composition, and soil structure (Havstad et al. 2007) – are the same properties that determine the fate of many pollinators. Thus, while ranchlands have high potential to support pollinator conservation, this can only be achieved through increased understanding of how specific management strategies affect pollinators and the resources that support them. This project will advance this goal by determining how adoption and intensity of rotational grazing, impacts pollinator habitat – including food and shelter resources – and communities.
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.
Cooperators
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Research
In 2023-2025 we quantified pollinators and their resources at eight sites maintained through high-intensity rotational grazing by the Dixon Water Foundation, (b) six continually grazed control sites, and (c) seven sites managed through low-frequency rotational grazing (Fig 1). We worked with three ranchers in addition to those listed in the original proposal. At each site, the rancher cooperator followed their typical management; this project did not necessitate any management changes. To quantify management, we digitized grazing charts from cooperators who provided them.
At each site we established eight 90m transects that radiate out from a central point. Sampling along these transects enabled capturing spatial heterogeneity within a site. In 2023-2025 we sampled pollinators, flowering plants, ground cover, and forage production (biomass) using methods successfully piloted in 2021 and 2022. We sampled pollinators and vegetation during each of Texas’ main bloom periods (spring, summer, fall) in 2023 and during the summer bloom period in 2024-2025, and biomass in the dormant season (December). Sampling for multiple years improved results, because insect and plant communities show inter-annual variation.
We sampled 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 along the transects (45 minutes in the morning and afternoon). All insects were pinned and identified in the lab, to calculate abundance, richness (number of species), and plant visitation networks for each site. We measured food and shelter resources via a quadrat method commonly used by pollination ecologists. We placed 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 identified all plant species currently in flower, counted the number of flowers of each species, measured vegetation cover (separately quantifying green grass, green forbs, senesced vegetation, and open canopy), and measured ground cover (separately quantifying bare ground, litter, stems, rocks, and wood such as tree trunks). We used 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 vouchered all flowering plant species at each site by uploading detailed photos to an iNaturalist project that the lab maintains. In 2024, we additionally measured vegetation cover using the Grassland Effectiveness Monitoring (GEM) protocol. This is a line-point intercept method developed through a collaboration of federal and state agencies, non-profits, and private companies to monitor grassland responses to practices like prescribed fires, grazing, herbicide application, plantings, and brush management. Implementing the GEM protocol at our sites allowed us to compare its cover measurements to those obtained with our quadrat-based method. We measured biomass by clipping all vegetation within a standard area, drying the vegetation, and weighing it. We placed a 0.5x0.5m quadrat 100m from the site center along four transects per site. At each location we cut and collected all vegetation occurring within the box defined by the quadrat. To better estimate potential pollinator shelter resources we separately collected standing matter (connected to the ground) and loose litter from within each quadrat. In the lab we measured dry weight.
We used these data to address several questions.
- Do available resources systematically differ among grazing management strategies?
- How do grazing management and resources impact pollinator communities and plant-pollinator interactions?
- Is there a strong trade-off between livestock production and conservation of plants and pollinators?
- How do different methods for measuring vegetation cover in grasslands compare?
We addressed questions 1 and 2 through a combination of (general) linear regression, multivariate indicator analysis that assessed impacts on community composition, structural equation modeling (SEM) that allowed us to determine both direct impacts of management on pollinators and indirect impacts via flowering plants, and analyses of interaction networks. This helped indicate whether each strategy can promote pollinators and identify habitat elements that are most important for pollinator conservation and thus are useful indicators of habitat quality. We addressed question 3 by determining the correlation between flowering plant richness and dormant season biomass (livestock forage). This is important because sustainable ranching must support both livestock and various wild animals. To address question 4, we conducted a formal methods comparison of our pollinator-focused vegetation cover protocol, the GEM protocol, and a relatively new Rangeland Analysis platform (RAP) set of spatial data layers from remote sensing. Comparisons were both qualitative and quantitative via logistic regression.
Across the project duration we found 189 flowering plant species and collected 8,263 insect specimens. This includes 3,612 Coleoptera (beetles), 2,292 Diptera (flies), 1,668 Hymenoptera (bees and wasps), and 396 Lepidoptera (butterflies and moths). This includes two Texas Species of Greatest Conservation Need (SGCNs) – narrow-leaf coneflower (Echinacea angustifolia) and Megachile amica – and one bee species that was last found in Texas in 1905: the blue sage bee (Xenoglossa cressoniana; Fig. 2).
A Master's thesis addressed questions 1 and 2. SEM indicated that high-frequency rotation increased flower abundance, and that this resulted in higher pollinator abundance (Fig. 3). Linear regression showed that flower richness was also higher under high-frequency rotation. Combining data collected through this grant with soil texture measured through a different grant showed that managing for plants and pollinators also needs to consider soil properties. Our sites were largely clay and clay loam (Fig. 4). Sites with higher soil sand content had higher flower richness and higher abundance of Virginia pepperweed (Lepidium virginicum), big-headed rabbit-tobacco (Diaperia prolifera), Texas toothleaf (Stillingia texana), dotted gayfeather (Liatris punctata), and hairy vetch (Vicia hirsuta). Abundance of two beetle species – Cryptorhopalum dermestid beetles and rugged flower weevils (Odontocorynus salebrosus) – was lower at sites with sandier soil. The natural history of these two species is not sufficiently described to know why they are sensitive to soil texture. Grazing management also altered plant-pollinator interactions, with interaction networks being more specialized in pastures experiencing high-frequency rotation than in continuously-stocked pastures. Specialized species are highly dependent on one or a few other species, and often indicate more intact networks. They tend to be the first to disappear under disturbance. We additionally found high interaction turnover among sites, driven largely by species turnover rather than interaction rewiring. This, and differences in community composition across the three grazing management strategies, indicate that each strategy supports somewhat different groups of plant and pollinator species. Our results suggest that maintaining a mosaic of management strategies across rangeland landscapes may best promote pollinator conservation, as long as some pastures are managed in ways such as high-frequency rotation that support more sensitive species and specialized interactions.
We addressed question 3 by looking for a correlation between the flowers that feed bees and the vegetation biomass that feeds livestock. Flowering plant abundance and richness did not correlate with winter forage yield (Spearman's rank correlations: flower abundance r = 0.28, p = 0.25; flower richness r = 0.22, p = 0.36). While flower abundance did differ with management, biomass did not (Kruskal-Wallis rank sum test: Χ22 = 4.46, p = 0.11). This suggests there might not be a trade-off in managing for grasses and forbs, which facilitates sustainable rangeland management that supports pollinator conservation.
A second graduate student addressed question 4 by conducting a formal methods comparison of three different protocols for measuring rangeland/grassland vegetation: USDA's Rangeland Analysis remote sensing platform (RAP), USFWS' Grassland Effectiveness Monitoring (GEM) protocol, and the quadrat-based protocol described above that is used by many pollinator conservation researchers. Qualitative comparison found that the protocols varied in whether and how they separated plants into functional groups (e.g., grasses versus forbs, alive versus senesced), whether they measure canopy gaps, the resolution they capture data at (10 x 10 m squares, points, 200 cm2 patches), and how much time they took to implement. The pollinator protocol does not measure canopy gaps, and our simulation analysis showed that such gaps cannot reliably be estimated from the proportion of the quadrat that has no canopy cover. The GEM protocol was quicker to implement in the field than the pollinator protocol. Quantitative analysis showed that the three protocols captured similar patterns in canopy and ground cover. However, the pollinator protocol recorded relatively more forb cover (and was the only protocol to reliably estimate flower richness and cover) while the GEM protocol found more grass cover. Our results highlight that each protocol has strengths and limitations, and that it is critical to clearly articulate management goals or research questions before selecting a method for measuring rangeland vegetation.
Educational & Outreach Activities
Participation summary:
We disseminated results from this project through oral and written presentations to ranchers, land managers, and academics. Outreach to ranchers and land managers leveraged existing activities hosted by the Dixon Water Foundation, adding a component on managing rangelands for pollinator conservation. The Dixon Water Foundation regularly supports local ranchers through customized demonstrations, field days, and workshops. We developed materials that disseminate our results during these events.
- On-ranch field day (Fig. 5) – The Dixon Water Foundation routinely hosts workshops on sustainable ranching for ranchers, NRCS staff, conservationists, Master Naturalists, and other groups. We presented this project during workshop organized by the Texas Farmer & Ranchers Soil-to-Profit Producer Training program (2024) and Holistic Management International's Grazing & Infrastructure School (2025). Attendees learned about pollinator habitat requirements and conservation on ranches and farms. Participants were able to see pinned pollinator specimens (item 4) and photos of pollinator-friendly habitat.
- Handout for Dixon Water Foundation staff to use – We created a handout about the benefits insects provide to rangelands, in both English and Spanish. We made this handout available at our 2025 presentation, and the files have been provided to the Dixon Water Foundation for use at additional field days and workshops they host. The handouts are also freely available on the Lichtenberg Lab website.
- Pollinator conservation poster – We designed both English and Spanish copies of a poster that promotes pollinator conservation. Dixon Water Foundation staff were provided with the poster files so they can display the poster at the Josey Pavilion at the their North Texas ranch and the main office at the West Texas ranch. Events hosted by the Foundation occur largely at these two buildings. Poster files are also freely available on the Lichtenberg Lab website.
- Pollinator display collection (Figs. 6, 7) – We made two displays that include pinned specimens of various flower-visiting insects, with educational labels. Insects came from surplus material collected by students in UNT’s Insect Ecology course and the Sam Houston State University Natural History Collections, and thus did not require collection of extra individuals. In October 2023 we previewed a preliminary version of the display box at a Ranching for Resilience Workshop hosted by the Dixon Water Foundation, where it generated significant interest. We have since brought it to other events hosted by the Dixon Water Foundation, and to sustainability-focused events for the general public such as the Heard Natural Science Museum's Bugs LIVE Opening Day Celebration.
- Plant identification guide – Due to interest by ranchers and NRCS staff, we published the lab’s local plant guide for non-experts through the Lichtenberg Lab's website.
- Peer-reviewed publications – We have submitted three publication to a peer-reviewed journals: Agriculture, Ecosystems & Environment; Applied Vegetation Science; and Journal of Lost Species.
- Conference presentations – Graduate students and PI Lichtenberg presented research results and conclusions at the Entomological Society of America 2023 and 2024 annual meetings. In 2024, two of these talks were part of a symposium on pollinators in grazing lands that Lichtenberg organized.
Learning Outcomes
General pollinator information
Project Outcomes
This project addresses the issue of providing beneficial insect habitat in agriculture. Through quantification of pollinators and their resources on ranches, we aim to show how grazing management affects pollinator conservation. Pollinator conservation, in turn, increases agricultural sustainability by improving environmental quality, enhancing quality of life, and, in some cases, helping sustain economic viability of ranches. Sustainable ranching relies on nature to provide livestock forage. Pollinators underlie processes such as plant reproduction, soil nutrient cycling, and water filtration that sustain forage production. Because of this keystone role, the results of this project will provide a missing link needed to reimburse ranchers for the value created by healthy rangelands, in addition to determining how specific management strategies (high- and low-intensity rotational grazing) affect a conservation priority group.
By quantifying rotational grazing’s impacts on pollinator communities and determining relationships among grazing management, forage production, specific resources, and pollinators this project will provide critical information for advancing grazing sustainability. It will provide the data necessary to conserve pollinators, and thereby support additional ecosystem services, and to compensate ranchers for the services that their ranches provide to society.