Assessing Impacts of Grazing Management on Pollinator Conservation in Rangeland

In 2023 we quantified pollinators and their resources at eight sites maintained through high-intensity rotational grazing by the Dixon Water Foundation, (b) five continually grazed control sites, and (c) six sites managed through low-intensity rotational grazing (Fig 1). This expanded number of sites will provide more robust results. At each site, the rancher cooperator follows their typical management; this project does not necessitate any management changes. To quantify management, we are digitizing grazing charts from cooperators who provide them.

At each site we established eight 90m transects that radiate out from a central point. Sampling along these transects enables capturing spatial heterogeneity within a site. In 2023 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), and biomass in the dormant season (December). Sampling for two years will improve 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 and diversity (e.g., richness) 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. 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 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.