PS 39-76 - Quantifying the biodiversity value of private working lands to regional prairie conservation

Abstract: Pollinator populations in the U.S. have been dropping precipitously over the last few decades, with potential impacts on both native plant communities and productivity of neighboring agricultural lands. Similarly, North American grassland birds have shown severe declines, with 53% disappearing in the last 50 years. These patterns of decline ring true for the grasslands of western Washington, which cover only 1-3% of their historic range due to invasive plants, altered disturbance regimes, and land development. Many of the farming communities in this region are struggling due these same pressures; identifying ways to support resilient ecosystems on these lands is vital for both people and wildlife. This project aims to quantify the biodiversity contribution of private rangelands to regional prairie conservation. Four properties in southwest Washington were chosen for evaluation, each with potential to support rare prairie species. In spring 2022, we collected data on plant species richness from replicated 15m x 15m gridded plots within grassland areas of each site. We also collected floral abundance data along transects within the same plots and performed transect walks once per month (May-June-July) to capture all pollinating insects observed contacting flowers. To determine avian species diversity, we centered 100 m radius buffers around the center of all vegetation plots and selected survey points to maximize coverage of the site. Surveyors recorded all adult birds seen and heard three times during the breeding season. We compared the findings from these four sites to values from native prairie preserves located within the same region to gauge the relative biodiversity contribution of these lands to regional prairie-oak conservation. We found an average of 69 herbaceous plant species across the private lands, 26% of which were native, including one federally listed species. Nearby prairie preserves had on average 38 herbaceous species, 55% of which were native. We found 35 bee and syrphid fly taxa on private lands, as opposed to over 100 bee and fly taxa on native prairie preserves. While this represents a low percentage, there were four bee taxa found on working lands that have never been found in the prairies, representing a potentially unique habitat niche. Finally, an average of 42 bird species were found at the private sites, which, while lower than the average 60 species found on native prairies, represents a substantial portion of represented species. These data capture the important role that working lands can play in regional conservation efforts.