Monitoring the Rare Dune Endemic Eureka Valley Dune Grass, Swallenia alexandrae (Poaceae)

Scoles-Sciulla, Sara; Shryock, Daniel; DeFalco, Lesley

doi:10.36967/2317740

Swallenia alexandrae is a long-lived perennial grass that occurs solely on active inland sand dunes within Eureka Valley at Death Valley National Park and was recently down-listed from Endangered to Threatened status. To standardize monitoring of population trends through time, National Park Service (NPS) established a 1-ha georeferenced grid system overlaid on the entire range of the species during 2007/08. Annual survey methods have shifted since that time in an effort to strike a balance between optimizing time to survey populations and a design that has appropriate statistical power to detect meaningful population change. Using local weather stations to adjust spatially-modeled weather (Parameter-elevation Regressions on Independent Slopes Model, PRISM) data, we found that greater abundance of S. alexandrae, as measured through density classes, was related to warmer than average maximum temperatures the July prior to spring surveys. This result is consistent with S. alexandrae and other warm season species whose growth is greatest during summer and fall. In contrast, a combined analysis using stage classes found that cooler and wetter than average winter/spring conditions prior to surveys were related to greater probability of plants in non-reproductive rather than reproductive status. Collectively, these results suggest that S. alexandrae population fluctuations reflect non-reproductive growth with negligible recruitment during the past decade, although focused demography studies remain crucial to estimate parameters for population viability. Lastly, we conducted a power analysis to compare different subsampling scenarios to inform future monitoring efforts. Power analysis simulations using quantitative and qualitative abundance measurements for S. alexandrae revealed that surveys using density classes were more powerful to detect population changes than those using direct counts. Importantly, a completely random subsampling design (10% of total grid cells annually surveyed during 2017–2019) includes too many unoccupied grid cells to capture meaningful trends in the population. Instead, 200 grid cells randomly selected from occupied grid cells (i.e., those with S. alexandrae present during prior whole-population distribution surveys) or weighted-stratified subsampling (drawing from proportionally more grid cells in the 0 and 1 density classes to capture their greater variability among classes) are needed to detect a 20% decrease in the population. In addition, 300–400 grid cells are needed to detect a 20% increase in the population under the same sampling designs. These occupied-only and weighted-stratified subsampling designs equate to slightly more field effort than the completely random subsampling implemented during 2017–2019 but are more powerful to detect population trends.

Monitoring the Rare Dune Endemic Eureka Valley Dune Grass, Swallenia alexandrae (Poaceae)

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