Abstract
The Southwestern United States desert serves as the host for several threatened and endangered species, one of which is the desert tortoise (Gopherus agassizii). The goal of this study was to develop a fine-scale, remote-sensing-based approach that indicates favorable burrow locations for G. agassizii in the Boulder City (Nevada) Conservation Easement area (35,500 ha). This was done by analyzing airborne LiDAR data (5–7 points/m2) and color imagery (four bands, 0.15-m resolution) and determining the percent vegetation cover; shrub height and area; Normalized Difference Vegetation Index (NDVI); and several geomorphic characteristics including slope, azimuth, and roughness. Other field data used herein include estimates of canopy area and species richness using 1271 line transects, and shrub height and canopy area using plant-specific measurements of ~200 plants. Larrea tridentata and Ambrosia dumosa shrubs were identified using an algorithm that obtained an optimum combination of NDVI and average reflectance of the four bands (IR, R, G, and B) from pixels in each image. The results, which identified more than 65 million shrubs across the study area, indicate that percent vegetation cover from aerial imagery across the site (13.92%) compared favorably (14.52%) to the estimate obtained from line transects, though the LiDAR method yielded shrub heights approximately 60% those of measured shrub heights. Landscape and plant properties were combined with known locations of tortoise burrows, as visually observed in 2014. Masks were created using roughness coefficient, slope percent, azimuth of burrow openings, elevation, and percent vegetation cover to isolate areas more likely to host burrows. Combined, the masks isolated 55% of the total survey area, which will help target future field surveys. Overall, the approach provides areas where tortoise burrows are more likely to be found, though additional ecological data would help refine the overall method.
Highlights
The desert tortoise (Gopherus agassizii) in the Mojave and western Sonoran Deserts was federally listed in 1990 as threatened under the U.S Endangered Species Act of 1973 (ESA) [1] because of population declines, habitat alteration, and habitat loss [2]
The method used to identify an optimum combination of Normalized Difference Vegetation Index (NDVI) and average reflectance (AVGr) thresholds from the imagery yielded an objective function (Figure 3a) showing where combinations led to lower root mean squared error (RMSE)
The use of LiDAR and aerial imagery presented some challenges in this desert environment, but the approaches taken here have produced a rich dataset that can be used for several purposes, including assessing potential burrow locations for G. agassizii
Summary
The desert tortoise (Gopherus agassizii) in the Mojave and western Sonoran Deserts was federally listed in 1990 as threatened under the U.S Endangered Species Act of 1973 (ESA) [1] because of population declines, habitat alteration, and habitat loss [2]. Population declines have continued in four of five designated recovery areas, in part because of reduced breeding rates, low density of adults, and habitat loss and fragmentation. Habitats suitable for the desert tortoise can be characterized as favorable for constructing burrows, foraging, breeding, etc. Areas suitable for desert tortoise burrows have been characterized through. Baxter [12] related tortoise habitat distribution in the southern Mojave Desert to plant communities and soil properties and compared it to random points. Results showed that 97% of tortoise burrows (either openings or tunnels) were associated with shrubs and that only 3% were entirely in interspaces
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