Spatial Analysis of Common Raven Monitoring and Management Data for Desert Tortoise Critical Habitat Units in California
AuthorXiong, Ally Pychy
AdvisorNussear, Kenneth E
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Common Ravens (Corvus corvax) are a native species in the Mojave Desert, but their populations have increased throughout the years due to resources provided by humans (ex. landfills, agriculture, standing water etc.). Increased densities of Ravens may have negative impacts on endangered or threatened species, such as the Desert Tortoise (Gopherus agassizii). We want to understand the Raven aggregations with respect to both anthropogenic and natural subsidies and their impacts on tortoise population. We have developed statistical models to provide spatial context to Raven nest density, nesting preferences, nesting success, and characteristics of offending nests throughout the Desert Tortoise Critical Habitat Units (CHU) in California. We analyzed nest survey data collected for nests on both anthropogenic and natural substrates using a variety of spatial methods to reduce autocorrelation bias, including spatial bootstrapping comparisons with null models, point process models, and geostatistical analyses. We found that nests on natural substrates tend to be located in areas that are have high Desert Tortoise suitability values, closer to agriculture, and in rougher terrain. Nests placed on anthropogenic substrates are more prominent in areas with high Desert Tortoise habitat suitability and closer to seasonal water resources. Fledgling success for both anthropogenic and natural nests were inversely correlated with temperature at the beginning of breeding season. We found negative effects on nest success relative to whether a nest was offending, indicating that these nests tended to be in areas with a suite of conditions that reduced success, and that the addition of tortoises into Raven diets did not in fact appear to improve the likelihood of success. This work will increase our understanding of the potential influence of anthropogenic features and subsidies in the desert, provide a spatial context on Raven threats to Desert Tortoise populations, and may have the potential to improve management tools and practices in managing Raven populations, which may ultimately aid in recovering Desert Tortoise populations.