An Ecological Niche Model to Predict Range Expansion of the Eastern Gray Squirrel in California

Abstract. The eastern gray squirrel, Sciurus carolinensis (EGS) has been introduced to California and has expanded its geographic range since initial introductions. In this study we projected the potential future geographic range of the EGS in California using Maxent to create an ecological niche model. Location data were obtained over the time period of 2004–2015 from museum specimens, wildlife rehabilitation centers, the California Department of Public Health, the California Roadkill Observation System, and non-iNaturalist citizen science observations. Research grade data from iNaturalist was obtained over the time period of 2004–2018. Range and habitat suitability maps were developed by mapping in ArcGIS. Three threshold selection methods were used to create different estimates of the potential future range of the EGS in California. The first method used the 10th percentile logistic threshold, the second used the minimum training presence logistic threshold, and the third used Jenks Natural Breaks. We propose that Jenks Natural Breaks has distinct advantages over the other two methods for estimating the potential future range of the introduced EGS in California, because it provides information on the habitat suitability ranking throughout California, whereas the other methods only provide a binary suitable/unsuitable map.