Habitat structure and extinction risk modeling of <italic>Sciurus griseus</italic> in long-term coexistence habitats of southern California

The native western gray squirrel, <italic>Sciurus griseus</italic>, has been extirpated in some of its former range due to habitat fragmentation, and the introduction of the invasive eastern fox squirrel, <italic>Sciurus niger</italic>, in the western U.S. Particularly in the greater Los Angeles metropolitan area of California, populations of the western gray squirrel have declined in habitat fragments where eastern fox squirrels have been introduced. A previous study created a Habitat Suitability Model (HSM) to predict the presence or absence of <italic>S. griseus</italic> and <italic>S. niger</italic> within urban habitats of southern California. My study tested the model by predicting the presence or absence of <italic>S. griseus</italic> and <italic>S. niger</italic> at additional sites. Predictions from the presence/absence HSM agree with on-site observations in all sites that could be used to test the presence/absence HSM. A long-term field test I continued at a botanical garden in Claremont, CA to test the presence/absence HSM was consistent with the prediction of squirrels coexisting to date. In addition, my study analyzed fine-scale details of habitat structure by creating an abundance-based HSM for six long-term coexistence sites. The abundance-based HSM shows that species of tree, particularly a low to medium percentage of oak trees and medium to high percentage of conifers, have a positive relationship with the relative abundance of western gray squirrels. The six long-term coexistence sites were also analyzed for extinction risks of <italic>S. griseus</italic> to determine relative timelines, and a suitable habitat size, for <italic>S. griseus</italic> existence with <italic>S. niger</italic>. The results show that sites with smaller, homogeneous habitat patches are more likely to produce extinction of western gray squirrels within the 100 year period analyzed. The findings from all of the analyses were used to make recommendations, using a landfill and regional park as examples, in the hopes of informing management decisions in urban areas of southern California to maintain populations of <italic>S. griseus