Proceedings of the Vertebrate Pest Conference

Eastern fox squirrels have been introduced to many regions within the western United States including areas within California, Idaho, Montana, Oregon, and Washington. In California, fox squirrels have been introduced to San Francisco, Fresno, Los Angeles, San Diego, Berkeley, Mount Diablo in Clayton, Bakersfield, and Santa Barbara. The geographic range of the eastern fox squirrel has expanded greatly both through natural dispersal and additional intentional introductions by humans. The fox squirrel has replaced the native western gray squirrel in certain habitats while the two species coexist in other habitats. Western gray squirrels exist by themselves in a third type of habitat even though eastern fox squirrels have occupied adjacent habitat for about 30 years. Habitat Suitability Models (HSMs) have been used to predict the presence and/or abundance of a particular species within a habitat. Analysis of 9 habitat variables using Discriminant Analysis produced a HSM where replacement, coexistence, and exclusion habitats can be identified using only 3 of the 9 habitat variables: average height of ground cover, percent canopy cover, and percent of total trees that are deciduous. A field study underway for over two years at Rancho Santa Ana Botanic Garden in Claremont, CA is testing the validity of the HSM with the study site predicted to be a long-term coexistence habitat. The ratio of western gray squirrels to eastern fox squirrels has gone from 3:1 shortly after the fox squirrel entered the habitat to 1:1, but after 2.5 years of coexistence both species are commonly observed in the study site. The HSM is also being tested through the analysis of additional habitats. Although coexistence of the two species is possible for many decades, the number of western gray squirrels in coexistence habitats is often small, coexistence habitats are geographically isolated from each other, and the probability of extinction of western gray squirrels from these habitats could be high. We are also looking for new habitats which could support western gray squirrels if individuals were moved to these habitats.