The distribution and ecology of the non-native tree squirrels Sciurus carolinensis and Sciurus niger in northern California

Access restricted to the OSU Community"Fox and eastern gray squirrel populations have become established in a number of localities in north central California. A review of studies of their feeding habits in their native range in the eastern United States shows that mast of half a dozen species provides the mainstay of their diet, but they are also opportunistic feeders, utilizing a wide variety of nuts, seeds, fruits, and fungi as they are available. Squirrels in California also show this opportunism, taking many items similar at the generic level to those taken in the east and others that are new. The yearly pattern usually seen in the east of reliance on mast in the fall and winter and green vegetation and other items in spring and summer prevails on the study sites in California. Thus although both species of introduced squirrels have adapted their diet to take foods available in California, this has not required a major change in their feeding habits. The fox squirrel population at Sunol has a breeding pattern very similar to that of fox squirrel populations in the east. The eastern gray squirrels at Bellota breed at a rate seen only in exceptionally good years among eastern populations. It is obvious that human disturbance to the habitats at the study sites at Sunoland Bellota has enhanced the squirrels' success. However, a consideration of their diet and of other areas they occupy indicates that they can survive in undisturbed areas, at least in the more mesic environments in northern California. Under situations favorable to the introduced species they are capable of replacing the native western gray squirrel, but the nonnative squirrels do not appear able to invade the drier woodland of California. As population growth continues and wooded environments become even more prevalent, additional contacts between the three species are likely."--Conclusions