ROOF RAT POPULATIONS IN SANTA CLARA COUNTY, CALIFORNIA

Northern Santa Clara County, California, was historically an agricultural community with orchard culture dominating the economy. Prunes, apricots, walnuts, and cherries were the principal crops. Orchards were disced clean during most of the summer growing season, and a cover crop of mustard or horse beans was grown beneath the trees in winter. About the only natural year around cover on the valley floor was the brush and grass which volunteered along the several creek banks. Under such conditions, roof rats (Rattus rattus) were not numerous. An occasional rat colony could be found in a fruit processing shed or in some of the older buildings of the several small towns, but wild or feral rats were rare. Truly feral roof rat colonies are uncommon in most of the United States. Even in southwest Georgia where cover conditions would appear ideal, an intensive study of roof rat ecology failed to uncover any feral roof rat popu¬lations (Ecke-1955). On the other hand, studies on Guam by Baker in 1946; and on Hawaii by Spencer and Davis in 1950 and by Kartman and Lonergan in 1955 have shown that feral roof rat populations thrive in tropical climates, particularly where their introduction does not place them in competition with native rodent populations. This competition factor may well be an important clue to the recent rat build-up in suburbia California, including Santa Clara County. Starting in the late 1940\u27s and continuing at an ever accelerating pace, Santa Clara County has experienced a flood tide immigration of people. Defense plants have attracted people and people have demanded homes. Because space was available and the weather ideal, most of these homes were designed to include spacious lots with patios and landscaping that go toward making outdoor enjoyment a part of California living. Inadvertently this trend in landscaping has greatly increased the available habitat--free of natural competition--for an increased roof rat population. In the past 10 years, beautifully landscaped yards have matured into vegetative harborages of vines and shrubs, and ornamental fruit trees together with remnants of old walnut and fruit orchards are supplying ample quantities of rat food. Such favorable habitat together with ideal climatic conditions has provided the stimulus for a roof rat population explosion in Santa Clara County