Page:Natural History of the Ground Squirrels of California.djvu/20

606 The species now under discussion is restricted in its distribution mainly to the state of California. It extends a little ways south into Lower California; and to the eastward it barely crosses the Nevada line in the vicinity of Lake Tahoe. To the northward in the coast belt it is cut off sharply by the Golden Gate and San Francisco Bay, but in the interior it extends to the headwaters of the Feather River. Southeastwardly toward the deserts the race beecheyi blends into the race fisheri, which in turn is wholly cut off by the hottest deserts beyond. (See map, fig. 17.) In the coast belt north of San Francisco Bay and from the upper Sacramento Valley northward the Beechey Squirrel is replaced by the Douglas Ground Squirrel.

Altitudinally the California Ground Squirrel ranges from sea level, as on the shores of Monterey Bay, up to an altitude of at least 8200 feet, as in the Yosemite National Park. It is most abundant on the plains of the San Joaquin and in the Coast Ranges and Sierra foothills. As regards life-zone, the metropolis of the species lies in the Lower and Upper Sonoran (see fig. 23). It is less numerous in the yellow pine belt (Transition zone), and is but rarely or sparingly represented in the Canadian zone, still higher on the mountains. Its preferences as to local conditions are not closely limited, except that it avoids dense chaparral and thick woods. It frequents pasture lands, grain fields, orchards, sparsely tree-covered slopes, small mountain meadows, rock outcrops on the tops of ridges, and even granite talus slopes. It is always most abundant, however, in the open situations, and its decided preferences are such that it thickly populates much of the best farming and grazing lands in the state, to the great reduction of their producing value from the human standpoint.

This squirrel secures shelter for itself and young, and safety from its enemies, by burrowing in the ground. Where possible it chooses to excavate its retreats in hillsides or in low earth banks. Here some at least of the necessary digging can be done in a horizontal direction. But, of course, those members of the species which live on the plains or on small flats or meadows in the foothills or mountains must dig down vertically for considerable distances to gain the requisite protection. Many of the squirrels which live in the granite country make their homes under large boulders or in rock taluses where a minimum of burrowing is necessary to insure safe retreats. On wooded hillsides special safety from enemies that dig is secured by location of the burrows under tree-roots or old stumps.

In the foothill region at the southern end of the San Joaquin Valley there seems to be a decided tendency on the part of the ground squirrel to select alluvial fans for home sites. This preference may be accounted for by the fact that the plants upon which the squirrel feeds make a better growth in the deep alluvial soil there than they do on the adjoining hillsides which are often steep and with but shallow cover of soil. For the same reason the alluvial fans afford easier digging to considerable depth and hence better protection. In seven burrows in different localities, in which the squirrels were gassed and then dug out (J. Dixon, MS), the extent, diameter and depth were found to vary and to depend largely upon the nature of the soil. In shallow adobe or clayey soil, underlaid by broken rock, the burrows were found to be short, of