Fig. 97. Food cache of northern pocket gopher (Thomomys talpoides tacomensis), from chamber four inches below surface of ground, Tacoma, Washington, December 1, 1940. Contents 575 grams (about 2 liters) of roots, principally quackgrass, Agropyron repens. (Fish and Wildlife Service photo by Victor B. [Scheffer], No. 1119.)
Another economic factor is the gopher's removal of certain weeds from grazing land. A number of introduced weeds form dense rosettes that prevent the growth of grass from several square inches of ground and themselves lie so close to the ground as to be unavailable as food to grazing stock. These weeds seem to be favored food of gophers which cut not only the rosettes but the roots of the weeds as well.
In irrigated parts of eastern Washington the gopher is a serious pest. It burrows in the banks of the main ditches, causing cave-ins and occasional breaks with resultant water loss. The mounds of gophers fill in the smaller channels and divert the streams. Constant attention is necessary to keep the ditches free of gopher mounds.
In uncultivated land the gopher is a distinct asset unless the land is near enough to cultivated land to serve as a reservoir of pests. In the mountains and on the desert the gopher cultivates and keeps the soil soft and stimulates the growth of water-retaining vegetation, thus preventing rapid run-off and erosion and keeping the flow of springs and streams constant. Boulders, logs, and other obstructions are undermined and, as a result of gopher activity, in time sink under the surface of the ground. Thus a larger surface is available for plant growth. Lastly the gopher furnishes an important food source for certain fur-bearing mammals and eases the predator pressure on game species.
The subspecies of pocket gopher occupying the Puget Sound area are highly restricted in their habitat preference; they occur only on the grassy prairies of the glacial outwash aprons. They do not occur in woods, brush or even small openings on the borders of the prairies. The alpine forms occupy the mountain meadows and are slightly less selective in their habits. It should be noted that in western Washington the forests are far more open at higher altitudes than at low elevations. The races inhabiting the desert are found in open areas, often in sandy places. They occur more rarely in areas where the soil is baked and claylike, and then usually in the vicinity of springs or watercourses. The race T. t. fuscus has a wide range of tolerance as regards environmental factors; it occurs near Wenatchee in essentially desert conditions, in alpine meadows of northeastern Washington, and in many habitats at intermediate elevations. It occurs also in brushy areas and is often abundant in open pine forests.
Since the gopher usually has a narrow range of tolerance as regards its environmental adaptations, this has resulted in considerably more isolation than is the case with other mammals, and has probably contributed to the formation of the many subspecies. Within the range of almost every race, microgeographic races, or local populations with distinctive characters, are found. Many subspecies of Thomomys are probably the result of chance fixation of genetic characters already present in a more genetically variable ancestral population, and the loss of other genetic factors. Such races might be considered degenerative (see [Dalquest] and [Scheffer], 1944: 24).