Almost every species of forest-dwelling animal had its range separated into two parts by the southward movement of the glaciers. Most of these forest-dwelling species were composed of relatively homogeneous stocks, although the yellow pine chipmunk probably was not. The extensive range of tolerance of the yellow pine chipmunk to altitude and climate and its unique habitat requirements cause it to meet radically different natural selective factors. The predators of the chipmunks near Wenatchee, Chelan County, would include: rattlesnake, gopher snake, badger, striped skunk, prairie falcon, red-tailed hawk and other predominantly desert-dwelling species. The chipmunks at Stevens Pass, in the mountains to the west, would have to contend with: marten, black bear, goshawk, bald eagle and other alpine predators. At the present time, the chipmunk of the eastern Cascades is racially distinct from that of the higher Cascades. Geologic and botanical evidence indicates that the Columbian Plateau was a desert in pre-Wisconsin Time. We suppose that a transition from alpine conditions in the Cascades to desert conditions on the Columbian Plateau existed even in pre-Wisconsin Times. We suppose also that the chipmunk existed in this transition area and in the Cascades before Wisconsin Time and in the southern and southeastern Cascades during Wisconsin Time. We further suppose that the differences separating the transition area race (Tamias amoenus affinis) from the mountain race (T. a. ludibundus) came about through natural selection and not as a result of geographic isolation. The principal difference between the two is the paler color of the race in the transition area.
The descent of the Vashon-Wisconsin glaciers, then, found two races of the yellow pine chipmunk in the Cascades. Chipmunks living north of the Columbian Plateau, in northern Washington and British Columbia, were probably forced southward onto the inhospitable plains of the plateau and exterminated. Farther east, north of northeastern Washington, chipmunks from the north were probably forced southward to compete with resident chipmunks.
The range of Tamias amoenus luteiventris in Washington, Idaho and Montana is most unusual (See [Howell], 1929; [Davis], 1939). From a compact range in Montana, two long fingers reach northward and westward. The western finger crosses Idaho to end in the Blue Mountains of Oregon and Washington. The northern finger crosses northern Idaho, northeastern Washington and extends on into southern British Columbia. Between these two fingers of the range of luteiventris another race (canicaudus) is found. This race occupies a more lowland area than does luteiventris. The range of luteiventris in the northern Rocky Mountains is extensive. Presumably this race occupied an area farther north in pre-Wisconsin Time and was forced southward to its present range by the Wisconsin glaciers. The original population occupying extreme eastern Washington and adjacent Idaho was Tamias amoenus canicaudus. The pre-Wisconsin range of this race might have been more extensive. At any rate, luteiventris which was driven southward displaced canicaudus, or some other race of chipmunk, from much of the Rocky Mountains south of the glacier. The northern chipmunks were adapted to more boreal conditions and perhaps otherwise better suited to environmental conditions of the northern Rocky Mountains. A small population of the older established race (canicaudus) persisted in lowland areas of eastern Washington and adjacent Idaho.
Regarding the range of canicaudus, surrounded by the range of luteiventris on three sides and faced by desert on the west, [Davis] (1939: 220) writes, "It may be that, of these two races, luteiventris has a greater range of tolerance to environmental conditions and, thus, is able to succeed in areas to which canicaudus is not adapted. This inference is supported by the fact that luteiventris occupies a large range which is diversified geographically and climatically, whereas canicaudus seems to be limited to a much smaller, more nearly uniform area." Seemingly canicaudus now exists only in an area ideally suited to it, and one where it can successfully compete with the generally more adaptable and successful luteiventris. The maximum extent of the glacial ice, then, found luteiventris the dominant chipmunk in the northern Rocky Mountains, with an isolated population of canicaudus in eastern Washington and adjacent Idaho.
The topography of the ground moraine exposed by the retreat of the Vashon-Wisconsin glaciers was a barrier to many species of mammals. The rough, rocky surface with thin soil probably first supported mosses and grasses, then brush, and later trees. The earlier stages of plant succession on the deglaciated ground probably presented ideal habitat for yellow pine chipmunks. Certainly the races immediately adjacent to the glaciers extended their ranges farther north than many species. In eastern Washington, T. a. luteiventris spread to the northeastern corner of the state and on into British Columbia. In the northern Cascades, T. a. affinis spread northward and eastward, across the Okanogan River, into northeastern Washington as far as the range of luteiventris. The chipmunk of the higher Cascades (ludibundus) likewise extended its range northward into British Columbia. In the northwestern Cascades of northern Washington and southern British Columbia, a richly-colored race, T. a. felix, now occupies a limited geographic range. This race doubtless originated from ludibundus stock but the method of its development is unknown. Perhaps in early postglacial time, selective factors developed in chipmunks of the western slopes of the Cascade Mountains the rich, dark color of felix. The ancestral ludibundus may have given rise to a pale race, affinis, in the arid eastern Cascades and a dark race, felix, on the humid western slope of the Cascades. This seems improbable for there is no trend to darker color on the western border of the range of ludibundus south of the range of felix, and instead, affinis may have given rise to ludibundus. A more appealing hypothesis is that a local mutation in some ludibundus stock so changed the range of tolerance of a portion of the population that it was allowed to enter the more dense habitat along the coast north of the Fraser River and, there, isolated by habitat selection, it developed the characters of felix. Population pressure later forced it eastward until the eastern border of its range again met the range of the ancestral race, ludibundus.
The chipmunks of the Olympic Mountains probably reached their present range from the Cascades. Their probable path of emigration was westward from Mt. Rainier, along the glacial outwash train of Nisqualli Glacier, to the moraine and outwash apron of the Vashon Glacier and thence to the Olympics. So similar are the chipmunks of Mt. Rainier and the Olympic Mountains that [Howell] (1929) included Mt. Rainier in the range of caurinus.
Briefly summarized, the probable pre-Vashon-Wisconsin distribution of chipmunks of the species Tamias amoenus in Washington was: ludibundus in the higher Cascades; affinis in the eastern Cascades; canicaudus in eastern Washington and adjacent Idaho; and luteiventris in the area north of the range of canicaudus. The descent of the Vashon-Wisconsin ice restricted but did not materially alter the ranges of ludibundus or affinis. On the east, luteiventris was forced southward to compete with canicaudus and displaced it over a large region, especially in mountainous areas. Following the retreat of the ice, luteiventris, affinis, and ludibundus extended their ranges northward over the deglaciated territory. A stock of ludibundus that moved westward from Mt. Rainier became isolated and gave rise to caurinus. In some less obvious development, ludibundus stock gave rise to felix north of the Fraser River in the Cascades.
Tamias ruficaudus.—Until a better understanding of the range of this chipmunk and its relation to other Tamias is gained, uncertainty will remain concerning its distribution in the past.
Tamias townsendii.—This is a typical coastal species that ranges southward, along the coast, to California. The lowland race of western Oregon and Washington (townsendii) probably occurred no farther north than southwestern Washington when the Vashon Glacier was in place. Chipmunks of this species in the Cascades and in the southern Olympic Mountains probably developed independently the slightly paler color that separates cooperi from townsendii. The tendency for species of the Pacific Coastal Fauna of the Cascades and the Olympic Mountains to be paler than their lowland relatives is widespread.
After the retreat of the ice, both races probably moved northward. Perhaps because of its alpine adaptations, cooperi has moved farther than townsendii. Also, townsendii, in the lowlands, ranges to the Fraser River, a barrier not encountered by cooperi.