The Transition Life-zone is the principal life-zone in Washington. It is divisible into three subdivisions: Humid, Arid-timbered and Arid-grasslands (Fig. 11) subdivisions. The Humid and Arid-timbered subdivisions of the Transition life-zone are closely related in some respects but different in others. They are separated by the Cascade Mountains. All of the Transition Life-zone west of the Cascades belongs to the Humid subdivision and the timbered Transition Life-zone east of the Cascades belongs to the Arid-timbered subdivision.

The Arid-grasslands are of minor geographic extent. Although this subdivision is relatively distinct as concerns the distribution of plants, insects and birds, it is of little importance as concerns the distribution of mammals. For the most part, the mammals occupying it are more representative of surrounding areas. Large parts of the Arid-grasslands have been taken over for agriculture, especially wheat raising. Perhaps the greatest extent of the Arid-grasslands existing in a natural state is along the eastern Cascade Mountains and along the eastern side of the Columbian Plateau. These are truly transition areas, situated where the arid pine forests are replaced by open, sagebrush desert.

The Upper Sonoran Life-zone includes the desert areas of Washington (figure 11 D). Its principal extent is the central Columbian Plateau. From the central Columbian Plateau, fingerlike projections of desert extend along the principal valleys.

The Arctic-alpine Life-zone occurs in the high Olympic Mountains and on the higher peaks of the Cascades. This is shown on the life-zone map, Fig. 11, as white, unshaded areas.

The mammalian faunas of the Hudsonian and Canadian life-zones resemble each other closely. The boundary between them is too complex to permit separating them on a small-scale map. Consequently they are mapped together on Fig. 11 as A.

The Humid Subdivision of the Transition Life-zone

This subdivision is remarkably uniform in composition over western Washington. The greatest difference is in precipitation. Rainfall along the coast is heavier than that in the interior. Consequently vegetation is more dense and luxuriant along the coast.

Four habitats may be distinguished in the Humid subdivision and further subdivision is possible. The dominant and most extensive habitat is the forest. Dominant mammalian species include: Peromyscus maniculatus, Sorex trowbridgii, Sorex obscurus, Tamiasciurus douglasii, Clethrionomys californicus, Aplodontia rufa, Glaucomys sabrinus and Odocoileus hemionus. Mammals are scarce and nocturnal forms prevail. As a rule, a line of mouse traps set in a forest habitat will take principally Peromyscus maniculatus with a few Sorex trowbridgii and Sorex obscurus and rarely a Clethrionomys californicus. In some places, especially where the moss is deep, a line of mouse traps will catch only shrews.

A second important habitat is the deciduous jungle. This differs from the forest habitat in that the dominant trees are of the deciduous type and in that understory vegetation, such as shrubs and annuals, is dense. The jungle habitat occurs in ravines and in valleys of streams and rivers and, in general, covers the lower, poorly drained portions of the Humid subdivision of the Transition Life-zone. Mammals are abundant and varied in the jungle habitat. The deer mouse (Peromyscus maniculatus) is the most common mammal but a line of mouse traps might also catch: Neurotrichus gibbsii, Scapanus orarius, Sorex vagrans, Microtus oregoni, or Zapus p. trinotatus. The mountain beaver, snowshoe rabbit, and Townsend chipmunk also occur there.

The prairies form a third habitat. These areas of native grasslands are of minor extent but are the principal home of several races of gophers and the Townsend mole (Scapanus townsendii). Deer and elk also browse on the prairies.

A fourth habitat is the marsh. In it there is here included the sphagnum bogs and marshy shores of lakes and streams. This habitat is characterized by damp ground, standing water, and dense vegetation. Typical mammalian species include the water shrew (Sorex bendirii), Townsend meadow mouse, muskrat and mink.

Other habitats, such as aerial for the bats and aquatic for the beaver and otter, might be listed.

The Arid Timbered Subdivision of the Transition Life-zone

In Washington the Arid timbered subdivision of the Transition Life-zone is the open, pine forest. Because of the aridity of this habitat, marshes and streamside thickets are uncommon, but where habitats of this kind do occur they have a fauna distinct from that of other habitats.

The pine forest habitat includes many diurnal species, such as the red squirrel, yellow-pine chipmunk, and Columbian ground squirrel. The white-tailed deer occurs here and, for most of the year, the mule deer. Snowshoe rabbits are usually present. Near rocks the bushy-tailed wood rat is common. Mice are scarce, probably because of the open nature of the surface of the ground. A night's trapping usually yields only a few Peromyscus maniculatus.

The mammalian fauna of the marshes and streamside thickets is similar. Shrews including Sorex vagrans and Sorex obscurus are uncommon. Meadow mice, including Microtus pennsylvanicus, Microtus longicaudus, and more rarely Microtus montanus, are taken.

The Arid Grasslands Subdivision of the Transition Life-zone

This subdivision is so much utilized by man where it occupies any considerable areas, and is of such a transitional nature elsewhere, that it is important for only a few native wild mammals. The sagebrush vole (Lagurus curtatus) seems to be confined to the arid grasslands. The white-tailed jack rabbit is now found principally in the arid grasslands, but its confinement there has resulted probably from competition with the black-tailed jack rabbit. The montane meadow mouse (Microtus montanus) is the only common, representative species. Many species from the Upper Sonoran Life-zone extend into the arid grasslands where conditions are suitable. These include Reithrodontomys megalotis, Perognathus parvus, Citellus washingtoni and Marmota flaviventris. A few species more typical of the Arid timbered subdivision of the Transition Life-zone stray onto the arid grasslands. Citellus columbianus and Microtus longicaudus may be included here.

The Upper Sonoran Life-zone

The sagebrush desert in Washington is relatively uniform in nature. Several different habitats may be distinguished, such as sandy areas, open sage, dense sage, stony ground, and talus. Qualitatively, however, the mammalian fauna of these areas is surprisingly similar. Quantitatively, there are great differences. For example, the grasshopper mouse is rare in the open sage areas with hard, claylike soil but common on drifted sand. The harvest mouse is common in dense sage but rare in open sage or in open, sandy areas. Mammals are abundant on the sagebrush desert and typical species include: the black-tailed jack rabbit, Nuttall cottontail, Ord kangaroo rat, Great Basin pocket mouse, Townsend ground squirrel, Washington ground squirrel.

Marshes are not uncommon on the Columbian Plateau and elsewhere in the Upper Sonoran Life-zone in Washington. They do not possess a fauna that is strictly Upper Sonoran but instead contain species more typical of the Arid-timbered subdivision of the Transition Life-zone. Meadow mice found in desert marshes include Microtus montanus and Microtus pennsylvanicus. The only shrew we have found is Sorex vagrans. The harvest mouse (Reithrodontomys megalotis) is often abundant in marshes.

The Canadian Life-zone

The Canadian Life-zone is a relatively uniform area in which the forest habitat is most important. Talus and swift, cold streams bring some typical mammalian species into the Canadian Life-zone from the Hudsonian. Mammals are usually common; they are abundant only in the upper portion of the Life-zone. Arboreal species and forms adapted to life beneath the forest cover are dominant. The Douglas squirrel, red squirrel, northern flying squirrel and Townsend chipmunk are typical arboreal species. Traps set beneath the trees might catch Peromyscus maniculatus, Clethrionomys gapperi, Neotoma cinerea, Sorex obscurus, or Sorex trowbridgii.

The Hudsonian Life-zone

In Washington the Hudsonian Life-zone is of greater diversity than any other. Frost and steep slopes have formed great masses of talus and, in this talus, certain species of mammals, such as Ochotona princeps, Marmota caligata and Marmota olympus are found. Other species, such as the golden-mantled ground squirrels, mountain chipmunk, bushy-tailed wood rat, red-backed mouse and long-tailed meadow mouse find the talus an ideal home. Glacial action has produced, in the Hudsonian Life-zone, numerous level or concave areas that contain small lakes and slow-moving streams. Dense, herbaceous vegetation is abundant nearby. Small mammals abound and a line of mouse traps will almost certainly catch a few such typical species as: Sorex palustris, Sorex obscurus, Microtus oregoni, Microtus richardsoni, Microtus longicaudus and Zapus princeps as well as the ever present Peromyscus maniculatus. The shrew-mole or heather vole might also be taken, though the latter is more apt to be found in nearby heather meadows.

Table 1. Distribution of mammals in Washington by Life-Zones.
A. Abundant. C. Common. R. Rare.

gibbsii minor