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.
Fig. 13. Humid subdivision of the Transition Life-zone, Headley's Marsh, five miles east of Granite Falls, Washington, June 4, 1938. (Fish and Wildlife Service photo by Victor B. [Scheffer], No. 60.)
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.
Fig. 14. Timbered arid subdivision of the Transition Life-zone, Kettle Falls on the Columbia River (now beneath Coulee Dam backwater), Stevens County, Washington, June 15, 1938. (Fish and Wildlife Service photo by Victor B. [Scheffer], No. 72.)