Geology and Historical Biogeography of the Olympic Peninsula

Abstract

A brief introduction to the geologic and climatic history of the Olympic Peninsula is presented, focusing on the Middle Miocene to present (the past 20 million years). The period is marked by a general cooling and drying trend, with a corresponding loss of the mesic broadleaf forest and the rise of conifer-dominated forests. The causes of the regional climate changes are discussed with an eye on the relative roles of topographic uplift versus larger global-scale changes in atmospheric circulation and sea surface temperature. The patterns of endemism and disjunction on the peninsula are presented to emphasize insular aspects of its biogeography. A list of 29 endemic taxa is compiled from literature searches, though it is acknowledged the list will continue to grow. Mapped distributions of the endemic taxa suggest long-term persistence of certain habitat types, including dry meadows and wet riparian forests, through periods of glacial cycles.