I investigated the determinants of distributional patterns in the mussel Mytilus californianus in the Strait of Juan de Fuca in Washington State. A three pronged approach including evolutionary, geomorphological, and population ecology research methods elucidated processes determining the density and distribution of the species as well as forces impacting its future trajectory. Primary conclusions include: 1. Changes in aerial temperatures in Washington State in the near future are unlikely to dramatically influence the abundance and distribution of Mytilus californianus. 2. The interaction of sea level rise with the geomorphological features of rocky coasts has the potential to dramatically change the distribution and abundance of shallow water depth restricted species like Mytilus californianus. 3. Distributional patterns of Mytilus californianus occurring at multiple spatial scales are a result of the integration of population vital rates as determined by environmental gradients, and variation in vital rates are sometimes scale dependent (growth and recruitment rates) and sometimes not (survival rates). 4. Mussel populations were most sensitive to variation in growth rates, least sensitive to variation in recruitment rates, and intermediately sensitive to variation in survival. 5. Mytilus californianus density is unlikely to be primarily controlled by variation in growth rate or recruitment rate.