Document Type


Date of Degree Completion

Fall 2020

Degree Name

Master of Science (MS)



Committee Chair

Kristina Ernest

Second Committee Member

Alison Scoville

Third Committee Member

Daniel Beck


Roads negatively affect wildlife by degrading and fragmenting habitat. The I-90 Snoqualmie Pass East project was established to improve traffic safety for both humans and wildlife. Here, the Washington State Department of Transportation is constructing wildlife crossing structures to increase ecosystem connectivity as part of this project. The goal of my study was to investigate shrews – very small mammals with presumably low mobility and dispersal capacity – at sites on the eastern slopes of the Cascade Range where wildlife crossing structures will be built in the future. My main objectives were to verify shrew species composition, assess population genetic structure relative to the highway, and determine whether shrew abundance varied among macro- and micro-habitats. Using live-trapping techniques, I captured 136 individual shrews at three different paired sites north and south of the highway in the summer of 2019. Initial field identifications documented six sympatric species: Trowbridge’s shrew (Sorex trowbridgii), montane shrew (S. monticolus), masked shrew (S. cinereus), vagrant shrew (S. vagrans), marsh shrew (S. bendirii), and the American water shrew (S. palustris). However, molecular analysis of the four terrestrial species determined that the study area was home to the Olympic shrew (S. rohweri) rather than the masked shrew (S. cinereus). This discovery represents a range extension for the Olympic shrew east of the Cascade Range and the surprising absence of the masked shrew. Analysis of population genetic structure for the three most abundant species indicated that montane shrew and Olympic shrew populations separated by the highway still act as one population. Trowbridge’s shrew populations had higher genetic variation, providing some evidence for population genetic structuring across the highway based on a limited sample size. Capture rates of shrews did not differ among habitat types or with most microhabitat characteristics. This study provides baseline information on an assemblage of shrew species near a major highway in central Washington and can be replicated in the future to assess potential effects of wildlife crossing structures on connectivity for these very small mammals.