Highways as potential barriers to movement and genetic exchange in small mammals /

Abstract

Small mammal populations separated by highways may be partially or completely isolated from one another due to low dispersal capabilities, low probability of surviving highway crossing attempts, and/or avoidance of areas adjacent to highways. The objective was to determine how population connectivity is influenced by highways of different widths and traffic levels for several small mammal species that may experience varying success in crossing highways. Mark-recapture techniques were used to compare movement adjacent to highways to movement across highways for southern red-backed voles (Clethrionomys gapperi), deer mice (Peromyscus maniculatus), yellow pine chipmunks (Tamias amoenus), and red-tailed chipmunks (Tamias ruficaudus) in forested areas of western Montana. In addition, genetic techniques were used to compare gene flow (movement plus reproduction) adjacent to highways to gene flow across highways for red-backed voles, deer mice, and vagrant shrews (Sorex vagrans). Overall, 2.5 times more individuals moved adjacent to highways than across highways, and more crossed 2-lane than 4-lane highways. Observed movements varied among species, with forest-associated species (red-backed voles and chipmunks) more inhibited by highways than habitat generalists (deer mice). However, decreased movement has not yet led to genetic divergence for voles separated by highways. Gene flow across highways in deer mice was highly variable among sites, with an 11% decline in gene flow evident at one 4-lane highway site, in spite of relatively high numbers of observed movements at this site. Shrew gene flow was reduced by both 2- and 4-lane highways, and surprisingly, effect sizes (up to 37% decline) were largest for this habitat generalist.