Movement and gene flow of northern flying squirrels across an interstate highway

Abstract

Roads are a pervasive cause of habitat fragmentation around the world. Roads can present barriers to movement through direct mortality, behavioral avoidance, or by acting as an impassable physical object in the landscape. The barrier effect of roads has been demonstrated for species from multiple taxa. Species inhabiting the interior of forests may be particularly sensitive to roads because of their inability or disinclination to traverse gaps in forest cover. We combined telemetry and molecular genetic techniques to examine the effects of a high-speed, high-volume highway on the movement and population genetic structure of northern flying squirrels (Glaucomys sabrinus) in the Cascade Mountains of Washington, USA. During 2009 and 2010, we trapped and radio-tracked flying squirrels (n = 17) to gather data on movement within their home ranges and to detect movement across the highway. Additionally, we tested for effects of the highway on genetic variation in the study area using DNA extracted from cheek cells of 59 squirrels and genotyped at 11 microsatellite loci. Seven of the 17 radio-tracked squirrels crossed the highway at least once during their nightly movements. The width of the gap between forest edges across the highway appeared to negatively influence crossing rates and no crossings were observed at a site where the average gap width exceeded 80 meters. Genetic analysis provided no evidence that either geographic distance or the presence of the highway was associated with genetic differences between sites at the landscape scale. Results suggest that populations on either side of the highway are well connected demographically and genetically, and that connectivity can be maintained if gaps in forest canopy associated with the highway are kept to a width within the gliding range of flying squirrels.