Drainage culverts as habitat linkages and factors affecting passage by mammals

1. Drainage culverts are ubiquitous features in road corridors, yet little is known about the efficacy of culverts for increasing road permeability and habitat connectivity for terrestrial wildlife. Culvert use by small- and medium-sized mammals was investigated along roads in Banff National Park, Alberta, Canada. An array of culvert types was sampled varying in dimensions, habitat and road features during the winters of 1999 and 2000. Expected passage frequencies were obtained by sampling relative species abundance along transects at the ends of each culvert. 2. Weasels Mustela erminea and M. frenata and deer mice Peromyscus maniculatus used the culverts for passage most frequently, whereas red squirrels Tamiasciurus hudsonicus and snowshoe hares Lepus americanus were the most common small mammals in the study area according to transects sampled near each culvert. 3. Species' performance indices (observed crossing vs. expected crossing) were calculated for five species by comparing their tracks inside and adjacent to 36 culverts. Culvert performance indices were significantly different between the five species: culvert attributes influenced species' use but different attributes appeared to affect use by different species. 4. At all scales of resolution (species, species group and community level), traffic volume, noise levels and road width ranked high as significant factors affecting species' use of the culverts. Passage by American martens Martes americana, snowshoe hares and red squirrels all increased with traffic volume, the most important variable. Coyote Canis latrans use of culverts was negatively correlated with traffic volume. Increasing noise and road width appeared to be negative influences on culvert passage by coyotes, snowshoe hares and red squirrels. 5. Structural variables partially explained passage by weasels and martens. Weasel passage was positively correlated with culvert height but negatively correlated with culvert openness. Martens preferred culverts with low clearance and high openness ratios. High through-culvert visibility was important for snowshoe hares but not for weasels. The passage by weasels and snowshoe hares was positively correlated with the amount of vegetative cover adjacent to culverts. 6. For many small- and medium-sized mammals drainage culverts can mitigate the potentially harmful effects of busy transport corridors by providing a vital habitat linkage. To maximize connectivity across roads for mammals, future road construction schemes should include frequently spaced culverts of mixed size classes and should have abundant vegetative cover present near culvert entrances. Further work is required to assess the effects of culverts on population demography and gene flow adjacent to large roads.