Individual study: The efficacy of culverts as road underpass corridors for small- and medium-sized mammals in Bow River Valley, Banff National Park, Alberta, Canada
Clevenger A.P., Chruszcz B. & Gunson K. (2001) Drainage culverts as habitat linkages and factors affecting passage by mammals. Journal of Applied Ecology, 38
Little is known about the efficacy of culverts as road underpass corridors and there role in habitat connectivity for terrestrial wildlife, even though some are installed specifically for this purpose. Drainage 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.
Study area: The study was undertaken in the Bow River Valley, Banff National Park (51°15' N, 115°30' W), along the Trans-Canada Highway (TCH) and highway 1A, approximately 120 km west of Calgary. Banff is the most heavily visited national park in Canada with over 5 million visitors per year. Most arrive by private vehicle or coach via the TCH, which is also a major commercial motorway. On average, daily traffic volume at the park east entrance is more than 14,000 vehicles per day, peaking at more than 35,000 per day in summer.
Culvert selection and characteristics: Small- and medium-sized mammal use of 36 drainage culverts along the TCH (distance = 55 km) and highway 1A (distance = 24 km) was quantified. Culvert selection was stratified by habitat type and culvert size. Only full-length culverts were sampled i.e. those spanning the road without openings in the central reservation. Each culvert was characterised by structural, landscape and road-related attributes
Culvert use: Data on small- and medium-sized mammal movement at the 36 culverts were collected during winter (January–April) 1999 and 2000. During this period each culvert was checked at least 12 times. The passage of animals was recorded using several sooted track-plates (75 × 30 cm) placed to cover the bottom of each culvert. Track-plates were checked weekly and each species presence recorded. An estimate of numbers crossings and the direction of travel was made. The presence of tracks in the snow within a 20 m radius of culvert openings was also noted. If these indicated the culvert was used but there was no register on the track-plates this was counted as passage.
Species recorded using culverts: Mammals for which tracks were recorded in this study were coyote Canis latrans, American marten Martes americana, short-tailed weasel (stoat or ermine) Mustela erminea and long-tailed weasel M.frenata, snowshoe hare Lepus americanus, American red squirrel Tamiasciurus hudsonicus, bushy-tailed wood rat Neotoma cinerea, deer mice Peromyscus maniculatus, voles (Arvicolinae) and shrews Sorex spp.
Weasels (both species) and deer mice used the culverts for passage most frequently, whereas American red squirrels and snowshoe hares were the most common small mammals in the study area according to transects sampled near each culvert.
Influence of culvert attributes: Culvert attributes influenced individual species use. Traffic volume, noise levels and road width ranked high as significant factors. Passage by American martens, snowshoe hares and red squirrels all increased with traffic volume, the most important variable. Coyote 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.
Structural variables partially explained weasel and marten passage. 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.
Conclusions: In this study, for many small- and medium-sized mammals drainage culverts appeared to mitigate to a greater of lesser degree the potentially harmful effects of busy transport corridors by providing underpasses which act as habitat linkage corridors. The authors conclude that 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.
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