Individual study: Elements promoting highway crossing structure use by small mammals in the Bow River Valley, Banff National Park, Alberta, Canada
McDonald W. & St.Clair C.C. (2004) Elements that promote highway crossing structure use by small mammals in Banff National Park. Journal of Applied Ecology, 41, 82-93
Wildlife corridors can provide connectivity in habitats that have been fragmented by human activities. A study was undertaken to assess how small mammals (mice and voles) used crossing structures across the Trans-Canada Highway in Alberta, in relation to crossing structure size, vegetation cover at crossing entrances and the distance between crossing structures and their home range.
Study site: Data was collected from 1 July to 15 October in 1999 and 2000 in the Bow River Valley, Banff National Park. Deer mouse Peromyscus maniculatus (a nocturnal grassland/forest species), meadow vole Microtus pennsylvanicus (a diurnal grassland species) and red-backed vole Clethrionomys gapperi (a nocturnal forest species), were caught using Longworth live-traps. Individuals were marked (numbered metal ear tags) sexed reproductive condition assessed, and weighed to assign age class (adult or juvenile).
Translocations: Individuals were first captured three times within 50 m of wildlife crossing structure entrances to establish their residency. They were then moved across the road to release points adjacent to crossing structures. Animals were translocated to both the up-drainage (n = 118) and down-drainage (n = 138) sides of the crossing structures. Responses were assessed regards: variation in: crossing structure size/type; vegetative cover at crossing structure entrances; and distance that displaced individuals would travel to return to there home range via crossing structures. Experiments were conducted as follows:
i) Crossing structure size/type: Individuals were translocated to within 2 m of the entrances to two, 15 m wide and either 75 m or 79 m long, wildlife overpasses which were covered with sparse trees and shrubs; nine 3 m diameter soft-bottomed arch-shaped underpasses which contained no vegetative cover inside or at the entrances with a mean length/range of 68 m/64-73 m; and nine 0.3 m diameter metal drainage culverts which contained introduced grasses near their entrances with a mean length/range of 65 m/63-72 m.
ii) Vegetative cover: Individuals were translocated across two wildlife overpasses (n = 13) and nine 3 m diameter underpasses (n = 26) and released within 2 m of brush piles (heavy, medium and no cover treatments, approximately 100%, 50% and 0% respectively, of the ground area 2 m inside and outside of the crossing entrances covered).
iii) Distance between crossing structures and home range of target individuals: Individuals were captured adjacent to the highway and translocated across the road to similar habitat in the vicinity of 3 m diameter underpasses. Animals were released at distances of 20 m, 40 m and 60 m from the underpasses so that they were always released at approximately 80 m distant from their capture location but at varying distances from the nearest culvert. Each individual was translocated only once.
Prior to release animals were marked with a small quantity of fluorescent powder (RadiantTM fluorescent pigment) which fell off as animals moved along the ground, permitting fine scale monitoring of movement paths. Individuals created a pigment trail that could be tracked for an average of 4.5 days (range 4.1-5.3 days). Traps were monitored at each individual’s original capture point for 4 days following its translocation to determine the proportion returning. Individuals that failed to return after 4 days was captured at or near its release point and returned manually.
No experimental individuals disappeared or were killed on the highway (n = 166) so each could be categorised as having either succeeded (n = 90) or failed (n = 76) to return. Those that had failed moved on average 19.5 m in a relatively straight line away from the highway.
Crossing structure type: The number of translocated animals that successfully returned to their original capture location in the crossing structure preference experiment was affected by crossing structure type and species but not by sex or translocation distance. All species were more successful returning through 0.3 m diameter culverts than 3 m diameter culverts and least successful returning across overpasses. Deer mice had a higher return success (100% culverts, 75% underpasses and 40% overpasses) than the other two species through all crossing types. Meadow voles had the lowest return success. Neither vole species returned using overpasses and none crossed the highway without using crossing structures.
Vegetative cover: Individuals were much more successful returning through crossing structures with heavy cover provided at the entrances than with medium or no cover. Again meadow voles exhibited much lower return success through all structures than deer mice or red-backed voles. No animals crossed the highway without using crossing structures. Path tortuosity also changed when different amounts of cover were provided. Individuals exhibited the most tortuous (convoluted) paths where heavy cover was provided and were most direct where there was no cover.
Distance between crossing structures and the home range: Return success was affected by the distance animals had to travel back to their home ranges using crossing structures, and by species. Individuals were less successful returning as crossing structure distance increased (very low return success exhibited by animals released 60 m from a crossing structure). Deer mice tended to return the most (20 m - 100%, 40 m - 90% and 60 m - 58%), meadow voles the least, with non returning when released at 60 m. Four of six deer mice failed to return to their territories when released 60 m from the crossing structures but successfully crossed the highway without using a crossing structure.
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