The Effects of Highways on Fragmentation of Small Mammal Populations and Modifications of Crossing Structures to Mitigate Such Impacts
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The Effects of Highways on Fragmentation of Small Mammal Populations and Modifications of Crossing Structures to Mitigate Such Impacts

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    • Abstract:
      Highway 93 south from Missoula to the Idaho border is currently being widened from 2 lanes to 4. This construction project has incorporated the use of small diameter, drainage culverts fitted with animal shelving in an attempt to mitigate for habitat fragmentation and loss of animal population continuity. The present research was conducted to determine the effectiveness of such shelving and to modify the design, where necessary, so that the widest variety of species could be served. Three 1.2 m diameter steel corrugated culverts were fitted with shelving and 3 additional culverts adjacent to these, without shelving, were chosen to serve as controls. Over the period November 1, 2001 to January 31, 2004 remote sensing 35 mm cameras, placed in each culvert, recorded animal activity. Water levels within culverts were measured on a weekly basis and environmental variables (temperature, light, and humidity levels) were recorded adjacent to these culverts at 10 minute intervals. Relative abundance of small mammal species adjacent to the entrances of each culvert were determined through use of live-trap transects. Over 4,500 photographs and 8,100 events of animal activity (movement past detectors) were recorded documenting culvert use by 14 different mammal species. In general, the amount of use by a species reflected its relative abundance adjacent to the culvert. When water was present in the control culverts, animal use was greatly limited, if not completely prohibited. Using photographs which illustrated the behavior of animals while using shelving, modifications were made to increase the effectiveness of the shelf surface and entrance ramp. Following modifications, shelving allowed for continued use of culverts even when water was present. Fourteen species of mammal were observed to use the shelves. Animal movement patterns were highly seasonal reflecting responses to cold temperatures, standing water levels which prohibited access to culverts (and shelving ramps), and species behavioral differences. Vegetative cover at culvert entrances greatly influenced culvert use; protective cover was required in order for most species to move to the culverts. Meadow voles did not use culverts until a protected tube was provided which could be accessed without having to leave the surrounding vegetation. There was no strong correlation between traffic volumes and animal activity. However, most species at these locations were nocturnal and thus their movements occurred at night when traffic was light. Animal use of larger culverts with, and without, animal shelves was also studied. Recommendations for shelf use and for modifications of other culvert types is provided.
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      For up to date information, see the US State DOT Wildlife Crossing Structures website at:
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