Road undercrossing use across structure sizes by a Pacific Northwest wildlife community

Background/Question/Methods

Roads fragment habitats and affect the stability and evolution of the surrounding wildlife community. In addition, wildlife collisions are a serious issue resulting in human costs from property damage, injury and death, as well as typically fatal results for wildlife. One mitigation strategy for these impacts is to construct passage structures, allowing wildlife to cross under the road, avoiding direct interaction with vehicles and decreasing problems of habitat fragmentation. To minimize effects on wildlife, the City of Wilsonville included 13 under-road passage structures of four designs, as well as fencing, in construction of Boeckman Road. Structures included 1.5’ and 2’ diameter concrete culverts, a 4’ x 9’ box culvert and a large bridge structure 5’-9’ tall and 400’ long. This study compared the movement activity, richness and diversity of the vertebrate wildlife community utilizing passage structures at Boeckman Road to assess species preference for passage size and design. We also examined transects away from the road to determine if detections in passages were representative of surrounding communities and movement levels. Animal detections were collected over two seasons using motion detection cameras and sand tracking.

Results/Conclusions

Twenty-three species were detected using passages: five non-passerine birds, one reptile, three amphibians, and fourteen mammals. Wildlife use of smaller crossings was incrementally less than use of larger crossings with the exception of the 4x9 box culvert that had issues with standing water within the passage. The bridge passage had five times more detections (1089) than the next highest used passage type and supported the highest diversity of species, 17, including six species that did not use other structures. The two foot diameter culverts also performed well with 13 species detected, including two unique species, across 167 detections. Two species were detected at transects away from the road that were not detected in passages, beaver and nutria. Total animal detections were highest at passage structures and lowest at the furthest (100M) transect, suggesting a funneling of animals to passage structures and indicating a general lack of road avoidance. Alternately, frequently detected species may prefer to move nearer the road. These results indicate that this project with multiple and variable passage types is a successful mitigation of road impacts on habitat connectivity. Larger crossing structures provide the highest amount of connectivity, but not complete connectivity. Diversity in microhabitat structure must also be considered and included in the design of large crossing structures to maximize wildlife community use.