The Effectiveness of Vertebrate Passage and Prevention Structures: A Study of Boeckman Road in Wilsonville

One of the most obvious impacts roads have on the natural world is direct mortality to individual animals that attempt to cross roads. A less obvious but likely more important impact of roads on many species is habitat fragmentation. The ability for individuals to travel between subpopulations is the key to genetic diversity as well as ultimately the survival of the species. Considering these impacts and management goals to increase habitat connectivity, increased attention has been given to creating road design that reduces wildlife collisions. Most studies, however, have been in other countries or regions and most focus on connectivity for just one or a few species and rarely in urban settings. This study examines the effectiveness of a variety of under-road passage structures for a community of animals in an urban wetland. Wildlife passage was taken into great consideration in the construction of the Boeckman Road Extension, the new road (2006) in Wilsonville, Oregon, examined in this study. As a result it includes a bridge, two box culverts and eleven round concrete culverts to facilitate wildlife passage as well as a mammal fence atop an amphibian/reptile wall. In order to monitor passage use the authors used a combination of sand-track monitoring and motion-detection cameras. They compared passage structure use to movement in the habitats surrounding the road by monitoring four habitat transects in addition to the passage structures (Road transect). They also conducted tag and release studies targeted to small mammals. Finally, camera data collected in passage structures were compared to camera monitoring at the end of the exclusionary fencing. A total of 26 species were detected in this study, 73% (19) of which were found to be using passage structures. 15% (4) of the identified species were detected on transects but not in passages and 11.5% (3) were detected in locations other than transects or passage structures. The bridge structure provided passage to the highest number of species (16), followed by the 24” and 18” culverts, which had a combined total of 11 species, and lastly the 9x4 culverts, which were flooded, with 2 species detected. The results indicate that large and medium mammals overall did not use passages less than would be expected based on their local movement in habitats. In pit and Sherman traps 16 of the 48 small mammals that were captured and tagged were re-caught, but only four of these moved from one transect to another, with two moving away from the road and the other two moving toward and under the road. Of the species detected with cameras at the passage structures and circumventing fencing, excluding small mammals, reptiles and amphibians (139), 19% (33) were found potentially crossing on the road surface. The proportion of deer detected using over-road crossings (23%) is of particular concern for driver safety. The bridge structure provided the greatest passage, in not only frequency of crossings but also species present. When maintenance factors, driver safety and species of greatest concern are included, the higher cost of the bridge structure would likely be even further balanced by the benefits. This study highlights the effectiveness and potential problems (and possible fixes) of some of the passage and prevention structures. Including passage and prevention structures such as these in road design is imperative for habitat connectivity and the need for this becomes even more apparent as Oregon’s native wildlife face an ever increasing urban landscape as well as the potential impacts of climate change.