Initial monitoring results of high-mobility species usage of I-90 Snoqualmie Pass Project wildlife crossing structures

Josh Zylstra, Washington State Department of Transportation
Mark Norman, Washington State Department of Transportation
Topic Area
Terrestrial Wildlife and Ecosystem Interactions with Transportation

The Washington State Department of Transportation (WSDOT) has constructed the first two of four planned phases of the Interstate 90 (I-90) Snoqualmie Pass East Project. The project ultimately will improve a 15-mile stretch of I-90 in the Central Cascades, addressing safety and capacity needs, and enhancing ecological connectivity - reconnecting habitats bisected by the highway and minimizing wildlife-vehicle collisions. To date, 11 large wildlife crossing structures have been constructed, with wildlife fencing installation planned at these locations in 2019. To assess the performance of habitat restoration efforts and the effectiveness of crossing structures, WSDOT committed to implementing a robust monitoring program, including pre- and post-construction camera monitoring of high-mobility species usage of crossing structure locations. We present here the initial post-construction results of that camera monitoring effort.

We focused our monitoring efforts on medium- to large-bodied, high-mobility mammals to address the following objectives: estimate rates, species composition, and timing of crossings through existing and constructed large culverts, underpasses, and the project's first wildlife overpass. We quantified crossing activity using remote Reconyx trail cameras deployed to detect across the entire width of the structures. Due to the large size of post-construction bridges, ranging from widths of 120 feet to 1000 feet, we also deployed network-connected longer-range Axis thermal cameras at several of these structures. At one of the bridges we ran a trail camera array concurrently with a single thermal camera to compare detection rates between the two camera methods.

We detected wildlife crossings at all post-construction bridge locations beginning in the first year of monitoring, with deer and coyotes being the first species to cross and also the most frequently detected species in future years. Elk began using two bridges three years after the first deer detections. Other species detected include bobcat, river otter, and raccoon. We have not detected any black bears using the crossing structures, though they are present in the adjacent habitat.

The data show wildlife crossings increasing year-to-year to rates comparatively much higher than pre-construction rates. We have found the use of networked thermal cameras to be an effective method for monitoring coyote-sized or larger wildlife over distances of up to 1000 feet. In a side-by-side comparison of trail cameras versus a thermal camera, the thermal camera produced higher detection rates of coyote-sized or smaller animals and comparable detection rates of deer-sized or larger animals. We plan to continue monitoring and data collection efforts at crossing structures for a minimum of four years following the installation of wildlife fencing at each location.

Abstract Keywords
I-90 Snoqualmie Pass East Project
thermal cameras