Movements and ecological connectivity of amphibians and reptiles were studied along the Interstate-90 corridor in the mountainous Snoqualmie Pass area of Washington State. The ongoing project by the Washington State Department of Transportation (WSDOT) includes construction or improvement of 24 potential animal crossing structures, including two wildlife overcrossings. Mitigation efforts in the area have include restoration of wetlands and associated habitats, often adjacent to potential crossing structures. In addition, there have been numerous culverts replaced with bridges and artificial stream beds, with more to be added in the coming years. Field studies of amphibians and reptiles in the area have shown that mitigation efforts have expanded habitat and enhanced connectivity for these species. Species have, however, responded differently in their use of newly mitigated sites and crossing structures.
Surveys revealed that newly created/restored wetlands were rapidly (<1 year) colonized and used for breeding by some amphibians, especially by the Cascades frog (Rana cascadae), Pacific chorus frog (Pseudacris regilla), and ambystomatid salamanders. Western toads (Anaxyrus boreas) are much more selective in establishing new breeding sites. However, radiotracking studies have shown that Western toads move large distances over the landscape (sometimes >200m in one day) and will use newly mitigated sites, including newly constructed undercrossings. Since Western toads starting breeding in a newly created pond near the completed overcrossing, juvenile toads used the overcrossing as they dispersed from the breeding pond.
Mark-recaptures methods demonstrated that Coastal giant salamanders (Dicamptodon tenebrosus) would not cross I-90 though a large box culvert (51 marked individuals over a 3-year period). However, when the culvert was removed and replaced by an artificial streambed with a natural substrate, salamanders moved into the new streambed within 5 days. Subsequent studies showed that salamanders not only move across the I-90 corridor through the new streambed, they also reside permanently within it.
Mark-recapture studies of reptiles in the area show that the highly mobile garter snakes (Thamnophis sirtalis and T. ordinoides) readily colonized newly created/mitigated habitats. Northern alligator lizards (Elgaria coerulea), however, may require several years to move into an area. Continued study, including through fluorescent power tracking, are examining whether the Northern alligator lizards take long to establish in new areas because of their specific habitat requirements and/or their low dispersal capacity.
Based on these findings, widely applicable strategies to enhance connectivity for amphibians and reptiles can be drawn. For example, establishing breeding sites near crossing structures can enhance their use by amphibians, thus improving genetic connectivity across metapopulations. Species-specific differences in dispersal ability and habitat requirements have a marked effect on the ability of amphibians and reptiles to become established in newly mitigated areas and need to be considered in the planning stages of transportation projects.