Road Ecology Center

Funding Source and Total Budget: Mountains, Recreation, and Conservation Authority and California Department of Transportation (CALTRANS); $65,000 Project Period: June 1997-June 2000 Underpasses have received increasing attention as a useful tool in increasing connectivity between natural areas separated by roadways. However, few studies have investigated the utility of underpasses for multiple wildlife species in a system highly fragmented by urbanization. We used track and remotely triggered camera surveys to monitor 43 underpasses across an urban wildlife corridor bisected by a series of roads. We measured a suite of landscape and structural variables to determine which factors promote or limit the exchange of large predators (coyotes and bobcats), urban mesopredators (gray fox, raccoon, and striped skunk), and ungulates (mule deer) through underpasses. Top predators were more likely and more frequent at underpasses in parts of the corridor that were wider and less fragmented. Mesopredators and ungulates were more likely and more frequent at underpasses in parts of the corridor that were narrower and more fragmented. Top predators and ungulates were more sensitive to smaller underpasses, using them less frequently, whereas mesopredators were more tolerant of underpass dimensions. In general, the landscape variables played an important role in determining both the probability and frequency of underpass use, whereas the underpass dimension variables were more important in determining the frequency of underpass use. The success of predator movement between natural areas separated by roadways will ultimately depend on situating large underpasses away from areas of high road density and residential development. Wider corridors will allow for the continued movement of large predators across the landscape, potentially alleviating increases in mesopredator abundance. These data have played a vital role in addressing connectivity needs in southern California, as we have been working closely with the California Department of Transportation (CALTRANS) to determine the effectiveness of underpasses across the region.