Wildlife use of underpasses and culverts crossing beneath highways in Southern California

High-speed roadways are major barriers to animal movement. Underpasses and culverts beneath roadways have been assumed to serve as corridors for wildlife, but few data are available documenting this. I monitored animal use of fifteen crossings beneath three major highways near Los Angeles, California. I found that carnivores, raccoons, skunks, opossums, mule deer, several domesticated animals, and a variety of other species used the corridors. No significant correlations were found between predictor variables (passage length, cross-sectional area, percentages of developed and wild habitats, human activity) and carnivore use. In contrast, corridor use by raccoons alone and by raccoons combined with other medium-sized mammals was positively correlated with surrounding developed habitat. Deer use was significantly associated with both the length and the cross-sectional area of the passages. These results verify that many animals do use these corridors, and the nature of the surrounding habitat and of the passage attributes are critical in determining which animals use them. Road kill surveys conducted along the same highways demonstrated that the number of road kills is inversely associated with traffic volume and that large wild mammals are most commonly killed along highways bordered by open areas whereas raccoons and other medium-sized wild mammals are most commonly killed near developed areas.