From the Largest to the Smallest Mammals - Safe Wildlife Passages Across a 4-Lane Highway Provides More Than Expected

Jennifer Moonjian, California Department of Transportation
Topic Area
Terrestrial Wildlife and Ecosystem Interactions with Transportation

The State Route 46 corridor is an important east-west regional highway in California, connecting the Central Valley to the Central Coast. The first phase of the Route 46 Corridor Improvement Project began construction in 2008 to widen a two-lane conventional highway to a four-lane divided expressway for over 25 miles. Caltrans designed wildlife undercrossings as part of the transportation project as a component of CEQA and as required by a Biological Opinion from the U.S. Fish and Wildlife Service for the Federally and State listed San Joaquin kit fox. Specifically, Caltrans constructed two large (12' x 11' and 12' x 12') concrete box undercrossings exclusively for wildlife and enhanced five more large box culverts (10' x 10' or larger) for the same purpose. To help guide wildlife to the structures and to prevent wildlife-vehicle collisions, exclusion fencing was installed around undercrossing locations.

Caltrans began camera-monitoring the structures in July 2011, once the first phase of construction was complete. To date, over a thousand animal crossings have been documented using these structures including the following species: deer, bobcat, badger, coyote, red fox, gray fox, racoon, opossum, skunk, ground squirrel, cottontail rabbit, feral cat, and feral pig.

The camera data shows that small to medium-sized mammals, such as racoon and red fox, began using the culverts immediately, but that mule deer took more time. The first deer was documented using a new undercrossing in May of 2014. However, deer only used the crossing a handful of times, and then no deer were documented again until June 2018. Since that time, deer have regularly used the structure, including females with their fawns. If monitoring of the structures had concluded after five years, the initial use of the structures by deer in 2014 would have suggested incidental use, and the more consistent use beginning in 2018 would not have been captured. This would have erroneously suggested that the undercrossing was not a successful structure for deer.

In addition to improving the permeability of the Route 46 corridor for wildlife movement, the structures are also now home to four species of native bats. After evidence of night roosting was discovered on the culvert walls, Caltrans began installing bat boxes inside the culverts in 2011 to provide day roosting habitat. In total, Caltrans has installed 14 bat boxes or Oregon wedges along the Route 46 Corridor, and all of them have exhibited evidence of bat use. A minimum of four species of bats have been documented using the boxes including pallid bat (California Species of Special Concern), free-tailed bat, big brown bat, and species of Myotis. Moreover, there is evidence some of the boxes are used as maternity roosts for pallid and free-tailed bats. As such, installing bat habitat on large culverts may be an overlooked mitigation opportunity to help compensate for the loss of crevice-roosting bat habitat.

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