Existing highways act as barriers to wildlife movement, and as a result, have caused habitat loss/fragmentation and reduced population size, population persistence, and genetic diversity for various wildlife species. According to National Park Service (NPS), this can lead to negative consequences for wildlife populations by separating previously connected populations, inhibiting species from reoccupying areas that have suffered local declines/extinctions, and affecting gene flow. Wildlife-Vehicle Collisions (WVCs) are another aspect of highways which can affect species directly via mortality, or indirectly through the prevention of reaching necessary resources (such as food, mates, etc.).
In an attempt to remedy these negative effects, the California Department of Transportation (Caltrans District 7) constructed the State Route 118 (SR-118) Undercrossing Improvement Project, which includes the improvement of five undercrossings (four culverts and one bridge) located between postmiles (PM) 9.7 and 14.5 along SR-118 within the unincorporated community of Somis (Ventura County), and the city of Moorpark. The road is currently one of the largest obstacles affecting wildlife movement within the Santa Monica-Sierra Madre wildlife corridor. Between 2011 and 2013 Caltrans and NPS collaborated and identified/ranked key undercrossings, and modified them to allow for crossing.
Improvements were funded through the California Natural Resources Agency’s Environmental Enhancement and Mitigation Grant Program (EEMP) and consist of the following:
- Installation of fencing along SR-118 to prevent wildlife (that are reluctant to use culverts as crossings) from crossing the road directly, and instead, funneling wildlife into a safer passage under the bridge at Mejico Creek Bridge as well as Long Canyon Creek
- Improvements also consist of construction of ramps which allow wildlife to scale the high ledges under culverts which have proved to be barriers for wildlife crossing at the Wailing Siren, Long Canyon, La Cumbre, and North Street culverts.
Each modification was designed to improve wildlife crossing within an essential wildlife linkage, reduce fragmentation within wildlife populations, and reduce the number WVCs, thereby improving safety for local motorists and wildlife. NPS and Caltrans’s goal was to evaluate the effects of the drainage modifications on wildlife movement and mortality on highway SR-118 by measuring wildlife mortality from vehicles, determining wildlife movement patterns in the area, identifying land use that may affect animals’ ability to cross, and evaluating the use of existing road crossing points, post-modification. We are in the process of comparing current crossing data with data which was gathered before modifications were constructed, in order to see the effectiveness of the modifications and to measure the amount by which wildlife permeability increased and wildlife-vehicle collisions (WVCs) reduced.