Wildlife Passage Modifications on State Route 118 in Ventura County

Celina Oliveri, Caltrans

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 or 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.).

Caltrans (District 7) is attempting to remedy the effects of highways by improving key undercrossings for wildlife movement along State Route 118 (SR-118) within Ventura County. SR-118 is currently one of the largest obstacles affecting the movement of wildlife in a critical wildlife linkage within the Santa Monica-Sierra Madre Connection by acting as a barrier for wildlife crossing, and allowing for WVCs. The purpose of the project is to increase habitat connectivity for native wildlife across SR-118 by increasing access to important wildlife corridors which have the greatest potential impact on wildlife within the region, as well as reducing the number of WVCs, thereby increasing the safety of motorists and wildlife.

Underpasses along SR-118 were monitored by NPS and the following was reported: 

  • Three of the underpasses (Mejico, Fox Barranca\North St., and La Cumbre) accounted for 97% of the documented crossings.
  • Bobcats rarely crossed SR-118 except in the vicinity of Mejico (best crossing point).
  • Rabbits (accounted for 45% of all animals found as WVC mortalities), followed by opossums, and California ground squirrels were the most common wildlife species found killed on the road.
  • On average, there was 1.35 animals killed/day as a result of WVC’s within the study area. 
  • NPS documented a total of 13 species crossing SR-118 at the underpasses.
  • Several of the underpasses have walls that prevent or reduce the ability of wildlife to use the underpasses, including Fox Barranca\North St., Long Canyon, La Cumbre, and Wailing Siren.
  • If pre-existing culverts are of appropriate size and in areas that wildlife can access, wildlife will often find a way to use them unless they are being blocked by large walls, such as with multiple crossing points along SR-118.

NPS also ranked the crossing points in order of importance for mitigation as follows: 1) Fox Barranca/North St., 2) Long Canyon, 3) La Cumbre, 4) Wailing Siren, and 5) Mejico Creek.

Improvements for this project consist of 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.  If modifications take place at these undercrossings, wildlife permeability will be improved and there will likely be fewer WVC’s at these locations where there are currently large numbers of road mortality.  

Abstract Keywords
barriers
connectivity
wildlife
highways