News & Media

Study finds culverts, other changes aid wildlife crossing Highway 23

by Teresa Rochester (Ventura County Star)
July 6, 2012

In answer to the less-pondered question of how coyotes cross freeways, biologists with the National Park Service found they used exclusion fencing and debris-free culverts under Highway 23 to do so safely.

There was an 88 percent decrease in coyote roadkill after Caltrans installed fences and one-way gates and cleared debris from culverts during its widening of the route, according to a seven-year study by the National Park Service.

The $90,000 study, funded by Caltrans, involved road mortality surveys of large, medium and small critters for 34 months before the construction of two additional lanes, followed by surveys conducted for 27 months after construction.

The study area was a two-mile stretch of Highway 23 between Olsen Road in Thousand Oaks and Tierra Rejada Road in Moorpark.

Officials with Caltrans and the National Park Service did not return calls for comment. However, in a prepared statement National Park Service biologist Jeff Sikich said roads cut across natural habitat, making it difficult for animals to move and survive. In recent years, there have been reports of mountain lions being struck and killed on Southern California freeways.

“These crossings can effectively reduce road mortalities and increase connectivity in fragmented landscapes,” Sikich said.

In the statement, Caltrans senior environmental planner Barbara Marques said the department is pleased with the results of the crossing improvements and “we are working to further improve critical wildlife-freeway crossings in the region.”

The improvements benefit not only animals but also people driving on freeways by reducing the number of animal-vehicle collisions, according to the statement.

The goal of the improvements was to reduce the mortality rate of medium-sized and large mammals. The improvements resulted in a sixfold increase in the number of successful wildlife crossings.

While coyotes experienced a drop in road deaths, from 43 to five during the study period of January 2004 through August 2011, other species did not experience a reduction, according to the study.

Rabbits, in particular, have not fared well. Sylvilagus spp. made up the most road deaths before and after the wildlife-proof fence was constructed, at 21 percent and 51 percent.

“Although the fence reduced road mortalities for coyotes, the lack of change in road mortality for other species suggests that smaller animals were finding ways through and or around the fence,” the study states.

Researchers found the one-way gates Caltrans installed were used by both wildlife and people in one direction. The gates are intended to direct animals away from the freeway. Some of the gates had damage by people.

As part of its improvements Caltrans also removed debris from two of three culverts under Highway 23 for animals to use as crossings. Use of the culverts by animals increased. The culvert that did not require any changes received six times more crossings by coyotes, raccoons and skunks when construction was completed, likely because the exclusion fence funneled them toward the culverts, according to the study.

Bobcats were detected at each of the culverts, but they were not found to go through the entire undercrossing, researchers wrote in the study.

They suggested that the lack of small animals, such as rabbits, squirrels and mice, using the culverts could be attributed to lack of protective cover, which would make the underpasses unsuitable.

The researchers suggested Caltrans implement design changes to encourage use by smaller mammals.