Included in the programmatic permit were standards for bat habitats. “We’d like to do something for the uplift of the species,” Crook says.
Bats are not a regulated species in Oregon, although in some states they are “endangered” or “threatened.” But bat numbers have been declining, so when the programmatic permit was being developed, ODOT worked with the Oregon Department of Fish and Wildlife and the U.S. Department of Fish and Wildlife to develop standards that could be rolled into the programmatic. The standard outlines a goal — to maintain, replace or improve roosting on bridges over waterways. This standard was only developed for bridges over waterways because this is where bats’ primary food source is.
ODOT wants to keep bridges bat-friendly because of the integral role the mammals play in the environment. In a single night, one bat may eat thousands of insects, including mosquitoes, which could carry West Nile virus, Crook says. Some bat species also serve as pollinators whose eating habits can help farmers reduce pesticide use, he adds.
“The Oregon Department of Transportation wants to keep bridges bat-friendly because of this beneficial role in the ‘web of life,’” Crook points out.
However, contemporary precast concrete structures lack the texture and crevices bats need to be able to roost, so ODOT biologists worked with their counterparts from the Oregon Department of Fish and Wildlife and the U.S. Forest Service to develop guidelines that help engineers design bridges with bats in mind. Instead of forcing existing populations of bats to relocate, ODOT integrates habitats into the bridge structures when they are rebuilt or adds them on in the form of bat boxes when a bridge is being repaired instead of being replaced.
“We developed plans to actually build bat habitats into the bridges,” Crook says. “As bridge designers are looking at bridge constraints and what kind of structure or material to use, they will consider incorporating a bat habitat. An integrated bat habitat is highly beneficial because it doesn’t need maintenance like wood does.”
The performance standard provides clear guidance to bridge designers about how to replace, maintain or improve suitable roosting habitats on bridge program projects, according to ODOT.
ODOT worked with regulatory agencies to identify which bats may or may not be on the bridge at the time of the bridge assessments by looking for current habitat use and whether the bridge could be used with slight additions or alterations. Biologists went out into the field with a flashlight at night to check the bridges for bat use and to identify which species were using the bridges, because this would impact the type of structure that would have to be incorporated into the bridge design or rehabilitation plans. “We developed plans to build bat habitats into the bridges themselves,” Crook says. “This walks the designer through what to consider. We are essentially building habitats for different bat species.”
A place to call home
At Better Roads press time, 88 bridges in the program were in compliance with bat habitation environmental performance standards, according to ODOT. The agency has verified that its bat habitats have been successful.
With the creation of a crevice habitat — one of three types of bat habitats on bridge structures — in the shallow box-beam bridge replacements to provide a roost for mouse-eared bats, inspectors found piles of guano, or bat waste, directly beneath the crevices shortly after construction. This served as evidence that bats were using the structures, the agency says.
The other two types of bat habitats that can be incorporated into a bridge include a cave-like or an “Oregon wedge” design.
With a crevice habitat, an integrated habitat is built into the concrete of the structure. This is a crevice type of habitat that incorporates box-beam girders or pre-cast concrete. A gap is placed between the boxes to provide a crevice that runs along the span of the bridge. “We can size the crevice appropriately for an ideal bat habitat,” Crook says, adding that 1/2 inch to 1-1/2 inches is standard for the crevice.
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