How state DOTs can maintain bridges while protecting local bat populations

How state DOTs can maintain bridges while protecting local bat populations
By Drew Powell and Lee Droppelman
Sep 15, 2021
There are over 617,000 bridges across the U.S. and 7.5% are structurally deficient. How can state DOTs maintain and upgrade these structures without harming the bats that roost in them? Acoustic deterrence can help.

State Department of Transportation (DOT) officials have a bat problem. The bat population in the United States is in decline, and perpetual human population growth has led to the destruction and fragmentation of natural winter and summer bat habitats—forcing these protected bat populations to seek alternative roosting options in roadway bridges and culverts.

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In our work for the Georgia Department of Transportation, we demonstrated that bridge treatment areas receiving ultrasonic broadcast over the course of three nights showed a reduction in bat usage up to 75%-95%.

In an effort to balance environmental compliance with the charge to maintain essential bridges and culverts, DOTs need a safe and effective way to exclude bats from these structures. Common methods for physically excluding bats on highway structures include installing escape tubes, netting or screens, expandable foam, plywood, and the use of artificial light as deterrence. Although these methods can be cost-effective for select construction activities, most require materials to be physically attached to the structure. In some cases, this could alter the structural integrity or unknowingly entomb bats within the roost. The implementation of physical barriers can also appear as an eyesore to the general public and bring attention to something that would otherwise go unnoticed. 

For DOTs seeking a less intrusive exclusion method, acoustic deterrence offers great promise. 

What is acoustic deterrence?

Acoustic deterrents were originally developed to reduce the growing number of bat mortalities caused by interactions with wind turbines, primarily pertaining to migratory species. The concept behind acoustic deterrence is a set of speakers that emit a constant ultrasonic broadcast that “jams” the bats’ ability to orient and forage. The ultrasonic broadcast creates a “bubble” of noise within a treated airspace that leads to auditory overstimulation.

We recently tested the efficacy of acoustic deterrence as a method of bridge exclusion in a series of four case studies for the Georgia Department of Transportation. Through our work, we concluded that acoustic deterrence is a valuable and useful tool to help reduce human and wildlife interactions, minimize physical harm to roosting bat species, and expedite state departments of transportation routine bridge maintenance processes.

For each of the four case studies we conducted, we took a slightly different approach to exclusion due to the variability in bridge structure design. But throughout the case studies, the treatment areas of the bridges that received ultrasonic broadcast over the course of three nights showed a reduction in bat usage up to 75%-95%. 

3 acoustic deterrence best practices for bridge maintenance projects

While each bridge maintenance project must be governed by the specific nuances of the structure, environment, and policies in place, there are universal best practices that state DOT officials should bear in mind when evaluating the use of acoustic deterrence for roosting bat colonies. 

  • Complete your exclusion measures before starting any disturbing maintenance activities. During torpor-inducing periods (low temperatures) or summer maternity roosting, unresponsive adults or non-volant pups may be unable to evacuate a roost during human disturbance. As such, bridge construction activities can place bats at risk of unintended mortality or promote unwanted human/bat interactions. The timing of maintenance activities on highway structures containing roosting bats is essential.
  • Consider the complexity of the exclusion. Although acoustic deterrence has been proven as an effective method of temporarily deterring bats, DOTs should evaluate the complexity of the structure features present before embarking on implementation. For example, bridges and culverts that contain multiple small, hard-to-access crevices or nearly inaccessible areas of the structure can result in challenging and potentially expensive exclusion projects. When trying to decide on the most effective approach to an exclusion project, a simple comparative cost-benefit analysis can prove helpful. 
  • Mitigate adverse effects to bat hearing. Because prolonged exposure to ultrasonic broadcasts from acoustic deterrents could potentially damage a bat’s hearing, no deterrents should be activated if the bats cannot safely and voluntarily evacuate the roost. This precaution also applies to situations where bats would not be able to readily move out of range of the treated airspace.

Folding environmental stewardship into transportation planning

Wildlife occupancy has been documented in highway structures throughout most states across the nation, and many of those structures require or will require routine maintenance. Due to this, cost-effective and cost-efficient methods of exclusion are necessary for DOTs to keep projects moving forward. Because bats are protected to some degree on a state level, and some species are even federally protected, ensuring the safety of bats utilizing bridges and culverts before any construction or maintenance is of utmost importance. 

The implementation of acoustic deterrence offers an advanced alternative that is temporary and does not require any alterations of the superstructure. It can also minimize physical “take” (defined under Section 3(18) of the Endangered Species Act as harassing, harming, pursuing, hunting, shooting, wounding, trapping, capturing, collecting, or attempting to engage in any such conduct) to any potentially roosting individuals.

State DOT timeframes for bridge maintenance can be narrow depending on geographic location, and when wildlife occupancy issues arise, these windows can become even smaller. The implementation of acoustic deterrence, paired with bats’ natural processes of nightly emergence, can provide contractors with a quick, nonlethal alternative for temporary exclusion. 

By continuing to incorporate environmental compliance and stewardship into their routine practices while leaning on less invasive exclusion methods like acoustic deterrence, state DOTs can increase biological conservation, decrease environmental impacts, and maintain the highway structures that play a critical role in day-to-day human life. 

Meet the authors
  1. Drew Powell, Ecologist
  2. Lee Droppelman, Senior Managing Director, Ecology

    Lee is an ecology expert with more than 20 years of experience conducting projects focused on threatened and endangered terrestrial species. View bio

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