Wildlife and Ecosystems

There may be a new mammal vying for the title of man’s best friend – and a new study is looking into how Texas Department of Transportation bridges may be key to this profitable mammal-human connection.  

Dr. Stirling Robertson, the biology team lead within Texas DOT’s natural resources management section, explained that bats help preserve the health of natural ecosystems and also provide substantial economic impacts by pollinating plants, spreading seeds and eating pests such as moths, beetles, mosquitoes, stinkbugs and termites. Some have been documented to consume as much as 85 percent of their body weight in insects every night – and bats can weigh anywhere from an ounce and a half to north of two pounds.

“Such voracious foraging on insects has definite economic impacts, especially for agricultural production,” he explained. “More than 100 million Brazilian free-tailed bats can fly nightly from caves and highway structures, like bridges and culverts, eating up all kinds of crop pests.”

In Texas’ Winter Garden Region southwest of San Antonio, a single Brazilian free-tailed bat will eat 20 insects a night. That translates to two cents per bat, per night, of ecosystem services as farmers do not need to apply additional pesticides to achieve the same yield of cotton. When extrapolated across that region, it translates to an annual “agro-economic value” on cotton ranging from $121,000 to more than $1.72 million. That’s compared to the total value of the crop in this region of $4.6 million to $6.4 million per year.

To obtain firm numbers on the economic benefits bats provide Texas and how bridge structures contribute to that benefit, the Texas DOT and Texas A&M University are in the midst of a three-year field study expected to last through May 2021 – though, due to the COVID-19 pandemic, that study may need to be extended as stay-at-home orders has shelved the agency’s field research for two months.

“Finding places to hang during the day can be a critical limiting factor for many of these temperate bat species,” Robertson pointed out. “This is why learning more about how and why bats interact with our bridges is important to the distribution and abundance of these important species.”

He said that, of the 33 species of bats that live in Texas, 18 have been documented and six potentially use Texas DOT highway structures as day roosts.

Based on several scattered records, a number of different bat species use bridges and culverts statewide in summer and winter, with studies of sites and species combinations indicating that many highway structures house more than 1,000 bats. 

“This sample is undoubtedly an underestimate of highway structure roost use in the state, but currently the frequency of this bat-highway structure interaction is unknown,” Robertson noted. That’s why Texas DOT began this study in partnership with Texas A&M; conducting a systematic inventory of bridges and culverts in the state to compare sites that have bats and those that don’t so experts can find what attracts bats to these structures.

Current research indicates that culverts that cross divided highways usually range about 200 to 400 feet long and are about 5 to 10 feet underground; creating “thermal qualities” that simulate the thermal qualities of caves, which could be a factor in the bat’s preference. By contrast, bridges provide numerous nooks, crannies and expansion grooves that offer tight spaces for bats to roost in.

A previous study by researchers at Boston University compared the development of Brazilian free-tailed pups raised in a cave to those raised under a bridge and found that the increased temperature of those bridges during the spring and summer resulted in pups that developed faster, weaned quicker, and had larger body sizes than those in a cave.

Undoubtedly, not all bridges or culverts are used by bats, thus Robertson hopes that Texas DOT’s ongoing study will help better illuminate the factors that attract bats to nest in them. “Better information on which bridges serve as important roosts for bats will also be extremely useful for the planning and timing of maintenance and construction of highway infrastructure,” he said.

Part 2 of this story will examine the work New Mexico DOT is doing to make its bridge structures more “bat friendly.”

The charismatic and familiar Monarch Butterfly serves as a “flagship species” for pollinator conservation – and a new report from the Transportation Research Board examines how transportation industry stakeholders can evaluate whether certain roadway corridors provide suitable habitats to aid in their preservation.

That report – NCHRP Research Report 942 Pre-Pub: Evaluating the Suitability of Roadway Corridors for Use by Monarch Butterflies – examines the potential for roadway corridors to provide habitat for monarch butterflies and provides tools for roadside managers to optimize potential habitat for monarch butterflies in their road rights-of-way.

This NCHRP report follows on the heels of a “historic agreement” finalized between the U.S. Fish and Wildlife Service and the University of Illinois-Chicago on April 8 that encourages transportation and energy firms to voluntarily participate in Monarch Butterfly conservation by providing and maintaining habitat on potentially millions of acres of rights-of-way corridors on both public and private lands.

The American Association of State Highway and Transportation Officials supported this effort in a two-page letter to the U.S. Department of the Interior on March 12; seeking “expedited approval” of voluntary national CCAAs to further encourage the creation of pollinator habitats in highway rights-of-way – especially the Monarch Butterfly.

“This decision gives state DOTs the ability to meet their highest priority to provide safe roads for the traveling public while simultaneously safeguarding the health of habitat for essential pollinators like the Monarch Butterfly,” noted Jim Tymon, AASHTO’s executive director.

A “historic agreement” finalized between the U.S. Fish and Wildlife Service and the University of Illinois-Chicago on April 8 will encourage transportation and energy firms to voluntarily participate in monarch conservation by providing and maintaining habitat on potentially millions of acres of rights-of-way corridors on both public and private lands.

Both signed an integrated, nationwide Candidate Conservation Agreement (CCA) and Candidate Conservation Agreement with Assurances (CCAA) for the monarch butterfly on energy and transportation lands throughout the lower 48 states.

The USFW noted in a statement that those are formal yet voluntary agreements between the agency and both public and private landowners to conserve habitats that benefit at-risk species and that it integrated both CCA and CCAA programs so energy and transportation partners and private landowners can provide conservation seamlessly throughout their properties, where there may be a mix of non-federal and federal lands.

A CCAA is for non-federal partners only and provides assurances to participants in the form of an “enhancement of survival permit” that no additional conservation measures will be required of them if the covered species later becomes listed under the Endangered Species Act (ESA).

The American Association of State Highway and Transportation Officials supported this effort in a two-page letter to the U.S. Department of the Interior on March 12; seeking “expedited approval” of voluntary national CCAAs to further encourage the creation of pollinator habitats in highway rights-of-way – especially the Monarch butterfly.

“AASHTO salutes the U.S. Fish & Wildlife Service for approving this essential agreement,” said AASHTO’s executive director Jim Tymon. “This decision gives state DOTs the ability to meet their highest priority to provide safe roads for the traveling public while simultaneously safeguarding the health of habitat for essential pollinators like the Monarch Butterfly.” 

“The regulatory protections provided by this CCAA allow transportation agencies to continue vegetation management practices with less concern that these actions will lead to an increase in the costs of regulatory compliance if the monarch is listed under the ESA,” the organization said in its letter.

The USFW said that agreement participants will carry out conservation measures to reduce or remove threats to the species and create and maintain habitat annually. And although this agreement specifically focuses on monarch habitat, the conservation measures will also benefit several other species – especially pollinating insects.

“Completing this agreement is a huge boost for the conservation of monarch butterflies and other pollinators on a landscape scale,” noted Aurelia Skipwith, USFW director, in a statement. “This is a great example of how … working proactively with our partners in the energy, transportation and agriculture industries to provide regulatory certainty for industry while addressing the conservation needs of our most at-risk species.”

“By engaging early in voluntary conservation, utilities and departments of transportation can avoid increased costs and operational delays as a result of a potential listing. This provides tremendous value to industry and will also yield big benefits to the monarch butterfly,” added Iris Caldwell, program manager of the University of Illinois-Chicago’s Energy Resources Center, which will administer the agreement.

“Not only is this the largest CCAA in history and completed on one of the fastest timelines thanks to our incredible partners, but it also represents an extraordinary collaboration between industry leaders and the U.S. Fish and Wildlife Service that can serve as a model for addressing challenges to other at-risk species,” Caldwell said.