The Importance of Bridges to Wildlife

by DL Ennis
WILDLIFE BRIDGES obviously serve a very important role in our ability to traverse over lakes, streams, rivers, roads, railroads and a multitude of other obstacles.

In our travels we typically drive across bridges and give little thought to what might be residing beneath them. But did you know that bridges are important for wildlife too? Some of you may be familiar with the highly publicized Congress Avenue Bridge in Austin, Texas where 1.5 million Mexican Free-tailed Bats and their young roost.

Or, perhaps you have seen Osprey or gulls nesting on bridges along the coast. But you may be surprised to know that there are a number of different species that have adopted bridges as a place to rest, feed, or raise their young.

Because bridges have become an important habitat structure used by wildlife, the Virginia Department of Game and Inland Fisheries (VDGIF) have begun documenting where and how wildlife is using bridges in Virginia.

What started out as “opportunistic sightings” has turned into a full-scale “Wildlife and Bridges” project. VDGIF is working with Dr. Bill McShea of the Wildlife Conservation Society and the Virginia Department of Transportation to better understand the types of bridges and associated habitats that draw wildlife to them.

Research done by members of the “Wildlife and Bridges” project has revealed, thus far, that seven species of birds, eight species of mammals, and one reptile use bridges.

The birds include Barn Swallow (Hirundo rustica), Northern Rough-winged Swallow (Stelgidopteryx serripennis), and Cliff Swallows (Petrochelidon pyrrhonota); Rock Dove or Domestic Pigeon (Columba livia); Eastern Phoebe (Sayornis phoebe); Osprey (Pandion haliaetus); and Peregrine Falcon (Falco peregrinus).

The mammals include rafinesque’s Big-eared Bat (Plecotus rafinesquii), Little Brown Bat (Myotis lucifugus), Big Brown Bat (Eptesicus fuscus), Eastern Pipistrelle Bat (Pipistrellus subflavus), Gray Bat (Myotis grisescens), and Northern Long-eared Bat (Myotis septentrionalis); Woodrats (Neotoma floridana); and Gray Fox (Urocyon cinereoargenteus).

The only reptile that has been found so far was a Black Rat Snake (Elaphe obsoleta obsolete) that was feeding on a bat. That is a total of 16 different species of wildlife that have been found utilizing bridges so far. It is expected that as the project progresses it will be found that more species use bridges.

While Virginia doesn’t have a situation like that at the Congress Avenue Bridge in Austin, it does have bridges that contain sizable wildlife populations. The actual number of individuals varies depending on the species.

Peregrine Falcons, for example, are highly territorial and will not tolerate another pair in their vicinity. Therefore, you could only expect one pair of peregrines at a bridge. However, bats and swallows are communal breeders; finding 20 to 50 Cliff Swallow nests or 100’s or even 1,000’s of breeding bats would not be unusual. With viewable wildlife becoming a popular pastime, through this project, it is hoped to be able to identify bridges that would be of interest to the naturalist.

The study has looked at bridges throughout the state and found that wildlife uses bridges across all of Virginia. However, when they looked at bridge use in the three major physiographic regions (coastal plain, piedmont, and mountains); some differences in bridge use were found among these regions.

The coastal plain has shown the least amount of use so far with only 18.4% of the bridges being occupied by four species of birds and one species of mammal. The mountain region follows with 34.8% of the bridges being occupied by five species of birds and seven species of mammals. In the Piedmont Region of the state 50% of the bridges are being occupied by five species of birds, three species of mammals, and one reptile. These are preliminary data and as additional bridges in each of the physiographic regions are examined these numbers will likely change.

An understanding of the types of bridges and surrounding habitats being used by wildlife will help the state to better manage the wildlife of Virginia. There may be situations where it would be beneficial to either enhance or discourage bridge use by wildlife.

By understanding the structural components of the bridge or surrounding habitat that attract or discourage wildlife management strategies can be developed for individual bridges or groups of bridges over larger areas. In a few cases the study has found rare or endangered or threatened wildlife using bridges.

The use of this artificial habitat may be a key component in helping to promote some rare species and add to their recovery. As the human population grows our need to understand human-wildlife interactions becomes more important in the use of our natural resources. This project is focused on adding to our knowledge and understanding of Virginia’s wildlife and how we can coexist.

Elsewhere in the US Canada and the UK—Wildlife Bridges, Wildlife Overcrossings, Tunnels, Wildlife Underpass Bridges, and Wildlife corridors—as they are called, are being built to give wildlife, and in some cases cattle, a way to cross busy highways without any danger to themselves or travelers.

For example, just east of Snoqualmie Pass in the Cascade Mountains of Washington State, the state Department of Transportation is proposing to expand Interstate 90 from four lanes to six. This stretch of freeway cuts across several north-south wildlife corridors that must be protected and restored to preserve healthy wildlife populations. State citizens and the US Congress have spent tens of millions of dollars over the last few years to acquire and protect habitat within these corridors.

I believe that most people will be familiar with the narrow concrete cattle bridges which cross some of the UK’s rural motorways which allow a farmer to move cattle across a motorway, without the need for the use of trucks and trailers. Cattle bridges can be fairly narrow, concrete affairs; as the cattle are herded by the farmer, and forced across together; and are more than happy to walk along in a tight group.

A more recent concept is to build so-called wildlife bridges to act as a crossover point for the motorway or a railway line. Whilst a traditional cattle bridge might only be 2, 3 or 4 meters wide, a wildlife bridge might be as wide as 20 or 30 or 40 meters or even wider. Like the wildlife bridges that have already been built in the US, the top of the bridge will be a green space, complete with grass and meadows and shrubs and perhaps even a few small trees. So far as the car or train driver is concerned, he or she is passing through a short tunnel. So far as the wildlife is concerned, they appear to be walking or running across a solid area of safe ground. They are less spooked by traffic noise from below; and, perhaps just as importantly, less pressured by other animals or prey species as they can maintain a larger separation distance between themselves.

What ever they are called, wildlife corridors are necessary because they maintain biodiversity, allow populations to interbreed, and provide access to larger habitats.

Wildlife Corridors connecting Core Reserves are crucial since they increase the effective amount of habitat that is available for species and effectively reverse habitat fragmentation. This is especially important for migratory animals and those with large home ranges. Larger habitats support greater Biodiversity, larger populations, and a wider range of food sources and shelter. They also allow populations to interbreed, improving long-term genetic viability. However, Wildlife Corridors cannot substitute for large areas of protected habitat like those in core reserve systems. –American Chronicle