Wildlife and Roads: Decision Guide Step 2.1.1

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2.1.1: Identify Species to Benefit from Potential Mitigation

2.1.2: Identify Ecological Processes (Water Flow, Animal Movement, Other)

2.1.3: Identify Landscape and Topographic Features That May Affect Movement and Mitigation

2.1.4: Identify Engineering and Maintenance Concerns

2.1.5: Weigh Cost Concerns with Potential Benefits

2.1.6: Identify Appropriate General Wildlife Crossing Type

2.1.7: Other Mitigation Options

2.1.8: References

2.1.1 Identify Species to Benefit from Potential Mitigation

In this step, the user examines the species selected in Step 1.2.4 (the single or multiple species most likely to benefit from crossing measures) and begins the process of determining the most effective type of crossings or other mitigation. It is possible to design mitigation strategies for multiple species use. Not every crossing will allow all species present to pass, but to the extent that several species can use a crossing, the objective of creating landscape permeability is closer to being achieved.

We presented in Table 1.2.4.4, a summary of animal needs for mitigation based on their movement, habitat, and biological issues related to the road and traffic. A simple starting point to assess whether a species or wildlife type needs the assistance of crossing structures is to ask one of the following two questions:

  1. If this is an aquatic culvert/bridge, are the flow characteristics of the stream (e.g., timing of flow, velocity, depth, seasonality) being simulated in the passage in ways similar enough to natural conditions that they allow both young of the year and adults to pass upstream at critical times and flows?
  2. If the concern is terrestrial wildlife, can the species move across the roaded areas without avoidance behaviors or, in the case of humidity sensitive amphibians, without desiccation? Importantly, is the average daily traffic flow measured in the hundreds of vehicles or less?

If you answer yes to either question 1 or 2, then there may not be a great need for mitigation measures at this time. Regardless, the situation may necessitate further investigation into species' abilities to move across the roaded landscape regardless of traffic. Future projections of traffic volumes and their putative effects on species movement are important considerations. In specific situations, it will be important to track developments in research that focuses on how the species react to the roadway and traffic. Understanding species natural history characteristics will be beneficial.

The process we use here in step 2.1.1 starts with classifying the size of animal and its trophic level (i.e., carnivore or herbivore) and considers its size and mode of movement. We give generalities as to what general type of crossing may be best for these types. In section 2.1.2 the different wildlife crossings types are further described so the user is better able to determine what may work best for the given situation.

Herbivores

Herbivores are plant eaters. The herbivores of most concern for safety reasons are deer, elk, moose, bison, antelope, and bighorn sheep. These animals are prey for carnivorous species (meat eating species) and generally respond with caution to underpasses that are tunnel-like or appear dark and closed. Additionally, there are differences in the 'willingness' of the different species of ungulates to use underpasses. For example, mule deer appear to use box culverts with minimum measurements of 5.5 m high by 18.3 m across. Elk and moose generally appear much less willing to do so. Work by Clevenger in Canada suggests that elk, moose, and deer tend to use overpasses much more willingly than underpasses. This does not mean that underpasses will not be used by ungulates, but what is clear is that the more open and large the underpass, the more likely ungulates will use it. Human presence in passages, in the form of frequent disturbance (vehicles, ATVs, walking) impacts the use of crossing structures by animals. Underpasses that might be used more frequently by ungulates will be more likely to be avoided with increasing human disturbance.

Bighorn sheep using bridge underpass in Arizona. Photo credit: K. Morgan, Arizona Game and Fish.
Bighorn sheep using bridge underpass in Arizona. Photo credit: K. Morgan, Arizona Game and Fish.

Elk using bridge underpass in Utah. Photo credit: S. Rosa
Elk using bridge underpass in Utah. Photo credit: S. Rosa.

Mule deer entering set of underpass bridges along Interstate 15 in Southern Utah.
Mule deer entering set of underpass bridges along Interstate 15 in Southern Utah.

Carnivores

Carnivores, or meat eating animals, are more likely to use underpasses smaller than those typically used by ungulates. Intermediate-sized carnivores, such as coyotes, bobcats, and wolves appear to use underpasses willingly, as do black bear. Again, with increased human presence or disturbance, use in impacted. Larger carnivores, such as grizzly bears typically use both overpasses and underpasses in Canada.

Understanding the differences in the natural history and preferences of herbivores and carnivores will help to inform the selection of different passages that are effective for different species such as predators and prey, and will aid in the design of multi-species passages that may meet many of the requirements of both. It is a non-trivial task to design a passage for predator and prey, but this has been done with two overpasses and select open underpasses in Banff National Park, Alberta, where grizzly bear, puma, and wolf have been photographed using the same areas as elk, moose, and mule deer (Clevenger and Waltho 2000, 2005). See pictures below.

Wolf using large box culvert underpass, in Banff National Park, Alberta, Canada. Photo credit: T. Clevenger.
Wolf using large box culvert underpass, in Banff National Park, Alberta, Canada. Photo credit: T. Clevenger.

Grizzly bear using overpass in Banff National Park, Alberta, Canada. Photo credit: T. Clevenger.
Grizzly bear using overpass in Banff National Park, Alberta, Canada. Photo credit: T. Clevenger.

Florida black bear in underpass, South Florida.
Florida black bear in underpass, South Florida.

Bobcat using Bennington Bypass bridge underpass, Vermont.
Bobcat using Bennington Bypass bridge underpass, Vermont.

Amphibians and Reptiles

Amphibians and reptiles, commonly referred to as 'herps' present their own challenges to passage, regardless of their roles as herbivores and carnivores. Amphibians need:

  1. to be able to find a passage in a matter of meters from the point at which they are intercepted by the fences that guide them to passages or the places they are moving from, such as an upland or pond;
  2. specific drift fencing or walls to 'guide' them to the passage;
  3. moist natural floor conditions along the passage to ensure they do not dry out along the passage;
  4. moist conditions that are not contaminated with vehicle run-off that could be absorbed through the skin; and
  5. light in the passage to help them see it leads to another area. Reptiles need similar passage requirements, although they are not limited by moisture concerns. See Scott Jackson's summary of amphibian and reptile crossings at: http://www.umass.edu/nrec/onlinedocs.html.

We know of no across-the-road walkways or overpasses have been built for these species. Overall, herps require much smaller passages under the roadway when compared to most other species. It is also worth noting that reptiles in particular may be subject to intentional killing by motorists as they cross the road. Ashley et al. (2007) conducted an experiment in Ontario with two wildlife decoys, (a plastic snake and a plastic turtle), and a disposable cup. Each was set individually on the yellow line between opposing lanes of traffic. While sitting concealed in the bushes nearby, the investigators observed motorists reactions to the three treatments (turtle, snake, and cup). Motorist response was documented as hit, miss, or rescue (some motorists stopped to pick up the turtle). Using log-linear analysis they found evidence that reptile decoys were hit at a higher rate than by chance with approximately 2.7% of motorists intentionally hitting them. This experimental situation was conducted on a site bordered by a United Nations World Biosphere Reserve Program wetland. The results of this study are mentioned here because if the results are extrapolated to other locations, then approximately 2.7% of drivers may be inclined to intentionally kill a turtle or snake on a road. If the relationship holds, and the volume of traffic on a specific road is multiplied by 2.7%, then one can obtain an average number of drivers per hour that may be inclined to intentionally hit these animals. Whether turtle and snake populations can sustain this type of mortality on roads would need to be investigated. These results suggest that installing reptile underpasses is a much more feasible and effective solution than erecting signs to warn motorists to slow for these creatures.

Newt on road in Vermont. Photo credit: C. Slesar.
Newt on road in Vermont. Photo credit: C. Slesar.


Painted turtle using culvert underpass in Portland, Oregon.

Small and Medium-Sized Mammals

Small and medium-sized mammals have been documented using underpasses of all shapes and sizes across North America. These animals are also further classified as predator and prey, so some of the same initial considerations with herbivores and carnivores in general need to taken into account. For instance, coyotes and fox may pass through a culvert without any vegetation or cover more readily than a rabbit or small mammal. This is more of a generality, not a fast rule. Cotton-tailed rabbits and small mammals have been photographed traversing long culverts with no cover in Florida (Dodd et al. 2004) and Montana (Foresman 2003). Current studies are exploring the use by small mammals of underpasses with added cover in the form of tree stumps to determine if use is increased (Bellis 2007). Foresman (2003) has that found small and intermediate-sized mammal use of existing culverts with water present if a grated metal shelf is placed in the culvert.

Badger culvert in British Columbia. Photo credit: N. Newhouse and T. Kinley.
Badger culvert in British Columbia. Photo credit: N. Newhouse and T. Kinley.

Badger using underpass tunnel in East Kootenay, British Columbia.
Badger using underpass tunnel in East Kootenay, British Columbia.

Woodchuck crossing track plates in Bennington Bypass, bridge underpass, Vermont.
Woodchuck crossing track plates in Bennington Bypass, bridge underpass, Vermont.

Grey fox using box culvert underpass, Florida.
Grey fox using box culvert underpass, Florida.

Flying Animals

To date, the only known bird passage in North America has been built in Arizona's Pima County for Burrowing Owls. The 'passage' is comprised of berms and vegetation built along the sides of the highway and in the median to facilitate flight over the flow of traffic.

Rick Ellis at AZ Owl Crossing
Pima County Planner Rick Ellis in Front of median with soil fill and trees planted to intercept flight of pygmy owl, which was present along route of this highway. The owls have since died out and the remaining male was brought into a captive breeding program.

This approach to changing the flight path of flying organisms is also being explored and created for endangered species of insects. Examples include Oregon's efforts to change air flow over a highway to help the Checkerspot butterfly, and Taiwan's protective netting and lane closures for the mass migrations of a purple milk weed butterfly (http://news.bbc.co.uk/1/hi/world/asia-pacific/6491255.stm). There is precedence in the United States for slowing speed limits in order to protect insects. In eastern Washington, the alkali bee is an important pollinator for the alfalfa crops that are bisected by county roads. The county commissioners agreed to reduce the speed limit to 20 mph during the seasonal daylight hours when the bees are traversing the road. See picture.

Underpasses have been documented being used by wading birds and birds that have a tendency to walk across the landscape as well (see photos below).

Reduced speed zone for alkali bee crossing, Washington. Photo credit: J. Cane.
Reduced speed zone for alkali bee crossing, Washington. Photo credit: J. Cane.

Wild Turkey using box culvert underpass in Florida.
Wild Turkey using box culvert underpass in Florida.

Arboreal Animals

Australia is the leader in creating rope and metal bridges for animals restricted to trees (arboreal). This is a relatively new technology, but initial studies have shown they worked in North Queensland (see http://www.villagevoice.com.au/article/20070108/NWS03/70108005/Bridges+to+slow+possum+deaths).

Arboreal Bridge being installed in Australia. Photo credit: Village Voice at www.villagevoice.com.au.
Arboreal Bridge being installed in Australia. Photo credit: Village Voice at www.villagevoice.com.au.

Aquatic Animals

Fish and aquatic species passage is beyond the specific scope of this decision guide, however, the use of multiple-species passages that pass both aquatic and terrestrial species is beneficial and cost-effective. In fact, many state departments of transportation are asking how these multiple passage types can be combined into one structure. If inclined to seek details on necessities of aquatic crossings, we refer the user to the overall North American Fish Crossings website (http://stream.fs.fed.us/fishxing/index.html), or their specific state or province guidelines. If the user is interested in including terrestrial passage in an aquatic structure, this decision guide can be of assistance. S. Jackson also summarizes needs for terrestrial and aquatic crossings at: http://www.umass.edu/nrec/onlinedocs.html. The USDA Forest Service has completed a guide to riparian road restoration. It helps readers understand the necessary steps for replacing older culverts with functioning ones. The guide can be found at: http://www.fs.fed.us/rm/pubs/rmrs_gtr102.html.

Salmon using recently installed weirs in culvert in Idaho. Photo credit: Z. Funkhouser.
Salmon using recently installed weirs in culvert in Idaho. Photo credit: Z. Funkhouser.


2.1.1: Identify Species to Benefit from Potential Mitigation

2.1.2: Identify Ecological Processes (Water Flow, Animal Movement, Other)

2.1.3: Identify Landscape and Topographic Features That May Affect Movement and Mitigation

2.1.4: Identify Engineering and Maintenance Concerns

2.1.5: Weigh Cost Concerns with Potential Benefits

2.1.6: Identify Appropriate General Wildlife Crossing Type

2.1.7: Other Mitigation Options

2.1.8: References

Decision Guide Overview Step 1: Resource Evaluation Step 2: Identify Solutions Step 3: Select & Create Plan Step 4: Construction Step 5: Monitor & Evaluate