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The Value of Wildlife Crossing Structures
By Liz Fairbank
As the number of roads and their traffic volumes continue to increase, mitigation measures are becoming increasingly important to minimize the risks to both humans and wildlife populations. One type of mitigation measure is the use of wildlife crossing structures that allow wildlife to cross either over or under roadways without coming into contact with traffic. The goals of building these crossing structures are to reduce the risks associated with wildlife-vehicle interactions, and to provide connectivity between habitat patches (Cramer and Bissonette 2005; Bond et al. 2008). There are many factors that influence the effectiveness of crossing structures, including their placement, frequency, and structural attributes. In this article, I review the current state of scientific knowledge on the effectiveness of crossing structures.
Types of Structures
There are several types of crossing structures that are used in highway mitigation, each with different levels of effectiveness and cost. Most fundamentally, these structures are either designed to allow animals to travel below the highway (underpasses) or over the highway (overpasses). The use of fencing in combination with crossing structures can help to guide and funnel wildlife towards crossing structures, increasing their use and keeping animals out of the roadway (Gagnon et al. 2005; Glista et al. 2009). Several studies have reported that crossing structures and fencing/barrier walls together have reduced road mortality by >80% (Dodd et al. 2004; Dodd et al. 2007; Huijser et al. 2011).
Wildlife underpasses are the most common and least costly mitigation option. They are typically not fauna-specific structures, and have the potential to be used by a wide variety of species (Mata et al. 2008). These structures can range greatly in size, anywhere from small pipe culverts (0.3-2m in diameter), to large underpasses crossing under road bridges (Glista et al. 2009). They are typically constructed of concrete, smooth steel, or corrugated metal (Glista et al. 2009). Large underpasses provide crossing opportunities for the greatest number of species, although small animals often show preference for small underpasses, presumably for security from predation (McDonald and St. Clair 2004; Glista et al. 2009).
Crossing structures can be sized to accommodate many different species groups, including amphibians. Photo © Marcel Huijser.
The dimensions of an underpass or culvert play an integral role in their use by certain species. Underpasses with high relative openness (short length, tall, and wide) are preferred by ungulates, as well as large carnivores, particularly grizzly bears and wolves (Clevenger and Waltho 2000 and 2005; Ng et al., 2004; Grilo et al. 2008; Glista et al. 2009). Other carnivores, such as black bears, coyotes, bobcats, and cougars have been found to also use, and possibly prefer, smaller and more constricted underpasses and culverts (Ng et al. 2004; Clevenger and Waltho 2005).
The vegetation approaching and around underpass entryway can affect crossing structure attractiveness and usage differentially by species type. For example, natural vegetation at underpass openings is beneficial for use by small to mid-sized mammals, but can deter ungulates if it restricts their vision (Glista et al. 2009). Visibility to the opening at the other side of the underpass is an important factor in underpass use, particularly by ungulates (Cramer and Bissonette 2005; Rosell et al. 1997 [as cited in Glista et al. 2009]).
As existing culverts and underpasses were often originally constructed for drainage purposes and may sometimes have standing water in them, modifying them with shelves or raised walkways has been found to be an effective way to ensure their use even when inundated with water (Foresman 2003; Brudin III 2003; Cramer et al. 2005; Bond and Jones 2008).
Overpasses provide wildlife with a wide bridge-like structure, connecting habitat on either side of a transportation corridor. Overpasses can range greatly in width from only a few meters to over 200m on each end (they are sometimes hourglass shaped), and are typically planted with natural vegetation including grass, shrubs, and trees to appear as a continuation of surrounding habitat (Glista et al. 2009, Corlatti et al. 2009). Wider overpasses are more effective than narrower structures, particularly for large mammals, and some studies suggest that structures should be >60m in width (Evink 2002 [as cited in Corlatti et al. 2009]).
Wildlife overpasses can accommodate a wider variety of species than underpasses and are generally the preferred crossing structure of deer, elk, and other ungulates (Glista et al. 2009). Other advantages of overpasses are that they are less confined, quieter, and maintain ambient environmental conditions such as moisture, temperature, and light (Jackson and Griffin 2000 [as quoted in Glista et al. 2009]). The drawback of overpasses is that they are typically the most expensive mitigation option because of their large size and high construction costs (Glista et al. 2009).
Wildlife overpasses can accommodate a wider variety of species than underpasses and are generally the preferred crossing structure of deer, elk, and other ungulates. A wildlife/nonmotorized transportation dual-use overpass. Photo © Marcel Huijser.
Factors That Influence Crossing Structure Effectiveness
Suitable habitat on both sides of the road is a necessary condition for all wildlife to cross, and areas with the highest quality habitat will often have the highest rates of crossing (Barnum 2003). Ng et al. (2004:504) agree, stating that regardless of other factors a crossing structure will be “of little value as a wildlife corridor if it does not connect suitable habitat.” This may be the most important factor for species with narrow habitat preferences, whereas species with broad habitat preferences (such as deer, elk and coyotes) may be more affected by other factors (Barnum 2003).
Distance to cover is another feature that will differentially affect crossing structure use by various species based on their habitat preferences. Deer and elk tend to prefer crossings in open areas away from forest cover, especially during the winter (Clevenger and Waltho 2005; Barnum et al. 2007). Moose have been found to prefer crossing in mixed cover types, where there is both forage and moderate cover (Barnum et al. 2007). Several studies found that, with the exclusion of red fox, the carnivores studied (including cougars, grizzly bears, black bears, coyotes, bobcats, gray fox, badger, genet, fisher, short and long tailed weasels, mongoose, stone marten and American marten) crossed most often in areas within or near more dense forest cover (Clevenger and Waltho 2005; Barnum et al. 2007; Grilo et al. 2008).
Drainage systems are known to be used as travel routes for wildlife and often contain a higher diversity of species, as they are also home to riparian species not present in surrounding upland areas (Clevenger and Waltho 2000; Puky et al. 2007). Carnivores in particular have a higher tendency to use crossing structures near drainage systems (although distance to forest cover remains a more significant factor) (Clevenger and Waltho 2000; Clevenger and Waltho 2005; Grilo et al. 2008). Conversely, in narrow valleys, ungulates were found to avoid underpasses near drainages, probably due to an increase in perceived predation risk rather than landscape attributes (Clevenger and Waltho 2000).
The proximity of crossing structures to human population centers and human activities/ use is shown to negatively affect their use by most wildlife, particularly large carnivores and other large mammals (Clevenger and Waltho 2000; Ng et al. 2004; Grilo et al. 2008). Thus restricting human use of crossing structures, especially at night, is essential in ensuring effective use by wildlife (Clevenger and Waltho 2000; Cramer and Bissonette 2005; Grilo et al. 2008).
Another important factor to take into consideration in the placement of crossing structures is the spatial and home range dynamics of the target species. This is especially essential for small mammals and less mobile species including reptiles and amphibians (McDonald and St. Clair 2004; Bissonette and Adair 2008; Glista et al. 2009). This means not only placing structures within appropriate proximity to the home range, but also with a frequency that is appropriate for the distance that species generally travel. As it may be financially impossible to place structures with the frequency needed to provide full habitat permeability, and because wildlife vehicle collisions tend to be clustered or restricted to crossing hotspots, appropriately spaced mitigation measures should be prioritized within these areas to yield the greatest increase in connectivity and decrease in wildlife-vehicle interactions (Bissonette and Adair 2008).
With an increasing understanding of the importance of restoring wildlife connectivity, mitigating highway fragmentation has come to the forefront of road ecology. Wildlife crossing structures have been shown to be effective at increasing permeability and reducing wildlife-vehicle collisions, but a number of factors can influence their success. By ensuring that highway planners choose the right type of structure and place them in the most appropriate places, we can build a sustainable highway system where both wildlife and humans can thrive.
— Liz Fairbank is a University of Montana Environmental Studies graduate student.