Using Wildlife Behavioral Traits to Design Effective Crossing Structures

 

Every species of wildlife behaves differently. Deer tend to like vegetative cover while pronghorn (antelope) seek open vistas. Small rodents seek dense vegetation and hug the ground while hawks fly over barriers. The classic ‘playing possum’ response to danger is one reason why opossums are well-represented among roadkill victims.

Using animal behavior to help design wildlife crossing structures is a successful way to ensure structures will be used by their intended design species. In contrast, ignoring behavior and expecting animals to behave like humans may result in an expensive failure.

Different species have mutually exclusive behavioral needs, such as a fisher’s desire for cover, and a moose’s desire for open space overhead to accommodate its large antlers. Although these two species live in the same type of habitat, their behavioral needs would not allow a single crossing structure to be effective, unless it mimics the natural condition of the area. This suggests that a large, open structure such as a high bridge with unsubmerged land, or a vegetated wildlife overcrossing, would serve many more species than a structure designed for one or two species. These types of structures are called Ecological Structures because they maintain the ecological integrity of the site. Ideally, ecological structures maintain the hydrology, soil substrate and vegetation of the site as well as the wildlife use of the area.

The following are examples of how behavioral traits can be used to design an effective crossing structure.

Basic Needs of All Species

All wildlife species have the basic need to find adequate food and water, shelter and mates. The movement necessary to fulfill these basic needs is usually what compels an individual to cross the extremely inhospitable landscape of a highway.

Most animals are in constant danger of being eaten by another animal, so much so that a species’ primary predator defense strategy is usually obvious from its body features. For example, if a mammal has particularly large ears, it probably detects its primary predator (or prey) by sound, so it will be particularly sensitive to unnatural noises. An animal that uses camouflage to avoid detection must have the appropriate substrate available or it will be vulnerable to predation; animals using this strategy tend to avoid areas where they are not camouflaged. Camouflage may be vegetation, rocky surfaces or many other types of substrates that can be incorporated into the crossing structure. A crossing structure designed to complement the design species’ primary defense strategy is likely to succeed.

Most wildlife species prefer to be away from human activities, particularly many of those that have special management status such as threatened or endangered species. Wildlife passages without human activities tend to be used more by shy species (A. Clevenger and N. Waltho, 2000).

Wildlife behavior often fulfills a need to conserve energy. Consequently, if given a choice many animals will travel along the edge of a watercourse instead of through the water, partly because it is more difficult to walk through water, particularly swiftly flowing water. Culverts tend to speed water flow and have few stepping-stones, so this phenomenon is exaggerated. A bridge or culvert with unsubmerged land alongside the water allows animals to easily travel without becoming wet.

Another typical way to conserve energy is to follow paths along gentle terrain. This is one reason why ridges and watercourses are favorite wildlife travel corridors. In steep terrain, many species will develop sidehill trails that follow the contour. The juncture between a cut and fill on a highway will often be the site of high roadkill, because animals follow a ridge or drainage until they reach the cut or fill barrier, then seek the first place they can cross. An underpass at the drainage along with diversion fencing at the cut/fill intersection can mitigate this situation.

Some Species Prefer Cover

Some species use vegetation or topography to hide themselves from danger. Usually these are the species used as prey by larger species, and range from very small rodents to black bears living in grizzly bear country. Topography and vegetation are the typical natural forms of hiding cover. Some species like fisher, wolverine, martens and squirrels will use large woody debris such as hollow logs as travel corridors. These species readily take to small culverts as crossing structures, particularly dry culverts.

Rodents are the food mill of the wildlife world, and are preyed upon by many predators larger than themselves. These small animals rarely venture into an area without cover nearby or directly overhead. Small culverts serve as conduits for them, as long as they have vegetative cover up to the mouth of the culvert. Larger underpasses can be made small-animal friendly by placing brush or stumps in a continuous path along the edge of the underpass.

Amphibians and reptiles use cover to protect themselves from the drying heat of the sun as well as from predators. Amphibians such as salamanders can readily use a bottomless culvert if it has adequate rock substrate to meet both moisture and hiding cover needs. Several species of salamanders travel through the interstitial spaces between loosely stacked rocks, so substrate size is a consideration.

Some amphibians, including frogs and salamanders, require light as well as appropriate moisture ranges. Slotted drain culverts provide a way to allow light into a culvert while maintaining moisture.

Overcrossings with natural vegetation are used even by very mobile groups of species such as woodland birds, because these birds are more comfortable with their preferred habitat than the openness of a cleared highway.

Some Species Prefer an Unobstructed View

Species that use their eyes and legs to avoid predation usually prefer open vistas. Jackrabbits, deer, elk and pronghorn are among those that depend on good visibility to give them a head start to run from perceived danger. Grizzly bears and mountain lions also prefer wide expanses. These are the species that open overcrossings serve best. However, openness can be simulated even in underpasses such as open span bridges and culverts if consideration is given to the appearance of openness. For these species, larger is better, and openness may be enhanced by proportionately increasing the size of the underpass as the length increases (the ‘openness ratio’). V-shaped underpasses such as single span bridges allow vegetation to continue further into the underpass than a culvert.

Some Species Have Limited Mobility or Sensory Capabilities

Numerous small animals are limited in their use of existing culverts because they are physically unable to reach them. Perched culverts effectively block access for salamanders, rodents, or mussels as well as fish.

The lack of ability of some groups such as small rodents, frogs and turtles to climb vertical walls may be used to advantage to divert them to a suitable crossing location. Lipped walls or pits with one-way diversions can be used. Lipped walls are particularly effective with snakes because they tip them over at the top, if placed high enough.

Migratory snakes use scent trails established over many years to guide them from one seasonal range to another. Using scented leaves as a guide may be a method of leading them into a crossing culvert.

Migratory and Dispersal Needs

Many animals, particularly those in colder climates, need to move long distances to fulfill all their seasonal needs. In most of western North America, deer and elk move many miles from summer to winter ranges, frequently crossing several highways in the process. Black bears use traditional travel routes so regularly that individual foot depressions can be found. Even smaller animals such as amphibians need to travel from their breeding ponds to dryer areas where they spend the rest of the year. Because these migrations are critical to the perpetuation of the species, migrants are strongly compelled to travel and often will vigorously try to cross highways at traditional locations. Using local knowledge of these migratory pathways to situate suitable crossing structures can greatly increase their effectiveness.

Young animals frequently disperse long distances from where they were born or hatched. These movements are often unpredictable in direction, and sometimes the location of a crossing structure for a dispersing animal such as a lynx or grizzly bear may be far from suitable habitat. The presence of suitable crossing structures in ‘linkage zones’ can allow young animals to locate and recolonize unoccupied habitat. The need to allow for possibly unsuspected dispersing animals is one excellent argument for making ‘ecological structures’ that allow all species to use them.

Daily movements to secure food or water are important incentives to cross highways. A local biologist’s knowledge of these locally important areas can be critical in reducing a localized mortality hotspot by building a suitable crossing structure.

Special Cases

Desert tortoises will continually push against fencing that appears permeable, even if a culvert is nearby. Bighorn sheep will ram fences if they are being pursued. Decreasing the appearance of permeability in a fence may reduce the damage done to the animal and fence.

Many species are powerful diggers. Badgers, coyotes and even deer can dig under diversion fencing if there is a small gap. These dug out spots are breaches for other animals to use to access the roadway. Fences can be buried to avoid this, with the depth of the buried section dependent on the type of species in the area.

Species such as bighorn sheep or elk that learn through herd interactions or dominant individuals can thwart intelligent designs. Matriarchal gregarious species such as elk look to older females to lead them into new situations. In several cases in Banff National Park in Canada, an older cow spread behavioral patterns that confounded such good ideas as railroad exclusion devices and one-way escape fences (B. Leeson, pers. comm.).

When faced with a long, linear barrier such as a cliff or a log, some species will approach the barrier and walk along the base. Bobcats, skunks and snakes will follow a linear barrier such as a fence or cement wall in like fashion, so this trait can be used to divert these species to a safe crossing.

Conclusion

Wildlife behavior can be used to design an effective crossing structure. The Wildlife Crossings Toolkit Database has many examples of projects incorporating wildlife behavior into successful designs.

Citations

Clevenger, Anthony P. and Nigel Waltho. 2000. Factors influencing the effectiveness of wildlife underpasses in Banff National Park, Alberta, Canada. Conservation Biology 14 (1): 47-56.

Sandra Jacobson, Wildlife Biologist, September 23, 2002