The Environmental Impact of Roads

Roads are increasingly common in today's world as human development expands and people increasingly rely on cars for transportation on a daily basis. The United States contains over 4 million miles of roadways and an estimated 20% of land in the country is impacted by the presence of roads.¹ This large network of roads has dramatically altered the landscape and can impact wildlife in a number of deleterious ways. In addition to causing mortality, roads can also shift population demographics and be a source of pollution into the environment. Studying the ecological impacts of roads is an important area of study in conservation biology and environmental science, as the impacts often extend far beyond the surface of the road itself.

Mortality & Population Declines

When animals cross roads, mortality is often the result. In fact, road mortality is the leading source of mortality to many wildlife populations and an estimated 1 million vertebrates die on roads every day in the United States.² This rate of mortality can severely threaten animals and has been identified as a leading cause of decline in some populations.

While the consequences of road mortality can be severe, many factors influence the degree to which roads impact particular animal populations. When a road crosses through an animal's preferred habitat, the chances increase for road mortality. For example, Highway 27 in Florida that passes over a lake inhabited by many turtles has been shown to have very high turtle mortality rates and be one of the most dangerous roads for wildlife in the country.³ Particular behaviors also put some animals more at risk. Chimney swifts eat insects and fly close to the ground as they follow prey. When these birds follow prey that fly over roads, it increases their chances of being struck by a car.⁴ Groups of animals like amphibians that have regular mass migrations are also particularly vulnerable. ⁴

Some animals are prone to road mortality due to their natural defense mechanisms, which although effective when dealing with natural predation, are often poorly equipped to handle the dangers posed by cars. Turtles often perceive cars as a threat and draw into their shell to protect themselves, which can put them at risk for getting struck by a car because they stay on the road longer.⁵ Snakes may also become immobilized when approached by a car and may remain immobilized for a minute after a car passes.⁶ Venomous snakes may be less inclined to flee from a perceived threat because they typically use venom for defense.⁶ Turkey vultures that normally fly away from predators may be unable to initiate flight quickly enough when approached by a fast-moving vehicle.⁷ The way in which animals naturally respond to threats may increase their risk of being stuck by cars.

Animals may also be attracted to the road surface. Reptiles like snakes and turtles sometimes bask on the warm asphalt of the road to regulate their body temperatures.⁵ Many scavengers prey on the carcasses of animals that have been killed on the road. These kinds of behavior increase the risk of mortality as they cause animals to spend more time around the road.

Rates of mortality are closely linked with movement patterns, as more movement generally incurs a greater chance of coming into contact with a road. Animals with large home ranges, such as Florida panthers with ranges of up to 630 km², have a high chance of encountering roads as they traverse such large distances.⁸ Movement during particular life stages can also result in peaks in mortality. These patterns are often associated with reproduction, as when gravid turtles undergo migrations to seek out a site to nest.⁹ In some snake species, the male can increase its home range up to 6 times as it searches for a mate. Many reptiles begin dispersing immediately after hatching, which also results in mortality peaks.¹⁰ In red foxes, the female repeatedly visits cubs at breeding sites, sometimes up to 10 times in a single night, which increases the chance for mortality.¹¹ Some studies have reported movement rates as the single greatest factor influencing the risk of road mortality.¹⁰

These specific factors that influence mortality can also result in demographic shifts in the population when particular segments of the population are killed. For example, aquatic female turtles make egg-laying migrations that males do not make, which puts them at a greater risk for mortality. As a result, turtle populations near roads can become male-biased as females are differentially killed.⁹ These types of shifts can further exacerbate population declines and threaten population viability. In some populations, the presence of roads has been identified as a component in the “extinction vortex” by amplifying the threats animals already face.¹²

Habitat Fragmentation & Alteration

In addition to causing direct mortality, roads can have a number of indirect impacts such as habitat fragmentation. This can result from either animals not being able to cross the road without being killed or through avoidance of the road. For example, some snakes have been shown to turn around and not cross the road when they encounter it.⁶ Some animals avoid the surface of the road even when there are no cars driving on it. Birds that typically fly short distances from one tree to the next may also be hesitant to fly across a large open space, which restricts their movements across roads.¹³

When roads create barriers to movement they can impact animal populations in many ways. One of these is through prohibiting gene flow. For example, in timber rattlesnakes, a study of genetics at hibernacula showed that in hibernacula that were blocked off by roads, genetic diversity was lower than in those that occurred across contiguous habitats.¹² Additionally, some male snakes follow trails of pheromones along the ground in order to locate mates. Roads can disrupt the pheromone trail and make it difficult for males to follow the trails and find a mate.¹⁴

Animals may also suffer by not being able to access particular habitats. In times of drought, roads can prohibit animals from reaching water.¹⁵ A study of turtles showed that roads could prevent gravid females from reaching their preferred nesting sites. As a result, they were relegated to suboptimal habitats where predation on their eggs was higher, which decreased reproductive success.¹⁶

In addition to fragmenting habitat, constructing a road alters the habitat. When a road runs through a forest, it creates an edge habitat along the portion of the forest that fringes the road. This can have consequences for birds, as predation rates on bird nests are sometimes higher in edge habitats. This is because predators can prey on nests better in the edge, where the forest canopy offers less protection to nests.¹⁷ Some species of turtles prefer to nest along the edge of the road because it creates an ideal nesting habitat. In highly degraded wetlands, the edge of roads may be the only viable nesting habitat left available to these turtles.¹⁸ Unfortunately, the nesting turtles put themselves at greater risk of mortality as they cross roads, and hatchlings may also be struck by cars as they disperse from the nest after hatching. Frogs have also been shown to experience higher predation rates closer to roads.¹⁹ Through altering the habitat and creating an edge, roads can impact animals even when direct mortality is not a result.

Pollution

Roads can also be a conduit for pollutants into the environment. The debris from tires on the road can decrease the time to metamorphosis of wood frogs.²⁰ Deicing salts that run off from roads into adjacent ponds can decrease survivorship of wood frogs and spotted salamanders.²¹ Frogs have been shown to have higher skeletal abnormalities closer to roads, possibly as a result of contamination.¹⁹ In addition to causing mortality, deicing salts can alter the behavior of frogs and decrease locomotor performance. This can impact fitness, as they may be less adept at catching prey or eluding predators.²² Roads also carry oils from cars that travel across them, which have the potential to harm wildlife when they enter the environment.

Pollution from roads extends beyond just chemicals, as light and noise pollution from roads can be detrimental as well. Noise from cars can impact birds by disrupting acoustic communication and interfering with warning signals, leading to bird population declines in the proximity of roads.²³ Not all birds are equally affected, however, as those that have song frequencies similar to car frequencies are more likely to be absent from roadside areas. In addition to decreasing the numbers of birds, road noise can alter the community composition of birds as certain species are differentially excluded.²⁴ Similarly, roads can interfere with the calling of frogs and make it difficult for them to find a mate.²⁵

Animals that rely on light for the control of biological activities can be impacted by the lights along roads. Robins that use sunlight as a cue to initiate songs in the morning can mistake lights for the sun and sing in the middle of the night.²⁶ Road lights can also alter the routes that bats fly. Exposure to artificial lights at night may delay the time to metamorphosis in some frog species.²⁷ Hatchling sea turtles that use light to navigate to the ocean can also be confused by road lights and go toward the road instead of going to the water. When this happens, they often die from dehydration, predators or get hit by cars on the road and they never reach the ocean. Nesting females that emerge onto the beach to lay their eggs can also become disoriented by road lights and have difficulty making it back to the water.²⁸

When land is cleared for roads, it often facilitates the spread of invasive species. Cane toads are incredibly invasive in Australia and they have used roadside areas for movement, which has increased their range.²⁹ The area along the road also provides an ideal habitat for invasive fire ants that build mounds by the road.³⁰ Several invasive species of plants also infiltrate habitats using roads. Roads can facilitate invasions because these plants face less competition from plants in a newly cleared area.

Mitigation

A number of mitigation strategies have been developed to decrease the harmful impacts of roads on wildlife. In cases where patterns of mortality are predictable during certain times of the year, road closures or speed limit reductions during these times may decrease mortality rates.¹⁰ Road closures during breeding migrations have been used to successfully decrease mortality of the Jefferson's salamander in Ontario. Since many gravid reptiles are killed while trying to find a nesting site, the construction of artificial nesting sites may prohibit them from needing to cross the road. For snakes, artificial hibernacula can also be constructed to reduce distance traveled and the chances of road mortality.

Decreasing traffic volume is not likely to be effective when animals avoid the physical surface of the road. In such cases, constructing alternative ways of crossing the road can be more successful. For smaller animals, culverts that pass under roads can allow for safe passage. Barriers along the road are also constructed to guide animals to the culverts. This has shown to be extremely effective in reducing road mortality in some cases. A drawback of culverts is that they primarily facilitate crossing by smaller animals. For larger animals, wildlife overpasses have been used to allow crossing. They sometimes have vegetation planted over them to mimic the natural habitat. Studies have shown that red deer and wild boar will use such structures to cross the road and usage may increase over time as animals become accustomed to them.³¹

Several factors must be taken into consideration to ensure the effectiveness of road crossing structures. The diameter of culverts may impact which animals will use it, as does the presence of vegetation around it.³² Frogs prefer crossings that are lined with gravel rather than pipe or concrete.³³ Because some animals avoid the noise of the road, the placement of the culvert in areas of lower traffic density may result in increased use. Appropriate design of crossings depends on the specific target species and will likely vary across different geographical regions. Research can provide the insight needed to construct wildlife crossings that facilitate passage over roads and eliminate some of the negative consequences of roads on wildlife.

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