The great paradox surrounding roadkill lies in the fact that the most noticeable victims are often the least in need of preservation. Basic probability tells us that you are more likely to encounter a common animal like a squirrel, raccoon, or white-tailed deer on the road, rather than a rare species. The casualties found on roads tend to come from urban, resilient, and ubiquitous animals. However, roadkill plays a significant role in the current mass extinction our planet is facing. Every year, American cars kill over 1 million large animals such as deer, elk, and moose, as well as up to 340 million birds. Billions of pollinating insects may also meet their end as roadkill. This includes many endangered species; in fact, a congressional report from 2008 revealed that at least 21 critters in the US, including the Houston toad and the Hawaiian goose, being existentially threatened by traffic. So, if the last-ever California tiger salamander were to meet its end, it’s likely to be on a wet road on a rainy spring night.
Reptiles and amphibians, known collectively as “herps,” are the poster children for roadkill’s hidden toll. The small size and secretive habits of herps mask their ecological dominance. In fact, many forests in the eastern United States have more salamanders than small mammals and birds combined. However, about one in five reptile species and two in five amphibian species are currently facing the threat of extinction. Snapping turtles and spotted salamanders, for example, are not on the verge of vanishing completely, but their populations have declined and become more isolated, retreating from our landscapes and our lives. Biologists refer to these local extinctions as “extirpations,” which are small losses that can accumulate over time to become a significant one. Wetlands advocate David M. Carroll has lamented this situation, referring to it as “the silence of the frogs,” an unsettling hush reminiscent of Rachel Carson’s Silent Spring.
There are several factors at play that make herps particularly susceptible to being roadkill. Reptiles and amphibians move slowly and, as cold-blooded animals, are attracted to warm surfaces such as asphalt or limestone. Turtles, for example, cross roads near lakes to lay their eggs, while snakes travel across highways to find hibernation spots. What’s worse is that some herps, like northern leopard frogs, do not attempt to avoid cars or race between them like deer do. These “nonresponders” are unfazed by traffic, even when it would be wise for them to exercise caution.
Amphibians are particularly vulnerable due to their unique life cycle. Frogs, toads, and salamanders live in two worlds—the water where they are born and the upland forests where they spend their adulthood after transitioning from gills to lungs. Moving between these two realms is a necessity for them. Amphibians are most active on spring nights when rain fills the temporary pools on the forest floor, calling them to mate. Wood frogs thaw and stir after spending the winter as cold and preserved as Popsicles, thanks to their natural antifreeze. Salamanders emerge from their underground hideouts, and tiny peepers make their presence known with powerful trills. Thousands of these tiny creatures embark on a journey toward wetlands teeming with gelatinous egg masses. This migration occurs over weeks in some places but in just a few nights known as Big Nights in others. During these Big Nights, a salamander will cross any road in its path, no matter the obstacles.
When an aggregation of mating herps attempts to cross a road, the result is what biologists have informally deemed a “massive squishing.” The statistics related to this squishing are both appalling and abstract, much like astronomical death tolls. For instance, nearly 28,000 leopard frogs were killed over two separate two-year periods on a causeway near Lake Erie. Up to 10,000 red-sided garter snakes were slain in a single season in Manitoba, and 2,500 toads met their fate on a French road. In Indiana, scientists discovered that reptiles and amphibians accounted for 95% of the crushed animals out of more than 10,000 counted roadkill victims. It’s likely that most vertebrate roadkill is made up of herps, rather than deer or squirrels.
One of the cruelest aspects of roadkill lies not in the number of animals it claims but in the specific victims. In wild ecosystems, natural selection weeds out the sick and old, ensuring the survival of the fittest. Roadkill, on the other hand, is an indiscriminate predator, taking out both strong and weak individuals. In Canada, for example, elk killed by vehicles tend to be healthier than those killed by wolves or cougars. A similar dynamic affects amphibians. Researchers in New York found that roadside ponds had smaller salamander egg masses, likely because young females were being crushed before they could reach reproductive age. This means that cars not only kill animals but also eliminate individuals that could contribute to the recovery of populations. Even a few deaths can have a significant impact. In Ontario, just nine annual roadkill incidents could ultimately wipe out a colony of black rat snakes. In central and western Massachusetts, a roadkill rate above 10% could lead to the elimination of any given group of spotted salamanders. By these standards, up to three-quarters of the region’s populations could be doomed.
For a long time, roadkill was not considered a significant threat. While cars were squashing millions of frogs each year, there seemed to be plenty left. Even Henry David Thoreau celebrated wagon-flattened animals in Walden, declaring his fascination with nature’s abundance. This view was shared by many biologists who considered roadkill to be a form of compensatory mortality, where the animals killed on the road would be balanced out by other factors such as predation or increased food availability. The idea that roads could have population-level effects on amphibians was not on anyone’s radar.
However, biologist Lenore Fahrig challenged this perspective. In the 1990s, she noticed that a quieter road near her Ottawa home had more frog roadkill than a busier road. This didn’t make sense since frogs readily crossed busy streets. Fahrig hypothesized that the lack of frog roadkill on the busy road was due to the population already being wiped out by traffic. Intrigued, Fahrig conducted a study in 1993, scanning roadsides for dead frogs and using auditory cues to identify live ones. Her findings confirmed her hunch: the busiest roads had the smallest remaining amphibian communities. This meant that given enough time and traffic, roadkill could indeed decrease and even eliminate populations. In other words, roadkill wasn’t just compensatory mortality, it was also additive mortality that affected populations in ways that nature’s processes couldn’t balance out.
Reptiles and amphibians are not only challenged by roadkill but also by other synergistic threats that compound their vulnerability. Urbanization, which leads to habitat loss, also increases traffic through wetlands, resulting in more roadkill on top of habitat destruction. As these populations face multiple threats, they become even more susceptible to extinction. Healthy animal communities can withstand natural fluctuations, but once they reach a critical point, a few instances of roadkill can be devastating. Once herp populations are depleted, it is rare for them to recover. The roads that divide wetlands and uplands disrupt the connection between these habitats, preventing frogs and newts from reaching breeding sites. The roads essentially separate land and water, leading to further declines in herp populations.
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