A dead animal’s remains at the roadside is a common sight in many parts of the world, from rabbits to deers to foxes. For centuries, trains, horse-drawn carriages, and other vehicles have been killing animals but it was only really given any notice in 1924. On July 13, biologist Dayton Stoner and his wife, Lillian, were on their way to a field station to do some bird banding. While traveling, they noticed that their route was strewn with “a considerable number of dead animals, apparently casualties from passing motor cars.”
The couple decided to count the dead animals they would be seeing.
According to The Atlantic, an online site that covers news, politics, culture, technology, health, and more, they reportedly saw 225 animals. The list included 53 red-headed woodpeckers, 19 flickers, 18 ground squirrels, 14 garter snakes, 12 cottontails, two weasels, and one woodchuck.
Stoner cautioned in a paper published in the journal Science that the automobile must be recognized “as one of the important checks upon the natural increase of many forms of life.” Since then, scientists have used Stoner’s concern for further research. They concluded that roads have surpassed hunting to become “the leading direct human cause of vertebrate mortality on land.”
"This is a relatively new source of fatality; and if one were to estimate the entire mileage of such roads in the state, the mortality must mount into the hundreds and perhaps thousands every 24 hours," California naturalist Joseph Grinnell said.
In Europe alone, it’s estimated that 194 million birds and 29 million mammals die on roads annually. Meanwhile, about 253,000 animal-vehicle accidents in the US are reported every year. The most cited pegs America’s roadkill toll at one million critters a day. However, experts say that we shouldn’t worry because the most killed animals on the roads are from the ranks of the superabundant such as raccoons, squirrels, and others. It’s not the same with rarer species, unfortunately.
For rarer species, roadkill can represent an existential threat. A study identified 21 threatened or endangered species that are jeopardized by the four-wheeled menace from the Houston toad to the Florida panther. While no studies have done a thorough worldwide count, rough estimates show that about 365 million vertebrates are killed every year. Some roadkill deaths are even intentional.
In an experiment, scientists put a fake snake, a fake turtle, or a piece of garbage on a highway in Canada. The researchers found out that fake animals got hit a disproportionate amount of the time, leading the scientists to calculate that 2.7% of drivers were intentionally hitting them. The same trend was also seen in Brazil and Australia.
Roadkill rates, however, are declining due to the COVID-19 pandemic. According to National Geographic, an American pay television network and flagship channel that is owned by National Geographic Partners, fatal collisions with deer, elk, moose, bears, mountain lions, and other large wild animals in the US during the peak of the lockdowns in March and April decreased by as much as 58%. A similar plunge was seen in dogs, sheep, and other domestic animals.
Fraser Shilling, co-director of the University of California, Davis Road Ecology Center, said that about 500 million vertebrates would not be killed on roads and highways if the current slowdown in the country lingers and nationwide traffic remains on average 50% lower for a year. "The longer it lasts, the more animals that would have died are not dying," she said.
Environmental Impacts of Roadkill
Knowing the extent of roadkill can help us understand the risks to protecting biodiversity and threatened species. In Brazil, which is thought to be the most biodiverse country in the world, researchers estimated that there were more than 10 million roadkill on its roads each year. This figure will not only impact the country but also our whole ecosystem. A 2018 study revealed that animals that are most at risk of being killed on roads are those that eat a more diverse diet and use a range of habitats. This is because they encounter roads more often and decide to cross them
"Roadkill is known to have an impact on biodiversity, but little work has been done to quantify this, and identify which animals are most at risk. We have shown for the first time that roadkill risk can be predicted based on species' characteristics or traits, such as their size, behavior, and ecological preferences,” Manuela Gonzalez-Suarez, a conservation biology lecturer at the University of Reading, said.
Gonzalez-Suarez said that roadkill could even cause some species to disappear completely. According to Euronews, an online site that provides European and world news, the long-term survival of several common animals in Europe are at risk due to busy roads. There’s a possibility that hazel grouse, a small bird species, and the russet ground squirrel could become locally extinct in places where roads put wildlife most in danger. This can reduce population abundance, limit dispersal, decrease genetic diversity, and eventually lead to local extinction.
The impact of roadkill is also seen through the deaths of large predators, which play an integral role in balancing ecosystems. A recent study found that there’s been a steep decline in California’s mountain lions due to habitat fragmentation by roads, leading to vehicle deaths and inbreeding among cut-off populations. Due to coronavirus lockdowns, the number of mountain lions killed by cars fell by 58%.
“In this case, there’s a tiny, tiny silver lining of the coronavirus that has slowed cars down or has reduced the number of cars,” Winston Vickers, a wildlife research veterinarian who directs the California Mountain Lion Project at the UC Davis Wildlife Health Center, said.
Unfortunately, the impact of roads on wildlife is often underestimated due to scavengers. While they perform a valuable ecosystem service by removing roadkill in urban areas, researchers are unable to properly estimate or analyze the impact of roads on wildlife. In a 2018 study, the team found out that scavengers were able to remove about 76% of the roadkill corpses within 12 hours.
According to Phys.org, an internet news portal that provides the latest news on science, lead author Amy Williams Schwartz concluded that the most important factor causing underestimations of roadkill numbers, particularly of small animals such as garden birds and rodents, is the removal of animals by scavengers.
"Our study demonstrates the frequency and speed at which scavengers can remove roadkill, and the extent to which we could underestimate the true number of casualties, but also suggests that many urban scavengers such as crows, gulls, and foxes could be providing an under-appreciated and largely unnoticed carcass removal service in our cities,” Schwartz said.