The great irony of roadkill is this: Its most conspicuous victims tend to be those least in need of saving. Simple probability dictates that you’re more likely to collide with a common animal—a squirrel, a raccoon, a white-tailed deer—than a scarce one. The roadside dead tend to be culled from the ranks of the urban, the resilient, the ubiquitous.
But roadkill is also a culprit in our planet’s current mass die-off. Every year American cars hit more than 1 million large animals, such as deer, elk, and moose, and as many as 340 million birds; across the continent, roadkill may claim the lives of billions of pollinating insects. The ranks of the victims include many endangered species: One 2008 congressional report found that traffic existentially threatens at least 21 critters in the U.S., including the Houston toad and the Hawaiian goose. If the last-ever California tiger salamander shuffles off this mortal coil, the odds are decent that it will happen on rain-slick blacktop one damp spring night.
The poster groups for roadkill’s hidden toll are reptiles and amphibians, known collectively as “herps.” The small bodies and secretive habits of herps conceal their dominance: Many forests in the eastern United States support more salamanders than small mammals and birds combined. But about one in five reptile species and two in five amphibian species are threatened with extinction, and many more are on their way. Snapping turtles and spotted salamanders aren’t on the verge of vanishing altogether, but they’ve become rarer and more isolated, retreating from our landscapes and our lives. Biologists refer to localized extinctions—a pond emptied of frogs here, a pool robbed of salamanders there—as extirpations, many small losses that over time can amount to a very big one. The wetlands advocate David M. Carroll has lamented “the silence of the frogs,” a hush as disquieting as the one that terrified Rachel Carson.
The forces muzzling the frogs and other herps are many—habitat loss, fungal disease, pollution—but it’s not a coincidence that herps are predisposed to become roadkill. Reptiles and amphibians move slowly and, as cold-blooded creatures, they gravitate toward warm surfaces, whether limestone or asphalt. Turtles lumber across lakefront streets to deposit their eggs; snakes slither over highways to huddle in hibernacula. Worst of all, some herps, such as northern leopard frogs, don’t attempt to race between cars, as deer do, or avoid roads altogether, as grizzly bears do. Instead, they’re what scientists call “nonresponders”: animals who are unfazed by traffic, even when prudence would serve them well.
Amphibians, whose name means “double life,” are especially susceptible. Frogs, toads, and salamanders belong to two worlds: the water in which they’re born and the upland forests where many species, upon swapping gills for lungs, spend their adulthood. When you have a toe in two realms, you must migrate between them. Amphibians move most frenziedly on spring nights, when rain refills the ephemeral pools that dimple forest floors and summons them to mate. Wood frogs that spent the winter as cold and hard as Popsicles, preserved by their own natural antifreeze, thaw and stir. Salamanders clamber from their catacombs. Peepers trill with a vehemence out of all proportion to their thumbnail-size bodies. Thousands of minuscule lives go on the march, called by wetlands that will soon be cloudy with gelatinous egg masses. In some places the emergence occurs over weeks; in others, in a few bacchanals known as Big Nights. And a salamander on a Big Night will cross any road in her path—come hell, high water, or Honda.
When an aggregation of libidinous herps boils over a road, the outcome is what some biologists call, none too scientifically, a “massive squishing.” Squishing statistics are both appalling and abstract, in the way that astronomical death tolls often are: nearly 28,000 leopard frogs killed over two separate two-year periods on a Lake Erie causeway; up to 10,000 red-sided garter snakes slain in one season in Manitoba; 2,500 toads flattened on a French road. In Indiana, scientists who counted more than 10,000 crushed animals found that 95 percent were reptiles and amphibians. You’ve probably never heard one pop beneath your tires, but in many places herps—not deer, not squirrels—make up most vertebrate roadkill.
One of roadkill’s cruelest aspects is not how many animals it culls: it’s which ones. Wild ecosystems weed out the sick and the old—the diseased fawn devoured by wolves, the senescent moose who collapses in a snowdrift. Roadkill, by contrast, is an equal-opportunity predator, as apt to eliminate the strong as the feeble. In Canada, for instance, vehicle-killed elk are healthier than animals slain by wolves and cougars. The same dynamic plagues amphibians. In New York, researchers discovered that roadside ponds held unusually small salamander-egg masses, likely because young females were getting crushed before they could grow into big, ripe breeders. Cars not only kill animals, in other words: They crush the very individuals who would otherwise help populations recover. Even a few deaths can add up. In Ontario, researchers have found, just nine roadkills a year couuld be enough to eventually rub out a cluster of black rat snakes; in central and western Massachusetts, a roadkill rate above 10 percent may eliminate any given group of spotted salamanders. By that standard, up to three-quarters of the region’s populations might be doomed.
For a long time, however, few considered roadkill a threat. Cars might have been pulverizing millions of frogs each year, but millions more crawled in the wings. Even Henry David Thoreau considered wagon-flattenings cause for perverse celebration. “I love to see that Nature is so rife with life that myriads can be afforded to be sacrificed … that tender organizations can be so serenely squashed out of existence like pulp,” Thoreau raved in Walden, “tadpoles which herons gobble up, and tortoises and toads run over in the road.”
Many biologists concurred. Roadkill was widely considered “compensatory mortality,” a form of death balanced by the scales of life. If more frogs were flattened, the thinking went, maybe fewer would get eaten by predators, or there would be more food for their tadpoles. “When you deal with a group like amphibians, which have a high reproductive rate, people just think, Well, they’ll be able to compensate for road mortality,” a Canadian biologist named Lenore Fahrig told me. “The idea that roads actually have effects on populations, I don’t think that was on anybody’s radar at all.”
Fahrig put the lie to that attitude. In the 1990s, she noticed that the herp carnage was worse on a quieter road near her Ottawa home than on the busier one—even though frogs readily cross busy streets, fewer cars had somehow produced more death. Fahrig puzzled over the conundrum and came up with a hypothesis: The busy road didn’t have many frogkills, because there weren’t many frogs left to kill. Traffic, she suspected, had already wiped out the local population. Curiosity piqued, Fahrig devised a study to test her hunch. In the spring of 1993, she and colleagues drove around Ottawa, scanning roadsides for dead frogs and toads, and stopping to listen for the trills, croaks, and squeaks of live ones. Sure enough, the busiest roads had the poorest remnant amphibian communities. Given enough time and traffic, roadkill could indeed diminish a population, even extirpate it. Roadkill was not exclusively a source of compensatory mortality: It could also be additive mortality, death that never came out in nature’s wash.
In a vacuum, reptiles and amphibians might be able to endure all of this. But they’re also bedeviled by “synergistic” threats, dangers that compound in pernicious ways. The same suburbanization that drains marshes also funnels more traffic through wetlands, piling roadkill atop habitat loss. As populations bow beneath comorbidities, they become more vulnerable. Healthy animal communities ride natural fluctuations like gulls bobbing in the surf, buffered from extinction by sheer abundance. Plunge too deep into the wave’s trough, though, and a few mishaps—a dozen SUVs on a soggy night, say—can be fatal. And landscapes, once drained of their herps, rarely refill. The roads that run between wetlands and uplands also sever the link between these realms, destroying any frog or newt brazen enough to strike out toward an empty pond. Roads disunite land and water, short-circuiting the experience of amphibiousness.
The diminishment of herps is a hard problem to grasp. Utter extinction, the fate that befell the passenger pigeon and the Carolina parakeet, is a concept universally understood. Yet extinction is rarely instantaneous, and the gradual ebbing of abundance that precedes it strains language. Some researchers have called such insidious losses “defaunation”; others know them as “biological annihilation.” The biologist E. O. Wilson favored “Eremocene”: the age of loneliness, a near and desolate future in which humankind bestrides an empty world, or perhaps drives over it.
This article has been adapted from Ben Goldfarb’s forthcoming book, Crossings: How Road Ecology Is Shaping the Future of Our Planet.