Roadkill Is a Problem

A road sign for newt crossing
Paul Quayle / Design Pics / Getty Images

Collisions between wildlife and vehicles are one of the environmental consequences of roads, and a serious public safety issue. It is only one aspect of road ecology, but roadkill is certainly one of the most visible. We all have observed dead deer, raccoons, skunks, or armadillos on the road. While it is certainly unfortunate for these individual animals, their population or species is generally not at risk.

Our concerns are usually limited to public safety and damages to vehicles. However, we hardly notice the countless small birds, small mammals, reptiles, and amphibians we hit or run over frequently. Here’s what we know about the conservation significance of roadkill for wildlife.

Birds

Songbirds are being killed by cars at high rates. Estimates vary, but sources put the yearly toll at 13 million birds in Canada. In the United States, a different study estimated 80 million deaths per year from cars. This is in addition to the hundreds of millions of birds killed every single year by communication towers, wind towers, house cats, and windows. This accumulation of stresses on bird populations may be enough to threaten some species over the long run.

Amphibians

Some amphibians that breed in ponds and wetlands, such as spotted salamanders and wood frogs, migrate in large numbers during a couple of wet spring nights.

On their way to their breeding ponds, they may cross roads in large numbers. When these crossings occur over busy roads, it can lead to massive mortality events. Eventually, some species can be locally extirpated (the term for local extinction) mainly because of these massive road mortality events.

Turtles

Because of how slow they are, turtles are vulnerable to cars. They often need to cross roads to move between wetlands, or to access nesting areas. In addition, the soft roadside dirt often attracts turtles looking for a sunny nesting spot. However, one of the biggest problems for turtle populations is the vulnerability associated with their population structure. Turtles are slow growing animals that start reproducing late in life, and produce few offspring every year. To balance this low productivity, they evolved a solid shell to ensure they can live a long time (some over 100 years) and have many chances at reproducing. That shell is no match to a car’s wheels, though, and adults that should enjoy a high survival are killed in their prime, leading to widespread population declines.

Mammals

Mammals that have small population are sometimes directly threatened by extinction from road mortality. The Florida panther, with less than 200 individuals remaining, has been losing up to a dozen individuals a year because of roadkill. Such a small population cannot sustain that level of pressure, and the State of Florida has implemented measures to reduce road mortality for panthers. Similar problems are experienced by other mammals such as mountain lions, European badgers, and some Australian marsupials.

Even Insects!

Road mortality may be a concern even for insects. A study published in 2001 estimated that the number of monarch butterflies killed by cars in the state of Illinois may exceed 500,000 individuals. These numbers are particularly troublesome in light of the recent steep decline in monarch populations range-wide (note that for anyone wanting to assist with monarch conservation, Monarch Watch is a great citizen science project).  

Sources

Bishop and Borgan. 2013. Avian Conservation and Ecology.

Erickson, Johnson, & Young. 2005. USDA Forest Service General Technical Report.

McKenna et al. 2001. Journal of Lepidopterists’ Society.