How Roads Affect World's Most Vulnerable Animals

Study finds which species are at risk of extinction due to vehicle collisions.

cheetah crossing road in India
A cheetah crosses a road in India. Nishant Shah / Getty Images

One of the reasons animals lose habitat is because of roads.

Key for infrastructure in order to move people and supplies, roads can be deadly for the wildlife that surround them.

A new study has identified four animal species that are the most likely to go extinct in the next 50 years if the same roadkill rates continue. Researchers pinpointed the leopard of North India, the maned wolf and little spotted cat of Brazil, and the brown hyena of Southern Africa.

The results were published in the journal Global Ecology and Biogeography.

“Research has shown that roads are another threat for many species. If species are already threatened by habitat loss and poaching, roads can make these species more vulnerable to extinction,” Clara Grilo, the study’s lead author and a postdoctoral research fellow with the Universidade de Lisboa in Portugal, tells Treehugger.

“There was the doubt about which species are more impacted by roadkill: the ones with high roadkill rates or the ones that are already threatened.”

For their study, researchers estimated the average proportion of terrestrial mammal populations killed annually on roads in a three-step process. First, they collected roadkill data on near threatened to critically endangered mammal species in North America, Central and South America, Europe, Africa, Asia, and Oceania.

They calculated increased extinction risk due to road deaths by taking into account information such as roadkill rates and population density, as well as traits like age of sexual maturity and litter size. Using these models, they created global roadkill vulnerability maps.

They found that the leopard (Panthera pardus) in North India had an 83% increased risk of extinction from roadkill. The maned wolf (Chrysocyon brachyurus) of Brazil has a 34% increased risk. The little spotted cat (Leopardus tigrinus) of Brazil and brown hyena (Hyaena brunnea) of South Africa have an increased extinction risk ranging from zero to 75%.

The findings revealed that deaths on roads are a risk for 2.7% of terrestrial mammals, including 83 species that are threatened or near-threatened. The researchers were able to identify areas of concern with species vulnerable to road deaths that have a high density of roads in parts of South Africa, central and southeast Asia, and the Andes.

Why Details Matter

The researchers were interested in information about litter size and age of maturity because some traits such as large litters and early age of sexual maturity can help species rebound from the cost of roadkill deaths, Grilo says.

But for animals like brown and black bears that have small litters and an older maturity age, road deaths can have a large toll on their populations.

“Using the phylogenetic models, we could predict which species are more vulnerable to roadkill and found that brown bear and black bears are particularly vulnerable,” Grilo says. “If there is at least 20% of the population road killed it can increase by 10% the risk of local extinction.”

In Florida, vehicle collisions are responsible for 90% of known bear deaths, according to the Florida Fish and Wildlife Conservation Commission.

Protecting Species

The researchers say they were not totally surprised by their findings.

“We were not completely surprised with the fact that species with low roadkill rates can be more endangered than species with high roadkill rates,” Grilo says.

“In general, the most abundant species can compensate for the loss of individuals because they have high reproduction rates (for example with a high number of litters per year or large litter size). We were somehow surprised by the number of species at risk and the number of species vulnerable if exposed to road traffic.”

Of the four species most affected, they didn’t necessarily have the highest mortality rate on roads.

“Even though these populations had relatively low roadkill rates, the abundance was also low,” Grilo explains. “Thus the impact on the population can be very high.”

Researchers say their findings are important and can be used to protect many species.

“From the point of view of conservation, we should look not only at the number of roadkill but also what proportion of the population is roadkilled,” Grilo says. “Thus, we should take into account the population density. If we just look at the number of roadkill we may protect abundant species and not the ones that are more impacted by the roadkill.”

View Article Sources
  1. Grilo, Clara, et al. "Conservation Threats From Roadkill in the Global Road Network." Global Ecology and Biogeography, vol. 30, no. 11, 2021, pp. 2200-2210., doi:10.1111/geb.13375

  2. "New Study Shows Impact of Roadkill on World’s Vulnerable Animal Populations." University of Central Florida, 2021.

  3. Clara Grilo, the study’s lead author and a postdoctoral research fellow with the Universidade de Lisboa in Portugal

  4. "Roads." Florida Fish and Wildlife.