Humans not directly responsible for North American reptile deaths

By Joshua Rapp Learn

A long-nosed leopard lizard (Gambelia wislizenii) appears in Arches National Monument. ©gilaman

Humans aren’t directly to blame for most North American reptile deaths, according to a large new meta-study that examines a host of research in the United States and Canada.

But they are likely still having an indirect effect on reptile deaths by introducing invasive species and climate change.

“We found that there wasn’t much of an impact of human footprint on reptile mortality. Most of the mortality was from natural causes than human causes,” said Jacob Hill a postdoctoral researcher in wildlife conservation at the State University of New York College of Environmental Science and Forestry and lead author of a new study published in Acta Oecologica.

The study is part of a larger examination using meta-analysis of other research to find the principle causes of death of birds, mammals and amphibians. For this study, Hill and his co-authors focused on reptile mortality.

They looked at 57 studies that tracked reptile populations through GPS devices or other means. The bulk of the reptile populations tracked in the U.S. and Canada were turtles and snakes. The team divided reptile deaths into different categories. Natural death categories included disease, starvation and predation, while human-caused deaths included categories such as directly killing reptiles for harvest, because they were perceived as a threat and vehicle collisions.

They found that 78% of the mortalities occurred due to natural causes while only 22% occurred directly from humans.

Of the direct causes, most reptiles were killed by vehicles. This is likely due to the species’ tendencies to bask on roads and highways, the researchers said. The primary defense turtles use — retreating into their shells — doesn’t work well against cars, Hill said, and snakes — especially venomous ones — don’t typically feel threatened by much at all, and they get hit when they refuse to give up ground to vehicles.

“[Vehicles] can be problematic for reptiles. They have a lot of traits’ that make them very susceptible,” Hill said.

The number of deaths from vehicle collisions didn’t necessarily go up in areas of higher human activity. This suggests that roadkill deaths are more likely due to reptile behavior than an increase in human population, he said.

The causes of reptile deaths differed greatly from the study they did on mammals, which include game species. Those species are often hunted and harvested more than reptile species.

The data in their study didn’t necessarily reflect the whole story of human impact on species, Hill cautions, since humans can indirectly cause reptile deaths by introducing invasive species that prey on them. This could be especially a problem since predation is the highest natural cause of death, according to the study. Climate change or habitat loss can also drive reptiles into marginal environments that can lower their overall survival rate.

Joshua LearnJoshua Rapp Learn is a science writer at The Wildlife Society. Contact him at jlearn@wildlife.org with any questions or comments about his article.

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