Researchers determined how roadkill persistence could affect population estimates
When vehicles flatten frogs on the road, the remaining amphibian pancakes can persist on the pavement for quite some time. This presents a conundrum for researchers trying to get an average count of wildlife strikes.
But a better understanding of the factors affecting decomposition and general persistence of carcasses can help improve amphibian population estimates and the understanding of the threats they face from road crossings.
“Both temperature and precipitation had a pretty big impact on how long the carcass would stay on the road,” said Sean Boyle, an assistant professor of conservation biology at Memorial University of Newfoundland and Labrador-Grenfell Campus.
Park managers wanted to resurface a road that runs through Presqu’ile Provincial Park, an area that encompasses a peninsula in Lake Ontario. Other researchers quantified mammal strikes on the road before its reconstruction, but no one had examined amphibian or reptile strikes.
As part of a side project during Boyle’s PhD at Laurentian University in Ontario, he decided to look at quantifying road mortality in amphibians, focusing on factors like vehicle-strike hot spots and how many individuals were being killed and when.
Part of understanding amphibian road mortality meant being able to quantify how long a carcass has been around. Boyle published a study in the Journal of Wildlife Management looking at what drove the persistence of amphibian carcasses on the Presqu’ile Provincial Park road.
In 2015, Boyle and his colleagues surveyed the road from May to the end of August. They examined 91 carcasses until they disappeared, mostly American bullfrogs (Lithobates catesbeianus), green frogs (L. clamitans), American toads (Anaxyrus americanus), gray treefrogs (Dryophytes versicolor), wood frogs (L. sylvaticus) and northern leopard frogs (L. pipiens)—the latter was the most common.
They found that on average, frog carcasses stayed on the road for 5.4 days. They eventually disappeared due to other vehicles constantly flattening them or flipping them off the road. Scavengers likely got others, and some gradually deteriorated over time. Sometimes, carcasses disappeared from a combination of these factors.
The longest carcass stayed on the road for 30 days.
Why do frog carcasses disappear from roads?
The team found that weather—temperature and precipitation—affected carcass persistence the most.
Carcasses stayed on the road longest during cold, dry periods. In contrast, carcasses disappeared the most quickly under wet and hot conditions.
These results suggest that researchers studying the threat of vehicle strikes on amphibians—and possibly other animals—should consider weather conditions during their study period and possibly work calculations accounting for the weather into their modeling.
Ultimately, the road was resurfaced in 2016, and mitigation measures like fencing and culverts for wildlife crossing were installed at wildlife collision hot spots. In another paper published earlier examining vehicle strikes before and after the road was redone, Boyle and his colleagues found that these measures reduced turtle and amphibian strikes by an estimated 70%.
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